Final Report Summary - CLIP (Enhancing Printed Electronics Applications by SMEs)
Executive Summary:
– Low cost conductive inks have been developed for screen printing and flexography printing based on a mixture of low cost copper flakes and a small fraction of nano copper. In combination with Photonic sintering, surface resistances comparable to thermal sintered thick film silver inks were realized. It has been proven that end user prices of low cost copper inks can be at least 50% lower than the prices of available thick film silver inks.
– In several demonstrators it has been shown that these low cost copper inks in combination with Photonic sintering result in comparable performance and in substantial cost decrease of the printed demonstrators. These low cost copper inks can also enable new PE applications such as for long distance readable UHF RFID´s.
– Developing of higher conductive multi-modal nano silver inks for ink-jet and Aerosol-jet can lead to substantial cost reduction by printing lower wet thickness (lower ink volume) in reference to available mono-modal nano silver inks.
– It is shown in demonstrators that the performance of these multi-modal silver inks is less critical in high conductivity demanding applications and can enable new PE applications such as low cost chip-less RFID tags and smart packaging sensors.
Project Context and Objectives:
The high cost of available silver thick film and nano-metal conductive inks is a bottle neck in several existing and potential PE applications. The high cost of these inks is not only the result of raw metal cost as in thick film silver inks , but also a result of the process cost to make the components of the conductive inks as in silver or copper nano particle inks. Nano particles are inevitable for making ink-jet (small diameter of inkjet head) and aerosol-jet printing inks (making the aerosol).
First main objective was development, formulation and feasibility assessment of several lower-cost components for silver thick film and silver nano-particle based conductive inks. Besides several sizes of silver nano particles for making bi-modal and tri-modal distributions also copper nano particles and silver coated copper nano particles were made by a wet chemical method. Available nano copper particles (IML: plasma enhenced vapour condensation process) and copper flakes (AVL: milling process) were optimised to be use as components in screen and flexography inks.
Second major objective is to develop optimized ink dispersions for printing and curing for a number of processes including screen print, flexography printing and aerosol-jet and ink jet.
The third main objective is to optimize the inks towards large area printing technology and towards techniques, which enable higher resolutions than in screen-printing. We realized during the project that the way of sintering is not only important to reduce the cost of large area printing, but is also strongly connected to the applicability of certain low cost alternatives of silver nano particles to realize the high conductivity. The way of sintering has also implications on the total cost due to the investment in specific equipment. We tested several Photonic sintering technologies and learned that laser sintering was most appropriate for obtaining high conductivities of copper based low cost inks printed on temperature sensitive substrates as PET film and paper.
The last major objective is the realization of the demonstrators based on these inks. Several demonstrators were made showing the cost benefits of the developed inks for ink-jet and aerosol-jet in the case of nano silver based inks and for screen and flexography printing in case of copper based low cost inks.
Project Results:
1. Base metals used for conductive inks
The most conductive base metals are gold, silver, copper and aluminum. Although there are some applications were gold and aluminum inks are used, silver and copper are the best candidates for developing low cost inks. (Fig 1) Gold is too expensive for commodity inks and is only used for its high inertness in biomedical applications.
Aluminum oxidizes very fast in air and the oxides are non-conductive. Aluminum inks are used in back electrodes for PV cells because thick film silver inks are too expensive, but one has to print very thick layers of it to meet the specifications for surface resistance.
Thick film silver inks have been used for decades in membrane switch and touch screen applications, but during the last 10 years the pure metal has increased 4-fold in price. Printers still using these silver inks because of its high conductivity and inertness (silver oxides are also conductive) and because there is today no real alternative.
Copper, which is a low cost conductive metal, has been tried as conductive component for thick film inks by using silver coated copper to prevent oxidation of the copper (copper oxides are non-conductive), but it seems to be difficult to cover the total copper surface of the particles resulting in low thermal stability of the printed inks, unless very thick silver coatings are depleted, resulting in comparable ink costs as pure silver inks.
2. Processes to make the metal components
Micron size metal particles are made by atomization or milling. The shape of the particles can be irregular, spherical or as flakes. Flakes are mainly used because of their higher package density and increased number of conductive contact points between adjacent flakes increasing in this way the conductivity of the printed ink.
In the development of CLIP inks we used micron size copper flakes from AVL (CLIP partner). These copper flakes (Fig 2) are made in ton scale for metallic luster paints (e.g. for car metallic lacquers) and because of that the price of it is very low.(10-15 Euro/Kg) .The copper flakes are protected from oxidation at room temperature by means of an organic coating also used as lubrication additive during the milling operation. AVL made especially for this project copper flakes with smaller lateral dimension (D100<5 micron)
Nano size particles are made by wet chemical (reduction of salts) methods or by plasma enhanced vapor condensation (Fig 3). Because of the small scale and the low volume and yield of the process nano size particles are very expensive and the price is mainly independent of the type of metal used (silver or copper). The nano size particles are coated (polymers) at the end of the process to prevent oxidation at room temperature and to improve the dispersion stability in liquids. The price of these nano particles will come down in the future if the scale of the process can be increased. This can only happen if the market for PE applications will grow substantially. IML made for this project specially coated nano copper particles of 45 nm mean diameter.
3. Concepts to make low cost screen and flexography inks
Screen and flexography inks can make use of micron size conducting particles, but in available thick film inks the polymer binder is limiting the number of contact points between the metallic particles and in this way is limiting the conductivity of the ink. Copper is easy oxidized at high temperatures and the non conductive copper oxide on its surface will also limit the conductivity. We used an ink composition consisting of copper metal flakes mixed with a small fraction of nano copper. This nano copper can be sintered to a pure highly conductive metal film at relative low temperatures and acts as conductive cement between the copper flakes. To prevent oxidation of the copper components we have to use very fast (in micro seconds) Photonic sintering techniques, so the copper surface of the micron and nano copper has not the time to be oxidized. The local high temperatures in the printed layer created by absorbing the energy pulses of the laser or xenon lamp sintering will also evaporate the other non metallic components of the ink without heating up the substrate too much. With ink compositions consisting of only 3% of nano copper and 59% of copper flakes it was possible to reach comparable surface resistances (2%-3% of bulk metal conductivity value) comparable to surface resistances of available thermal sintered thick film silver inks (3%-6% of bulk conductivity value) (Fig 4). If we calculate the total costs of making these inks, this results in a total cost of 271 $/kg compared to a total cost of 995 $/Kg for a typical thick film silver ink. So end-user prices of these low cost inks can be at least 50% lower costs than the thick film silver inks at comparable conductivities of the printed ink.
4. Concepts to make lower cost ink-jet and Aerosol-jet inks
For ink-jet and Aerosol-jet inks we tested a concept based on multi-modal distribution of nano particles. By mixing 3 nano-silver distributions (40nm, 5 nm and <1 nm) in geometric calculated ratios it was possible to reach a dense close packed structure. (Fig 5A, 5B) As the smaller particles between the larger ones sinter at lower temperature this tri-modal distribution will result in a denser structure even after sintering at lower temperatures compared to a typical mono-modal distribution with a mean diameter of 40 nm. At the same sinter temperature and time the tri-modal distribution will be more conductive compared to a mono-modal distribution. (Fig 6)
This was also observed in demonstrators based on inks with a tri-modal distribution in reference to the same demonstrator printed with available mono-modal nano silver ink (Cabot ink). Specific resistance of an ink-jet ink based on a tri-modal distribution was 39% lower than a commercial available ink-jet nano silver ink of Cabot. (Fig 7)
5. Demonstrators and applications
5.1. Demonstrators made with the low cost screen and flexography printing ink
EL-display
PEL started to print EL-displays on paper because this is a less critical application regarding surface resistance specifications because flexography inks are printed much thinner than screen printing inks (only 1% of bulk metal conductivity after xenon flash sintering is necessary). The front bus bars and back electrode were printed with the flexography ink; the transparent front electrode was printed with Orgacon (PEDOT) ink. Although the relative high surface resistance of the electrodes and bus bars, the EL display could be driven by a simple solid state inverter and the different segments lighted-up well. (Fig 8)
Other EL displays were printed on PET film with screen printed copper ink and the printed bus bars were now sintered with laser sintering leading to much better conductive tracks (Fig 9). With this ink much larger and brighter EL displays can be printed. (Fig 10)
As the cost of back electrode and bus bars of front electrode can be as high as 50% of the total ink costs for printing an EL display, substantial cost savings can be made by using the low cost copper ink instead of a thick film silver ink. (estimated cost reduction of inks to print an EL display = 25%)
Intelligent packaging demonstrator
A wireless pharmaceutical package is based on conductive ink printed on a cardboard based blister inlay, which is connected to a cellular phone module embedded in the package. This enables the tracking of one pill at the time of removal from the blister, whereby data is sent to the cellular module and than forwarded wirelessly, even instantaneously if required, using GSM or GPRS cellular networks, to electronic health record systems. This allows real-time tracking and intervention by physician and so enables physicians to make timely changes to patients’ medication.
The screen printing copper ink was used to print the conductor circuit pattern on the cardboard of the intelligent package.
The printed circuits were characterized by DC measurements. The measured resistances are in the range of 10-150 Ohm/sq depending on the length. This resistance rage is within the requirements for functional package.
UHF chip antenna
ACREO printed with the screen print copper ink a UHF RFID chip antenna on cardboard paper (Fig 11 and 12). The printed antenna was Photonic sintered with a CO2 laser. Surface resistance of the sintered ink = 100 m/sq or 3% of the bulk conductivity of pure copper metal. The antenna properties are in range of the calculated values and reading distances of 5-6 meter between antenna and detector could be realized.
This demonstrator proves that these low cost copper inks can even be used in very critical (high conductivity demanding) UHF antennas.
The use of RFID technology has increased during last years. This technology is a possible enabler for wide spread communication between objects using the internet. Internet-of-things is the commonly used expression for this foreseen massive communication technology and printed UHF antennas in combination with silicon IC´s are a possible low cost solution.
These antennas are intended for far field industrial applications especially for large items level tracking in supply chain (cardboard cartons containing water and metal objects and freight (wood and metal pallet) transportation.
5.2. Cost calculation of a printed RFID antenna
A cost calculation comparison was made for printing of typical coil shaped RFID antenna using low cost copper screen print inks and available thick film silver inks. In this calculation also investment costs for screen printing units, drying and sintering units was included. We compared the current situation where thick film silver inks were printed at 1m/min, dried and sintered in a thermal drying unit, with printing of a low cost copper ink in-line dried in a thermal oven and in-line sintered in 2 consecutive Xenon flash units at 10 m/min. (Fig 13)
Out of the results it can be concluded that printing, in-line thermal drying and in-line Photonic sintering of the antenna with the copper ink results in substantial lower cost than printing the thick film silver ink in combination with thermal curing and sintering. Most of the cost savings are in the cost of the inks (50% lower cost /pc). The total process cost for printing the antenna is 30% lower cost/pc in case of printing with the copper ink.
5.3. Demonstrators and applications of ink-jet and aerosol-jet printing inks
Chip-less RFID tags (Deliverable D15)
Chip-less RFID tags (Fig 14) were printed with ink-jet (KTH) and Aerosol-jet (SIRRIS) on PEL paper and Kapton. These chip-less tags consists of several LC resonant circuits. The capacitor dimension (L2) will determine the resonant frequency of each LC circuit
4-bits chip-less tags were printed (2 layers of tri-modal ink) and sintered at 150C for 1 hour on PEL paper and 200C/1 hour on Kapton. Excellent resonant response was measured and this could easily be read out. The same 4-bits tag printed with mono-modal Cabot ink had far less defined resonant peaks and could not detected even printed at double thickness (2 consecutive printed and dried layers). (Fig 15)
This result proves that the tri-modal ink has better conductivity as a commercial available mono-modal ink. The surface resistance of sintered (at 200° C/1h) tri-modal and Cabot ink reference was measured. The tri-modal ink had a 39% lower surface resistance which means that printed ink thickness or printed volume can be 39% lower in reference to the commercial available mono-modal Cabot ink.
Chip-less RFID tag are much cost sensitive because there is no chip in the tag, it consists only of printed LC circuits. When these tags can be printed at 39% lower thickness this results in a direct 39% price decrease of the tag.
Hybrid sensor systems for smart packaging (Deliverable D15)
A humidity sensor circuit was printed with ink-jet and aerosol-jet on PET film and PEL paper using the tri-modal ink (Fig 16). The printed circuit was sintered for 1 hour at 150C. The sensor element consists of a printed cam electrode coated with MW-CNT (multi wall carbon nano tubes) .The printed circuit connects the different components: chip, switch, LED indicators and the battery
The humidity sensor is operational although the printed circuit was printed with one layer and sintered at a relative low sinter temperature of 150 C (Fig 17). The interconnection of the chip with the printed circuit was realized by printing on a not-flat surface (on the epoxy glue to mount the chip) (Fig 18).
The humidity sensor circuit is an example of a smart packaging system which can sense and inform the customers or suppliers on the status of the products inside the packages and on the ambient conditions of the product.
This is a very broad field of application of PE and is viewed as a cost -effective approach to manufacture low cost sensor devices on temperature sensitive substrates as paper and plastic.
6. Conclusions
– Low cost conductive inks have been developed for screen printing and flexography printing based on a mixture of low cost copper flakes and a small fraction of nano copper. In combination with Photonic sintering, surface resistances comparable with thick film silver inks were realized by printing the inks at comparable wet thickness. It has been proven that end user prices of low cost copper ink can be at least 50% lower than the prices of available thick film silver inks.
– In several demonstrators it has been shown that these low cost copper inks in combination with Photonic sintering result in comparable performance and result in substantial cost decrease of the demonstrator. These low cost copper inks can also enable new PE applications even for long distance readable UHF RFID´s.
– Developing of higher conductive multi-modal nano silver inks for ink-jet and Aerosol-jet lead to substantial cost reduction by printing lower wet thickness (lower ink volume) in reference to available mono-modal nano silver inks.
– It is shown in demonstrators that the performance of these multi-modal silver inks is less critical in high conductivity demanding applications and can enable new PE applications as low cost chip-less RFID tags and smart packaging sensors.
Potential Impact:
All original main objectives of the CLIP project were reached at the end of the third period.
Silver nano particles were developed and used to make low cost inks for high resolution printing technologies as ink-jet, aerosol-jet. Copper flakes and nano copper particles were developed for high throughput printing technologies as screen and flexography printing. Specific inks for the different printing technologies were designed and tested. Demonstrators based on these inks were printed and tested.
Results of the project were disseminated at workshops and conferences.
For high resolution printing technologies ink-jet and aerosol-jet the printed ink volume of silver nano based inks can be decreased by 40% at comparable surface resistance in reference to commercial available silver nano inks. As nano silver based inks are the main cost component of high resolution printed electronics applications this cost decrease enables the application and commercialization of new and existing Printed Electronic applications that were not affordable due to high cost of nano silver inks.
For large area printing technologies; screen and flexography printing a low cost copper ink was developed based on a mixture of low cost copper flakes and nano copper particles. In reference to available thick film silver inks it is possible manufacture and commercializes these inks at less than half the end user prize at comparable surface resistance and printed thickness. Although these inks have to be cured with Photonic curing technologies, the investment in Photonic curing can be easily depreciated as Photonic curing can increase processing speed in reference to thermal curing and enable R2R printing processes. By using these low cost copper inks , existing PE applications which suffer from the high cost of thick film silver inks can now printed at a lower cost and new cost sensitive PE applications that were not affordable before because of high silver ink prices can be made at substantial lower cost enabling the application and commercialization of it.
Printed Electronic applications in general can benefit from these low cost inks and this can result in a breakthrough of this technology, leading to volume increase and more and new applications. In time this can result in more local production and more employment in Electronic and Printing Industry in Europe.
Dissemination activities
The consortium has been disseminating since the first year of the project on the project of CLIP.
Consortium partners such as Nanogap, IML spoke at different conferences such as Printed Electronics 2011 & 2012.
Seminars:
Master Class: The Future of Conductive Printing (based on results of CLIP)
A dissemination event was organised on 27 November in Diepenbeek, Belgium. The main goal was to bring the latest status on the CLIP project results and compare this with current commercial inks in the market. An overview of the state of the Art situation of conductive inks was provided. In total 5 presentations were given by the different RTO partners of CLIP, with certain support of SME partners such as IML and Nanogap.
It was clear that such a focused master class showed the reality info conductive printing. The feedback from the audience and delegates during the presentations and afterwards in the networking session showed that a project such as CLIP was bringing value to the market by revealing the whole reality. This meant that we were able to show the real bottlenecks but also the opportunities for the market.
The ESMA conference AFIP: Advanced Functional & Industrial Printing
On 6 & 7 March 2013, ESMA organized its European conference for Advanced Functional & Industrial Printing in Düsseldorf, Germany. It is a global conference for the printing industry to bring knowledge and innovation together in about 20 presentations during 2 days. The conference brought together about 150 delegates from 16 countries, 26 exhibitors and 5 non-commercial exhibitors.
During the second day, there was a special CLIP session with 3 presentation blocks. The presentation was given by the CLIP consortium.
Table 1: AFIP Conference, CLIP Presentations
The presentations were of a high level but very practical oriented and supported with nice videos. The feedback from the delegates was excellent on the CLIP topics. The commercial interest was being confirmed. In general, conductive inks and solutions were an important topic for the industry.
The consortium has also a table top at the exhibition to show all results and demonstrators that were available at that time.
CLIP presentation at Printed Electronics
IDtechEX, leader in conferences for Printed Electronics, organized during 17& 18 April 2013 their annual European conference in Berlin. They had over 1400 delegates and over 50 exhibitors during these 2 days.
Louis Bollens present at the opening of the second day an excellent presentation with the complete overview of the approach and results of the CLIP program. The number of delegates was exceeding the number of chairs and over 300 people followed his presentation which was based on the previous presentations held at the AFIP conference.
ESMA organized a CLIP consortium booth at the exhibitions which was supported by PEL with several demonstrators. Several visitors showed high interest and made contact with either one of the consortium partners. Beside ESMA also IML and PEL had a booth at this exhibition. IML gave a separate presentation based on own developed conductive inks.
CLIP presentation for The Royal Society of Chemistry, London, UK in April
Title: Printed Electronics and Project CLIP– Some Recent Developments for Lower Cost Conductive Inks and Novel Functionalising Process Technologies.
Peter Collins from PRA gave 8 April a presentation at The Royal Society of Chemistry in London.
Workshops
The Master Class in November 2012 was maybe the first attempt of a workshop on the subject. However, PEL is given real workshops and trainings on this subject including the information that has been build-up by CLIP.
KTH in Sweden is also very active in holding workshops and they will have a workshop on the results of their demonstrators build for the project such as the humidity sensors.
ACREO also had some workshops, but closed ones with specific partners when they developed special demonstrators such as UHF antennas for a special customer.
Sirris integrated in their Printed Electronics seminar a workshop for conductive printing.
Exploitation
Since the start of the CLIP project in 2010, some commercial projects have been initiated but were very often expensive in additional equipment (ex. Novacentrix). This isn’t helping the market to launch easy-to-use and lower cost conductive inks. Lower cost inks must be lower cost in both the whole process (not only ink level price) but also in post processing (sintering, with less energy or shorter times).
Many applications are waiting for much cheaper printed conductive tracks or areas such as antennas, bus bars, contacts… A significant drop in production cost could make a difference and offer some commercial solutions available for the broader market. This is the fundamental corner stone of Printed Electronics.
The right mechanism
During the project, it became clear that we found two major keys to unlock opportunities. The first one is the conductive particles and the combination of conductive particles (micro & nano, or bi- and tri-modal structures). Second key is the knowledge about sintering: photonic curing seemed to be the best solution but not the most economical. The extra studies within the project revealed the power and efficiency of laser sintering. Laser sintering is still unknown within sintering solutions and the excellent results showed high potential in combination with the developed inks.
Transfer
It is clear that copper based inks are not going to sell as hot cakes because of the need for photonic sintering and new handling techniques. Therefore, a selective part of the consortium partners will work on the idea to promote and transfer the knowledge towards potential users and ink manufacturers in workshops. They all will sign a NDA but will get a full explanation on sintering (photonics, xenon and laser), formulations, and effect of nano particles on sintering. The key is to sell particles and formulations which have been evaluated during the CLIP project. Even larger ink corporations have no longer full R&D budgets for the wide variety of developments demands entry solutions to stay in the market!
ESMA can play an important role with its long list of ink manufacturers as members. They are still running behind on the facts in conductive coppers inks. Once a few different players have signed up for different formulations and pay a license fee, the CLIP consortium can keep fine tuning their expertise in sintering, such as laser sintering and formulations that are even further optimized.
Commercialisation
Different studies have shown that conductive inks are in the top 3 of the most lucrative development areas for Printed Electronics. The use of conductive printing could extend the world of integrated circuits, PCBs and other developments. The focus of the CLIP consortium is mainly on particles, this is main business for partners such as AVL Metal Powders, Intrinsiq Materials and Nanogap. The ink manufacturers will also be more open in building their own formulations with the particles and general knowledge of the CLIP project, if CLIP project partners are seen as suppliers rather than competitors to them.
The consortium should also sell their expertise to companies involved in laser technology and extend their business with sintering. One of the approached companies is another member of ESMA, Xeikon Basys. They have high-speed laser systems on drums. The right adaptation of the equipment could bring inline sintering systems to the market. Many other smaller laser companies could also take profit of these developments.
IML has invested already with own resources in several developments of laser sintering systems during the CLIP project and they could lead the sales in large and medium sintering systems.
Change is in the air
The copper based inks will find their position in the market not only due to lower costs but also due to the large interest in an alternative solution in the market in PE. This has been recognized on all conferences but adaptation is not done overnight. A roll-out plan and aggressive introduction will help to bring the change.
Table 2: Projected Growth
The minimal potential within the consortium shows a turnover of 81 million euro over the next 5 years with the sales of 3 SME consortium partners. The number is expected to easily quadruple in that period when more 3rd parties and OEM (original equipment manufacturer) step into the sales process.
In the next graphics, the global market of conductive material for PE is shown, including transparent (ITO replacements, Ag nano wires…) for touch screen applications. PV is till taking the biggest part of the sales but is currently staying with the conservative Ag inks and technology. The goal of the Demonstration project CLIP 2 was to open this market to change and provide complete implementation of Cu based inks or inkjet solution. It is hard in a production line to make the switch from traditional Ag based screen printed fingers and bus bars in PV towards a CU based printing with sintering technology. It is not a 1 to 1 replacement but many critical factors such as investment, knowledge, reliability need to be controlled and available before changes can be fully implemented. Once the production environment has accepted this technology, the sales can grow in much quicker rate and bring enormous results.
List of Websites:
http://www.clip-fp7.eu/ contact details are Peter Buttiens via pb@esma.com and Luc Vander Heyden via webmaster@esma.com
– Low cost conductive inks have been developed for screen printing and flexography printing based on a mixture of low cost copper flakes and a small fraction of nano copper. In combination with Photonic sintering, surface resistances comparable to thermal sintered thick film silver inks were realized. It has been proven that end user prices of low cost copper inks can be at least 50% lower than the prices of available thick film silver inks.
– In several demonstrators it has been shown that these low cost copper inks in combination with Photonic sintering result in comparable performance and in substantial cost decrease of the printed demonstrators. These low cost copper inks can also enable new PE applications such as for long distance readable UHF RFID´s.
– Developing of higher conductive multi-modal nano silver inks for ink-jet and Aerosol-jet can lead to substantial cost reduction by printing lower wet thickness (lower ink volume) in reference to available mono-modal nano silver inks.
– It is shown in demonstrators that the performance of these multi-modal silver inks is less critical in high conductivity demanding applications and can enable new PE applications such as low cost chip-less RFID tags and smart packaging sensors.
Project Context and Objectives:
The high cost of available silver thick film and nano-metal conductive inks is a bottle neck in several existing and potential PE applications. The high cost of these inks is not only the result of raw metal cost as in thick film silver inks , but also a result of the process cost to make the components of the conductive inks as in silver or copper nano particle inks. Nano particles are inevitable for making ink-jet (small diameter of inkjet head) and aerosol-jet printing inks (making the aerosol).
First main objective was development, formulation and feasibility assessment of several lower-cost components for silver thick film and silver nano-particle based conductive inks. Besides several sizes of silver nano particles for making bi-modal and tri-modal distributions also copper nano particles and silver coated copper nano particles were made by a wet chemical method. Available nano copper particles (IML: plasma enhenced vapour condensation process) and copper flakes (AVL: milling process) were optimised to be use as components in screen and flexography inks.
Second major objective is to develop optimized ink dispersions for printing and curing for a number of processes including screen print, flexography printing and aerosol-jet and ink jet.
The third main objective is to optimize the inks towards large area printing technology and towards techniques, which enable higher resolutions than in screen-printing. We realized during the project that the way of sintering is not only important to reduce the cost of large area printing, but is also strongly connected to the applicability of certain low cost alternatives of silver nano particles to realize the high conductivity. The way of sintering has also implications on the total cost due to the investment in specific equipment. We tested several Photonic sintering technologies and learned that laser sintering was most appropriate for obtaining high conductivities of copper based low cost inks printed on temperature sensitive substrates as PET film and paper.
The last major objective is the realization of the demonstrators based on these inks. Several demonstrators were made showing the cost benefits of the developed inks for ink-jet and aerosol-jet in the case of nano silver based inks and for screen and flexography printing in case of copper based low cost inks.
Project Results:
1. Base metals used for conductive inks
The most conductive base metals are gold, silver, copper and aluminum. Although there are some applications were gold and aluminum inks are used, silver and copper are the best candidates for developing low cost inks. (Fig 1) Gold is too expensive for commodity inks and is only used for its high inertness in biomedical applications.
Aluminum oxidizes very fast in air and the oxides are non-conductive. Aluminum inks are used in back electrodes for PV cells because thick film silver inks are too expensive, but one has to print very thick layers of it to meet the specifications for surface resistance.
Thick film silver inks have been used for decades in membrane switch and touch screen applications, but during the last 10 years the pure metal has increased 4-fold in price. Printers still using these silver inks because of its high conductivity and inertness (silver oxides are also conductive) and because there is today no real alternative.
Copper, which is a low cost conductive metal, has been tried as conductive component for thick film inks by using silver coated copper to prevent oxidation of the copper (copper oxides are non-conductive), but it seems to be difficult to cover the total copper surface of the particles resulting in low thermal stability of the printed inks, unless very thick silver coatings are depleted, resulting in comparable ink costs as pure silver inks.
2. Processes to make the metal components
Micron size metal particles are made by atomization or milling. The shape of the particles can be irregular, spherical or as flakes. Flakes are mainly used because of their higher package density and increased number of conductive contact points between adjacent flakes increasing in this way the conductivity of the printed ink.
In the development of CLIP inks we used micron size copper flakes from AVL (CLIP partner). These copper flakes (Fig 2) are made in ton scale for metallic luster paints (e.g. for car metallic lacquers) and because of that the price of it is very low.(10-15 Euro/Kg) .The copper flakes are protected from oxidation at room temperature by means of an organic coating also used as lubrication additive during the milling operation. AVL made especially for this project copper flakes with smaller lateral dimension (D100<5 micron)
Nano size particles are made by wet chemical (reduction of salts) methods or by plasma enhanced vapor condensation (Fig 3). Because of the small scale and the low volume and yield of the process nano size particles are very expensive and the price is mainly independent of the type of metal used (silver or copper). The nano size particles are coated (polymers) at the end of the process to prevent oxidation at room temperature and to improve the dispersion stability in liquids. The price of these nano particles will come down in the future if the scale of the process can be increased. This can only happen if the market for PE applications will grow substantially. IML made for this project specially coated nano copper particles of 45 nm mean diameter.
3. Concepts to make low cost screen and flexography inks
Screen and flexography inks can make use of micron size conducting particles, but in available thick film inks the polymer binder is limiting the number of contact points between the metallic particles and in this way is limiting the conductivity of the ink. Copper is easy oxidized at high temperatures and the non conductive copper oxide on its surface will also limit the conductivity. We used an ink composition consisting of copper metal flakes mixed with a small fraction of nano copper. This nano copper can be sintered to a pure highly conductive metal film at relative low temperatures and acts as conductive cement between the copper flakes. To prevent oxidation of the copper components we have to use very fast (in micro seconds) Photonic sintering techniques, so the copper surface of the micron and nano copper has not the time to be oxidized. The local high temperatures in the printed layer created by absorbing the energy pulses of the laser or xenon lamp sintering will also evaporate the other non metallic components of the ink without heating up the substrate too much. With ink compositions consisting of only 3% of nano copper and 59% of copper flakes it was possible to reach comparable surface resistances (2%-3% of bulk metal conductivity value) comparable to surface resistances of available thermal sintered thick film silver inks (3%-6% of bulk conductivity value) (Fig 4). If we calculate the total costs of making these inks, this results in a total cost of 271 $/kg compared to a total cost of 995 $/Kg for a typical thick film silver ink. So end-user prices of these low cost inks can be at least 50% lower costs than the thick film silver inks at comparable conductivities of the printed ink.
4. Concepts to make lower cost ink-jet and Aerosol-jet inks
For ink-jet and Aerosol-jet inks we tested a concept based on multi-modal distribution of nano particles. By mixing 3 nano-silver distributions (40nm, 5 nm and <1 nm) in geometric calculated ratios it was possible to reach a dense close packed structure. (Fig 5A, 5B) As the smaller particles between the larger ones sinter at lower temperature this tri-modal distribution will result in a denser structure even after sintering at lower temperatures compared to a typical mono-modal distribution with a mean diameter of 40 nm. At the same sinter temperature and time the tri-modal distribution will be more conductive compared to a mono-modal distribution. (Fig 6)
This was also observed in demonstrators based on inks with a tri-modal distribution in reference to the same demonstrator printed with available mono-modal nano silver ink (Cabot ink). Specific resistance of an ink-jet ink based on a tri-modal distribution was 39% lower than a commercial available ink-jet nano silver ink of Cabot. (Fig 7)
5. Demonstrators and applications
5.1. Demonstrators made with the low cost screen and flexography printing ink
EL-display
PEL started to print EL-displays on paper because this is a less critical application regarding surface resistance specifications because flexography inks are printed much thinner than screen printing inks (only 1% of bulk metal conductivity after xenon flash sintering is necessary). The front bus bars and back electrode were printed with the flexography ink; the transparent front electrode was printed with Orgacon (PEDOT) ink. Although the relative high surface resistance of the electrodes and bus bars, the EL display could be driven by a simple solid state inverter and the different segments lighted-up well. (Fig 8)
Other EL displays were printed on PET film with screen printed copper ink and the printed bus bars were now sintered with laser sintering leading to much better conductive tracks (Fig 9). With this ink much larger and brighter EL displays can be printed. (Fig 10)
As the cost of back electrode and bus bars of front electrode can be as high as 50% of the total ink costs for printing an EL display, substantial cost savings can be made by using the low cost copper ink instead of a thick film silver ink. (estimated cost reduction of inks to print an EL display = 25%)
Intelligent packaging demonstrator
A wireless pharmaceutical package is based on conductive ink printed on a cardboard based blister inlay, which is connected to a cellular phone module embedded in the package. This enables the tracking of one pill at the time of removal from the blister, whereby data is sent to the cellular module and than forwarded wirelessly, even instantaneously if required, using GSM or GPRS cellular networks, to electronic health record systems. This allows real-time tracking and intervention by physician and so enables physicians to make timely changes to patients’ medication.
The screen printing copper ink was used to print the conductor circuit pattern on the cardboard of the intelligent package.
The printed circuits were characterized by DC measurements. The measured resistances are in the range of 10-150 Ohm/sq depending on the length. This resistance rage is within the requirements for functional package.
UHF chip antenna
ACREO printed with the screen print copper ink a UHF RFID chip antenna on cardboard paper (Fig 11 and 12). The printed antenna was Photonic sintered with a CO2 laser. Surface resistance of the sintered ink = 100 m/sq or 3% of the bulk conductivity of pure copper metal. The antenna properties are in range of the calculated values and reading distances of 5-6 meter between antenna and detector could be realized.
This demonstrator proves that these low cost copper inks can even be used in very critical (high conductivity demanding) UHF antennas.
The use of RFID technology has increased during last years. This technology is a possible enabler for wide spread communication between objects using the internet. Internet-of-things is the commonly used expression for this foreseen massive communication technology and printed UHF antennas in combination with silicon IC´s are a possible low cost solution.
These antennas are intended for far field industrial applications especially for large items level tracking in supply chain (cardboard cartons containing water and metal objects and freight (wood and metal pallet) transportation.
5.2. Cost calculation of a printed RFID antenna
A cost calculation comparison was made for printing of typical coil shaped RFID antenna using low cost copper screen print inks and available thick film silver inks. In this calculation also investment costs for screen printing units, drying and sintering units was included. We compared the current situation where thick film silver inks were printed at 1m/min, dried and sintered in a thermal drying unit, with printing of a low cost copper ink in-line dried in a thermal oven and in-line sintered in 2 consecutive Xenon flash units at 10 m/min. (Fig 13)
Out of the results it can be concluded that printing, in-line thermal drying and in-line Photonic sintering of the antenna with the copper ink results in substantial lower cost than printing the thick film silver ink in combination with thermal curing and sintering. Most of the cost savings are in the cost of the inks (50% lower cost /pc). The total process cost for printing the antenna is 30% lower cost/pc in case of printing with the copper ink.
5.3. Demonstrators and applications of ink-jet and aerosol-jet printing inks
Chip-less RFID tags (Deliverable D15)
Chip-less RFID tags (Fig 14) were printed with ink-jet (KTH) and Aerosol-jet (SIRRIS) on PEL paper and Kapton. These chip-less tags consists of several LC resonant circuits. The capacitor dimension (L2) will determine the resonant frequency of each LC circuit
4-bits chip-less tags were printed (2 layers of tri-modal ink) and sintered at 150C for 1 hour on PEL paper and 200C/1 hour on Kapton. Excellent resonant response was measured and this could easily be read out. The same 4-bits tag printed with mono-modal Cabot ink had far less defined resonant peaks and could not detected even printed at double thickness (2 consecutive printed and dried layers). (Fig 15)
This result proves that the tri-modal ink has better conductivity as a commercial available mono-modal ink. The surface resistance of sintered (at 200° C/1h) tri-modal and Cabot ink reference was measured. The tri-modal ink had a 39% lower surface resistance which means that printed ink thickness or printed volume can be 39% lower in reference to the commercial available mono-modal Cabot ink.
Chip-less RFID tag are much cost sensitive because there is no chip in the tag, it consists only of printed LC circuits. When these tags can be printed at 39% lower thickness this results in a direct 39% price decrease of the tag.
Hybrid sensor systems for smart packaging (Deliverable D15)
A humidity sensor circuit was printed with ink-jet and aerosol-jet on PET film and PEL paper using the tri-modal ink (Fig 16). The printed circuit was sintered for 1 hour at 150C. The sensor element consists of a printed cam electrode coated with MW-CNT (multi wall carbon nano tubes) .The printed circuit connects the different components: chip, switch, LED indicators and the battery
The humidity sensor is operational although the printed circuit was printed with one layer and sintered at a relative low sinter temperature of 150 C (Fig 17). The interconnection of the chip with the printed circuit was realized by printing on a not-flat surface (on the epoxy glue to mount the chip) (Fig 18).
The humidity sensor circuit is an example of a smart packaging system which can sense and inform the customers or suppliers on the status of the products inside the packages and on the ambient conditions of the product.
This is a very broad field of application of PE and is viewed as a cost -effective approach to manufacture low cost sensor devices on temperature sensitive substrates as paper and plastic.
6. Conclusions
– Low cost conductive inks have been developed for screen printing and flexography printing based on a mixture of low cost copper flakes and a small fraction of nano copper. In combination with Photonic sintering, surface resistances comparable with thick film silver inks were realized by printing the inks at comparable wet thickness. It has been proven that end user prices of low cost copper ink can be at least 50% lower than the prices of available thick film silver inks.
– In several demonstrators it has been shown that these low cost copper inks in combination with Photonic sintering result in comparable performance and result in substantial cost decrease of the demonstrator. These low cost copper inks can also enable new PE applications even for long distance readable UHF RFID´s.
– Developing of higher conductive multi-modal nano silver inks for ink-jet and Aerosol-jet lead to substantial cost reduction by printing lower wet thickness (lower ink volume) in reference to available mono-modal nano silver inks.
– It is shown in demonstrators that the performance of these multi-modal silver inks is less critical in high conductivity demanding applications and can enable new PE applications as low cost chip-less RFID tags and smart packaging sensors.
Potential Impact:
All original main objectives of the CLIP project were reached at the end of the third period.
Silver nano particles were developed and used to make low cost inks for high resolution printing technologies as ink-jet, aerosol-jet. Copper flakes and nano copper particles were developed for high throughput printing technologies as screen and flexography printing. Specific inks for the different printing technologies were designed and tested. Demonstrators based on these inks were printed and tested.
Results of the project were disseminated at workshops and conferences.
For high resolution printing technologies ink-jet and aerosol-jet the printed ink volume of silver nano based inks can be decreased by 40% at comparable surface resistance in reference to commercial available silver nano inks. As nano silver based inks are the main cost component of high resolution printed electronics applications this cost decrease enables the application and commercialization of new and existing Printed Electronic applications that were not affordable due to high cost of nano silver inks.
For large area printing technologies; screen and flexography printing a low cost copper ink was developed based on a mixture of low cost copper flakes and nano copper particles. In reference to available thick film silver inks it is possible manufacture and commercializes these inks at less than half the end user prize at comparable surface resistance and printed thickness. Although these inks have to be cured with Photonic curing technologies, the investment in Photonic curing can be easily depreciated as Photonic curing can increase processing speed in reference to thermal curing and enable R2R printing processes. By using these low cost copper inks , existing PE applications which suffer from the high cost of thick film silver inks can now printed at a lower cost and new cost sensitive PE applications that were not affordable before because of high silver ink prices can be made at substantial lower cost enabling the application and commercialization of it.
Printed Electronic applications in general can benefit from these low cost inks and this can result in a breakthrough of this technology, leading to volume increase and more and new applications. In time this can result in more local production and more employment in Electronic and Printing Industry in Europe.
Dissemination activities
The consortium has been disseminating since the first year of the project on the project of CLIP.
Consortium partners such as Nanogap, IML spoke at different conferences such as Printed Electronics 2011 & 2012.
Seminars:
Master Class: The Future of Conductive Printing (based on results of CLIP)
A dissemination event was organised on 27 November in Diepenbeek, Belgium. The main goal was to bring the latest status on the CLIP project results and compare this with current commercial inks in the market. An overview of the state of the Art situation of conductive inks was provided. In total 5 presentations were given by the different RTO partners of CLIP, with certain support of SME partners such as IML and Nanogap.
It was clear that such a focused master class showed the reality info conductive printing. The feedback from the audience and delegates during the presentations and afterwards in the networking session showed that a project such as CLIP was bringing value to the market by revealing the whole reality. This meant that we were able to show the real bottlenecks but also the opportunities for the market.
The ESMA conference AFIP: Advanced Functional & Industrial Printing
On 6 & 7 March 2013, ESMA organized its European conference for Advanced Functional & Industrial Printing in Düsseldorf, Germany. It is a global conference for the printing industry to bring knowledge and innovation together in about 20 presentations during 2 days. The conference brought together about 150 delegates from 16 countries, 26 exhibitors and 5 non-commercial exhibitors.
During the second day, there was a special CLIP session with 3 presentation blocks. The presentation was given by the CLIP consortium.
Table 1: AFIP Conference, CLIP Presentations
The presentations were of a high level but very practical oriented and supported with nice videos. The feedback from the delegates was excellent on the CLIP topics. The commercial interest was being confirmed. In general, conductive inks and solutions were an important topic for the industry.
The consortium has also a table top at the exhibition to show all results and demonstrators that were available at that time.
CLIP presentation at Printed Electronics
IDtechEX, leader in conferences for Printed Electronics, organized during 17& 18 April 2013 their annual European conference in Berlin. They had over 1400 delegates and over 50 exhibitors during these 2 days.
Louis Bollens present at the opening of the second day an excellent presentation with the complete overview of the approach and results of the CLIP program. The number of delegates was exceeding the number of chairs and over 300 people followed his presentation which was based on the previous presentations held at the AFIP conference.
ESMA organized a CLIP consortium booth at the exhibitions which was supported by PEL with several demonstrators. Several visitors showed high interest and made contact with either one of the consortium partners. Beside ESMA also IML and PEL had a booth at this exhibition. IML gave a separate presentation based on own developed conductive inks.
CLIP presentation for The Royal Society of Chemistry, London, UK in April
Title: Printed Electronics and Project CLIP– Some Recent Developments for Lower Cost Conductive Inks and Novel Functionalising Process Technologies.
Peter Collins from PRA gave 8 April a presentation at The Royal Society of Chemistry in London.
Workshops
The Master Class in November 2012 was maybe the first attempt of a workshop on the subject. However, PEL is given real workshops and trainings on this subject including the information that has been build-up by CLIP.
KTH in Sweden is also very active in holding workshops and they will have a workshop on the results of their demonstrators build for the project such as the humidity sensors.
ACREO also had some workshops, but closed ones with specific partners when they developed special demonstrators such as UHF antennas for a special customer.
Sirris integrated in their Printed Electronics seminar a workshop for conductive printing.
Exploitation
Since the start of the CLIP project in 2010, some commercial projects have been initiated but were very often expensive in additional equipment (ex. Novacentrix). This isn’t helping the market to launch easy-to-use and lower cost conductive inks. Lower cost inks must be lower cost in both the whole process (not only ink level price) but also in post processing (sintering, with less energy or shorter times).
Many applications are waiting for much cheaper printed conductive tracks or areas such as antennas, bus bars, contacts… A significant drop in production cost could make a difference and offer some commercial solutions available for the broader market. This is the fundamental corner stone of Printed Electronics.
The right mechanism
During the project, it became clear that we found two major keys to unlock opportunities. The first one is the conductive particles and the combination of conductive particles (micro & nano, or bi- and tri-modal structures). Second key is the knowledge about sintering: photonic curing seemed to be the best solution but not the most economical. The extra studies within the project revealed the power and efficiency of laser sintering. Laser sintering is still unknown within sintering solutions and the excellent results showed high potential in combination with the developed inks.
Transfer
It is clear that copper based inks are not going to sell as hot cakes because of the need for photonic sintering and new handling techniques. Therefore, a selective part of the consortium partners will work on the idea to promote and transfer the knowledge towards potential users and ink manufacturers in workshops. They all will sign a NDA but will get a full explanation on sintering (photonics, xenon and laser), formulations, and effect of nano particles on sintering. The key is to sell particles and formulations which have been evaluated during the CLIP project. Even larger ink corporations have no longer full R&D budgets for the wide variety of developments demands entry solutions to stay in the market!
ESMA can play an important role with its long list of ink manufacturers as members. They are still running behind on the facts in conductive coppers inks. Once a few different players have signed up for different formulations and pay a license fee, the CLIP consortium can keep fine tuning their expertise in sintering, such as laser sintering and formulations that are even further optimized.
Commercialisation
Different studies have shown that conductive inks are in the top 3 of the most lucrative development areas for Printed Electronics. The use of conductive printing could extend the world of integrated circuits, PCBs and other developments. The focus of the CLIP consortium is mainly on particles, this is main business for partners such as AVL Metal Powders, Intrinsiq Materials and Nanogap. The ink manufacturers will also be more open in building their own formulations with the particles and general knowledge of the CLIP project, if CLIP project partners are seen as suppliers rather than competitors to them.
The consortium should also sell their expertise to companies involved in laser technology and extend their business with sintering. One of the approached companies is another member of ESMA, Xeikon Basys. They have high-speed laser systems on drums. The right adaptation of the equipment could bring inline sintering systems to the market. Many other smaller laser companies could also take profit of these developments.
IML has invested already with own resources in several developments of laser sintering systems during the CLIP project and they could lead the sales in large and medium sintering systems.
Change is in the air
The copper based inks will find their position in the market not only due to lower costs but also due to the large interest in an alternative solution in the market in PE. This has been recognized on all conferences but adaptation is not done overnight. A roll-out plan and aggressive introduction will help to bring the change.
Table 2: Projected Growth
The minimal potential within the consortium shows a turnover of 81 million euro over the next 5 years with the sales of 3 SME consortium partners. The number is expected to easily quadruple in that period when more 3rd parties and OEM (original equipment manufacturer) step into the sales process.
In the next graphics, the global market of conductive material for PE is shown, including transparent (ITO replacements, Ag nano wires…) for touch screen applications. PV is till taking the biggest part of the sales but is currently staying with the conservative Ag inks and technology. The goal of the Demonstration project CLIP 2 was to open this market to change and provide complete implementation of Cu based inks or inkjet solution. It is hard in a production line to make the switch from traditional Ag based screen printed fingers and bus bars in PV towards a CU based printing with sintering technology. It is not a 1 to 1 replacement but many critical factors such as investment, knowledge, reliability need to be controlled and available before changes can be fully implemented. Once the production environment has accepted this technology, the sales can grow in much quicker rate and bring enormous results.
List of Websites:
http://www.clip-fp7.eu/ contact details are Peter Buttiens via pb@esma.com and Luc Vander Heyden via webmaster@esma.com
