For decades, polymer thick-film (PTF) systems have provided a low cost, non-fired option for screen-printing simple electronic circuits. The ability to apply these types of pastes on temperature sensitive substrates such as PET, polycarbonate, polyimide, and other polymers has facilitated a variety of applications, for instance membrane touch-switch keypads, buss bars for touch screens, various types of sensors, and flexible circuitry. Polymer thick-film is also one of the primary technology solutions utilized in the rapidly emerging Printed Electronic market, where flexible, durable materials are paramount to the success of these technologies. One of the largest emerging markets for polymer thick-film is wearable electronics, where engineers are designing “smart fabrics” with active circuitry for medical monitoring, performance enhancement in sports, and personal comfort. Polymer thick-film pastes include silver pastes for conductors, carbon pastes for resistive applications, silver-silver chloride fillers for glucose sensors, and dielectric pastes.

The major challenge with PTF silver conductors is that they are not conducive to soldering. This hinders the ability to attach components, leads, dies, wires, or other features to the prints. As copper is solderable, one possible solution would be a copper polymer thick-film metallization; however they start oxidizing at the typical paste curing temperatures, 110 – 130°C, rendering them unsuitable for the vast majority of conductive applications. In order to meet these challenges, Heraeus has developed a new line of solderable polymer thick-film conductors based on a high-performance silver-coated copper conductive filler. These metallizations are solderable, resistant to solder leaching, and result in sheet resistivities approaching that of pure silver polymer conductors. The prints do not degrade in performance when cured at temperatures as high as 200°C. The new product line was formulated to accept different types of solders, especially traditional SAC-305, which provides a complete matched solution for designers. The new metallization opens up new applications given its ability to print polymer circuitry on a variety of substrates including aluminum, steel, FR4, Kapton, Mylar, and glass. The technology also allows for the fabrication of more complex circuitry on these types of substrates, giving circuit designers a powerful new tool in their toolbox in applications such as LED lighting, sensors, and heaters. Finally, these materials may provide a lower-cost option for solderable flexible polymer end terminations for components used in vibration sensitive applications, for instance the automotive industry.

In our paper, we will present the properties of the new pastes and printed conductors. Performance testing includes surface resistivity, solderability, solder leach resistance, voiding, and adhesion on two substrates: FR4 and Kapton. Furthermore, we show that the solderable PTF conductor will provide a potential cost- savings over the current technology used on FR4 boards, stamped copper films. By replacing the stamped copper with our solderable PTF conductor, manufacturers will have the advantage of replacing a subtractive process for etching their patterns with an additive, environmentally friendly process, not only saving processing time but eliminating a large, dangerous copper waste stream. Finally, we will summarize the applications that the new technology enables.

The first mention of screen-printable low-temperature curable polymer thick-film (PTF) pastes as a replacement for fired thick-film and etched copper circuitry was cited in a patent filed in 1944. The vision for this PTF was to provide a lower cost alternative for these technologies, especially etched copper, which generates a significant quantity of waste. However, the technology did not start to get traction until the 1970s, when electronic OEM's started to adopt plastics into their devices, for instance, membrane touch keypads [1]. PTF has since enabled wider adoption of polymer substrates in electronics, where they provide flexibility and low cost alternatives. Today, PTF is used for component end-terminations, printed buss bars for touch screens, various types of sensors, and flexible circuitry. Polymer thick-film is the primary technology solution for the rapidly emerging Printed Electronic market. Wearable electronics, where engineers are designing “smart fabrics” with active circuitry for medical monitoring, performance enhancement in sports, and personal comfort, is a new target for PTF. Polymer thick-film pastes cover the entire suite of materials required for an electronic circuit: silver pastes for conductors, carbon pastes for resistive applications, silver- silver chloride fillers for glucose sensors, and dielectric pastes.

One major challenge that hinders wider adoption of PTF in circuit design is that the silver conductors are not conducive to soldering. This prevents the attachment of components, leads, dies, and other features to the prints. Although conductive adhesives may replace solder, their adhesion strength is lower and requires different processing from traditional circuit board manufacture. One possible solution is to replace silver with copper filler, which is easier to solder to; however, they are prone to oxidation, especially during paste curing at 110 – 130°C, rendering them unsuitable for the vast majority of conductive applications.

In order to meet these challenges, Heraeus has developed a new solderable polymer thick-film conductor, CL21-11130, based on a high-performance silver-coated copper conductive filler. The technology is adaptable to different types of solders, providing matched solutions for circuit designers. We will show that the new metallization is solderable with excellent solder leach resistance, and stable at curing temperatures as high as 200°C. We will further show its performance in traditional pick-and place and solder reflow testing on Benchmarker II boards. Finally, we will summarize potential cost advantages as well as several potential applications for the new technology.

The new solderable polymer thick-film paste is prepared in the same way as traditional thick-film pastes. The raw materials are weighed and mixed together, followed by milling on a three-roll mill for several passes at successively tighter gaps. This serves to homogenize the paste and the shearing action of the rolls fully disperses the solids within the paste matrix. After milling, the paste is tested for fineness of grind, viscosity, and percent solids. Typical paste properties for our solderable PTF conductor paste are a fineness of grind with a fourth scratch at 15 um or less, a target viscosity of 120,000 – 140,000 centipoise, 95% solids, and a metal loading of 90.5%.

For electrical properties and for solder leach resistance, the paste is printed onto a Kapton substrate using a 280 mesh screen with a L03-02 pattern (Fig. 1) and cured at 175 – 200°C. A multimeter is used for resistance and a Bruker microscope to determine the print thickness. When compared to a silver-based solderable polymer thick-film paste, the sheet resistivity of CL21-11130 after 180°C curing is 30 mΩ/□/25um, only slightly higher than the reference silver paste (26 mΩ/□/25um).

Fig. 1:

L03-02 pattern for testing the solderable polymer paste

Fig. 1:

L03-02 pattern for testing the solderable polymer paste

Close modal

Fig. 2 shows solder wetting using SAC-305 solder. As compared to the reference silver paste, CL21-11130 shows excellent solder wetting, compared to virtually no solder wetting on the silver paste.

Fig. 2:

solder wetting of (a) CL21-11130 and (b) reference silver paste

Fig. 2:

solder wetting of (a) CL21-11130 and (b) reference silver paste

Close modal

Solder leach resistance is tested by dipping the cured print on into a SAC305 pot at 250°C for 3 – 10 seconds. Fig. 3 shows the solder leach resistance of CL21-11130 dipped for 3, 5, and 10 seconds. As demonstrated, the new paste shows excellent wetting and solder leach resistance.

Fig. 3:

SAC-305 solder leach resistance of CL21-11130

Fig. 3:

SAC-305 solder leach resistance of CL21-11130

Close modal

Initial testing: to determine the suitability of the material in a traditional printed circuit application, CL21-11130 was screen-printed onto the backside of a Benchmarker II FR4 board (Fig. 4) using a 280 mesh screen and cured at 185°C for 30 minutes. The cured thickness was 38 um. After curing, no-clean and water soluble SAC-305 solder paste was stencil printed onto the printed pattern and populated with various size components ranging from 0402 to 1206 using an ASM component pick and place machine. Two solder reflow profiles were tested: 250°C for three minutes and 250°C for six minutes. Half of the boards were processed in air and the other half in nitrogen atmosphere. After reflow, boards went through thermal cycling 200 times between 150°C and −55°C with dwell times at each temperature for 10 minutes. Die shear adhesion was then tested both on initial and thermally cycled boards and compared against a reference etched copper Benchmarker II board. Fig. 5(a) shows the etched copper board populated with components after solder reflow, while Fig. 5(b) displays the populated board with solderable polymer CL21-11130.

Fig. 4:

Benchmarker II board

Fig. 4:

Benchmarker II board

Close modal
Fig. 5:

Solderable polymer printed board populated with components

Fig. 5:

Solderable polymer printed board populated with components

Close modal

Die shear adhesion was examined for each type of board varying the reflow conditions (“short profile” = 3 minutes vs. “linear profile” = 6 minutes), atmosphere (air vs. nitrogen), and type of solder used (no clean vs. water soluble). The data displayed in Fig. 6 shows initial die shear adhesion for the 0402 chips. For these chips, the solderable polymer demonstrates an approximately 50% reduction when compared to etched copper, however after 200 thermal cycles, a general increase in die shear is observed whereas on etched copper it tends to decrease. This result suggests that if the polymer is cured at a slightly higher temperature, die shear adhesion may be improved. The results also suggest that the solderable polymer-based boards may have improved reliability over etched copper-based boards. No clear differences are seen with respect to the use of water- soluble flux vs. no-clean as well as whether the reflow was conducted in nitrogen or in air.

Fig. 6:

die shear adhesion for 0402 chips at a variety of conditions for (a) solderable PTF and (b) etched copper

Fig. 6:

die shear adhesion for 0402 chips at a variety of conditions for (a) solderable PTF and (b) etched copper

Close modal

Fig. 7 shows the die shear adhesion for the 0603 chips. Performance for the solderable polymer on the larger chips is somewhat better with shear force about 60% that of the etched copper board. However, after thermal cycling, the gap closes significantly with the shear force for the solderable polymer approximately 80% that of the etched copper. 0805 chips exhibit a similar trend (not shown).

Fig. 7:

die shear adhesion for 0402 chips at a variety of conditions for (a) solderable PTF and (b) etched copper

Fig. 7:

die shear adhesion for 0402 chips at a variety of conditions for (a) solderable PTF and (b) etched copper

Close modal

Curing optimization: based on the first round data, the 3- minute reflow was selected and higher curing temperatures were investigated. To simplify the study, only the no-clean SAC-305 solder was used and reflow was conducted in air. Curing temperatures ranging from 185 to 205°C and times of 30 and 60 minutes were investigated. Fig. 8 shows solder wetting and demonstrates that on FR4, the best solder wetting occurs at 195°C 30 minutes, with longer times and higher temperature starting to show deterioration in wetting.

Fig. 8:

solderable PTF solder wetting on FR4

Fig. 8:

solderable PTF solder wetting on FR4

Close modal

Die shear for the 0402 chips is shown in Fig. 9(a) as a function of temperature and shows a positive trend with increasing cure temperature/time for initial die shear. After 200 thermal cycles, a more dramatic improvement is observed. An even stronger effect is seen with the 0603 chips (b), where after thermal cycling, the solderable PTF paste surpasses the die shear on etched copper at higher curing temperatures/times. Additionally, the failure mode improves, with lower temperatures showing pad delamination from the substrate while at higher temperatures, failure is via fracture within the solderable PTF body. Larger chips show the same trends observed with the smaller chips.

Fig. 9:

die shear adhesion for (a) 0402 and (b) 0603 chips as a function of temperature with the etched Cu reference on the left

Fig. 9:

die shear adhesion for (a) 0402 and (b) 0603 chips as a function of temperature with the etched Cu reference on the left

Close modal

Void percentage of the solderable polymer was checked via x-ray analysis (Nikon model XTV160) and quantitated by the Pad Array Void Analysis Tool. Four readings were taken and averaged together. A typical target for void% in solder paste is 20% maximum. Table I shows the void% as a function of curing temperature. As is clearly seen in the data, lower cure temperatures and times favor lower void percentage after thermal cycling.

Table I:

Void percentage as a function of temperature for initially cured parts and after 200 thermal cycles

Void percentage as a function of temperature for initially cured parts and after 200 thermal cycles
Void percentage as a function of temperature for initially cured parts and after 200 thermal cycles

For end users less concerned by the voids, higher temperatures are preferred in order to maximize die shear adhesion. However, if voids are a concern, curing temperatures are currently limited to 195°C for 30 minutes. Either way, future work will focus on lowering the void percentage at higher temperature curing in order to exploit the potential for higher adhesion.

Kapton is a trade name of a polyimide film that is stable at temperatures up to 400°C. As a result, it is considered an excellent substrate for printed circuits and, unlike FR4, may be used for flexible applications. Therefore, the CL21-11130 enables the ability to print complex electronic circuits on this substrate. Printing, curing, and testing was performed in the same fashion as on FR4 boards. Fig. 10 shows solder wetting on Kapton. Wettability in all cases is at least “good.”

Fig. 10:

solderable PTF solder wetting on Kapton

Fig. 10:

solderable PTF solder wetting on Kapton

Close modal

Fig. 11 shows the die shear adhesion for (a) 0402 and (b) 0603 chips on Kapton. 0402 chips show the same improving trend as curing temperature and time increases as well as improvement after thermal cycling. The sheer adhesion is similar to or slightly less than that on FR4. The 0603 chips also show the same improvement with temperature, although thermal cycling at lower temperature and time show a degradation of adhesion. The sheer adhesion is similar to that on FR4. Again, the failure mode improves, with lower temperatures showing pad delamination from the substrate while at higher temperatures, failure is via fracture within the solderable PTF body. Larger chips show the same trends as with the smaller chips.

Fig. 11:

die shear adhesion for (a) 0402 and (b) 0603 chips on Kapton as a function of temperature

Fig. 11:

die shear adhesion for (a) 0402 and (b) 0603 chips on Kapton as a function of temperature

Close modal

Table II shows the void% as a function of curing temperature on Kapton. At the higher temperatures, voiding is worse than on FR4, with only 185°C 30 minutes having an acceptable level of voids after thermal cycling. As in the case of FR4, future work will focus on improving the void percentage at higher temperature curing.

Table II:

Void percentage as a function of temperature on Kapton for initially cured parts and after 200 thermal cycles

Void percentage as a function of temperature on Kapton for initially cured parts and after 200 thermal cycles
Void percentage as a function of temperature on Kapton for initially cured parts and after 200 thermal cycles

Cost comparison: initial cost calculations show the material cost for the solderable PTF conductor to be under $1/Benchmarker II board. This compares favorably to the cost of a completed FR4 board based on etched copper, which can range from $2 - $23 per board. The solderable PTF as a result may be a low cost alternative for FR4 copper plated boards.

The solderable polymer has a positive temperature coefficient of resistance (TCR) as shown in Table II, which suggests that a printed conductor using the solderable polymer would be suitable as a replacement for low-temperature copper foil heaters.

Table III:

TCR effect of the solderable PTF conductor - values at the various temperatures are in ohms, TCRs are in parts per million

TCR effect of the solderable PTF conductor - values at the various temperatures are in ohms, TCRs are in parts per million
TCR effect of the solderable PTF conductor - values at the various temperatures are in ohms, TCRs are in parts per million

The excellent performance on Kapton substrates opens up new possibilities for printing complex electronic circuitry onto alternative polymer substrates. Although not tested yet, it is anticipated that the solderable polymer may also be used on aluminum substrates, which would support low cost LED lighting. The ability to solder components onto a polymer substrate may also support low power LED's.

The solderable PTF conductor also is conducive to digitalization of board designs, which opens up the possibility for low cost, customized board designs to support lower volume applications. The ability to make quick turns from one design to another is especially attractive for these types of markets. The technology also supports 3-D designs and 3-D printing, especially with a micro pen-printable version of the ink. This would provide promising new avenues for novel low cost lighting, medical components, board-to-board connections, and low temperature heaters.

Finally, the flexibility of polymer thick-film metallizations facilitates electronics used in environments with high vibration, for instance, component attachment for automotive applications.

Heraeus has developed a new solderable polymer thick-film conductor, CL21-11130, which shows excellent promise in not only substituting out etched copper boards, but also allows printing complex electronic circuitry onto alternative substrates. The characterization data on the paste shows the performance to approach that of etched copper on FR4 boards. Additionally, the new product allows for component and die attach to alternative polymer substrates such as Kapton. The flexibility of the curable polymer thick-film reduces the impact of vibration on the electronic circuitry by providing a buffer between the solder joint and the substrate. These attributes along with the fact that the technology is adaptable to digitalization and customization facilitates numerous potential applications and markets. Future work on the solderable polymer thick-film conductor will focus on optimizing the curing profile as well as possible optimization to the paste formulation for maximum die shear adhesion results. In addition, printing finer layers of the solderable polymer may also help with die shear adhesion as it will allow for any gasses formed during the curing process to escape more easily, reducing voiding. Finally, paste modifications will focus on alternative polymer substrates as well as aluminum.

Mi Mgoc Chung – preparing the formulations and paste testing

Karl Pfluke – solderability and adhesion testing; thermal cycle testing

[1]
K.
Gilleo
,
Polymer Thick Film
,
New York, NY
,
Van Nostrand Reinhold
,
1996
,
p
11