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December 2015 • SMT Magazine 81 When soldering the aluminum substrates, the solder pot temperature required an increase due to the increased mass of the substrates along with the high thermal conductivity of the aluminum. The rapid cooling experi- enced due to these substrate properties caused SAC305 to leave an excess amount of solder on the substrate. For all solderability parts on aluminum, the temperature of the solder pot was set to 250°C. The alumina substrates were soldered at the same temperature as the FR-4 samples (235°C). Adhesion characteristics When considering a thick film for an appli- cation, the conductor's durability is a priority. Many thick film build-ups have to operate un- changed for thousands of hours. Polymer con- ductor pastes are typically used in consumer products which are replaced on a regular sched- ule. They do not have the same requirements as high-temperature thick film materials used in applications such as military or automotive. Even though the use of these materials is dif- ferent, they still need to be durable under the same requirements. After testing the initial ad- hesion of the polymer paste, three aging tech- niques were also observed: 125°C thermal ag- ing, 85°C/85% RH and thermal cycling (-55°C– 150°C). The results of all aging data show a polymer conductor that will be durable on a number of substrates through various aging conditions. These results are especially promising on FR4, where adhesion has remained relatively un- changed through all aging tests. Low-Temperature Pastes There are many circumstances that are lead- ing to an increased interest in low-temperature substrates and low-temperature processing, including new technologies in the electronics market, along with the rise in processing costs for high-temperature materials. Due to these changes in the industry, manufacturers are seek- ing alternatives to traditional thick film pastes. To meet these demands, polymer systems are being developed that can withstand the testing and processing of cermet systems. Test results have proven the availability of a solderable conductor with good conductiv- ity and high adhesion. The adhesion results are particularly high for FR-4 applications; how- ever, good results are also seen with other sub- strate materials. These results show that there are new low- temperature alternatives that fit within the new demands of the electronics market. The polymer paste not only meets the industry demands of a Pb-free soldering poly- mer, but also fulfills the global requirements of RoHS- and REACH-compliancy. SmT LoW-TemPerATUre THIcK FILm PASTeS PermIT LeAD-Free SoLDerInG ArTiCle Steven Grabey is a research engineer at Heraeus Electronics. Researchers under Stephan Buecheler and Ayodhya n. Tiwari from the laboratory for Thin Films and Photovoltaics at Empa-Swiss Federal laboratories for Material Science and Technology have come up with a procedure that makes it possible to produce thin film tan- dem solar cells. The methods are suitable for large area, low cost processing. Flexible plastic or metal foils could also be used as substrate in future. This marks a major milestone on the path to mass production of high-efficiency so- lar cells with low cost processes. The researchers create the top solar cell perovskite film with a low-temperature pro- cedure at just 50°C. This promises an energy- saving and cost-saving production stage for future manufacturing processes. The tandem solar cell yielded an efficiency rate of 20.5% when converting light to electricity. Already with this first attempt, Empa researchers have emphasized that it has lots more potential to offer for better conversion of solar spectrum into electricity. Tandem Solar Cells are Simply Better

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