High-Reliability Solder Alloys’ Impact on Pb-free TF Conductors

Victoria DeLissio
Heraeus Precious Metals North America Conshohocken LLC
PA, USA

Fired-on thick film conductors continue to be the preferred choice for electronic circuits in high-reliability applications. The demand for high-performance thick film circuits is consistently growing, especially in sectors such as automotive, aerospace, and military, making the need for reliable solder joints critical. The transition to lead (Pb)-free solder joints in thick film applications has been accompanied by reliability challenges, specifically with SAC (tin-silver-copper) alloys that have raised concerns in thermal aging and cycling performance. To address the reliability concerns, a series of test vehicles were manufactured to accurately evaluate the reliability of thick film and solder joints according to rigorous standards.

These test vehicles – assembled on alumina oxide substrates – involved the combination of Pb-free silver-bearing thick film conductors, including Ag, AgPd, and APt alloys, with various SAC options such as SAC305 (used as a control), “HiRel A” alloy, and “HiRel B” alloy. The primary objective of this paper is to determine the most effective Pb-free solder/thick film combination through assessing these test vehicles after undergoing thermal cycling from -40°C to 150°C, as well as thermal aging at 150°C, via electrical, optical, and mechanical testing to meet the demands of high-reliability applications.

The main objective of this paper was to identify a solder (specifically Sn-based and high reliability) and thick film combination to improve overall reliability of thick film hybrids. This was done by conducting rigorous thermal testing. The work conducted for this paper served as an initial exploration into this area of study. It demonstrated that certain solder and thick film conductor combinations displayed favorable results across various tests and test groups. Specifically with HiRel B that has advantages such as being able to withstand high operating temperatures up to 175°C and the mixed metal conductors that showed better performance in most testing compared to an Ag-only conductor, even at high Ag ratios.

Factors such as using optimal reflow profiles and solder pad geometries could further improve the reliability of HiRel B in thick film circuits. Further work and continuous improvement projects for this study include investigating the above factors as well as a study into the formation of the intermetallic layer between the solder and thick film conductors.