Reliability of SAC Solders under Low and High Stress Conditions

Abstract

High thermo-mechanical fatigue resistance (TMFR) is essential for tin silver copper (SAC) solders when used in automotive applications due to the large changes of operation temperature. To obtain higher TMFR for SAC solders the silver content is increased up to 4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup> . In addition, additives like nickel, bismuth, antimonite or indium are added to the SAC solder to increase the TMFR by solid solution and precipitation hardening. In this paper, the effectiveness of the additives compared to the silver content is investigated under high and low stress conditions. A reliability study containing five different SAC solders was performed. Nine different high-power LED packages (seven ceramic and two lead frame packages) are investigated. The LED packages were soldered on aluminum metal core printed circuit boards (Al-PCB). After assembly, the initial inspection was performed using X-Ray, Scanning Acoustic Microscopy (SAM) and Transient Thermal Analysis (TTA). Afterwards, the test boards were exposed to standard automotive temperature air-to-air shock tests (-40°C/125°C). The crack growth is analyzed by SAM and TTA after specific numbers of shock cycles (50, 100, 200, 500, 750, 1000, 1250, 1500). The failure criterion is defined by the increase of the thermal resistance, measured by TTA, and correlated with the cracked area detected by SAM and cross sections. Due to the large difference of the coefficient of thermal expansion (CTE) between ceramic and the Al-core of the PCB, the solder is exposed to high stresses in case of the ceramic packages whereas in the case of lead frame packages only low stress occur. In the high stress case, the Ag percentage dominates the TMFR. The solder, with the highest silver content, (SAC3.8Cu0.7Sb1.5Bi3.0Ni0.1) has the highest TMFR. In comparison, the additives have a minor influence. The SAC1.0Cu0.7Bi1.6In0.2 does not perform better than the SAC1.0Cu0.5. However, for the case of extreme CTE mismatch, i.e. ceramic package with thin film solder pad metallization, the advantage of the Ag rich solder is comparably small. The SAC3.2Cu0.7Sb5.5 solder performs best in this case. For the low stress case, all solders perform very well and don't reveal a significant increase of thermal resistance. Applying the criterion of 20% increase of thermal resistance no failures occur until 1500 cycles. Nevertheless, compared to the Ag rich solders, the SAC105 shows slightly increased thermal resistance after 1500 cycles in some cases, which was not observed for the other solders.