Thermal Fatigue Endurance of Lead-Free Composite Solder Joints over a Temperature Range of −55°C to 150°C

Abstract

The thermal fatigue endurance of two lead-free solder/plastic-core solder ball (PCSB) composite joint structures in low-temperature co-fired ceramic (LTCC) modules was investigated using a thermal cycling test over a temperature range of À55°C to 150°C. The investigated solder alloys were Sn-7In-4.1Ag-0.5Cu (SAC-In) and 95.5Sn-4Ag-0.5Cu (SAC). Three failure mechanisms were observed in the test joints. Transgranular (fatigue) cracking mixed with minor intergranular cracking was the dominant failure mechanism at the outer edge of the joints in both test assemblies, whereas separation of the solder/intermetallic compound (IMC) interface and creep cracking occurred in the other parts of the test joints. The propagation rate of the transgranular crack was lower in the SAC-In joints compared with in the SAC joints. Furthermore, the SAC solder seemed to be more prone to separation of the solder/IMC interface, and more severe intergranular (creep) cracking occurred in it compared with in the SAC-In solder. In the thermal cycling test conditions, the better thermal fatigue endurance of the SAC-In solder composite joints resulted in a 75% higher characteristic lifetime compared with the SAC composite joints.

Enhanced thermal fatigue endurance and lifetime prediction of lead‐free LGA joints in LTCC modules

“…The typical crack path and the recrystallized microstructure of the test joints after the TCT over a temperature range of −40‐125°C are shown in Figure 2. The recrystallization behaviour and the intergranular (creep) cracking of the test joints were consistent with the thermomechanically loaded collapsible BGA joints with ternary SnAgCu alloys and the non‐collapsible solder joint configurations with the PCSB and SAC‐In solder (Hendersson et al , 2004; Mattila et al , 2004; Terashima et al , 2004a, b; Sundelin et al , 2008; Nousiainen et al , 2008a, 2009, 2010b). On the other hand, the recrystallized zone of the LGA joints with the Sn4Ag0.5Cu and Sn3Ag0.5Cu0.5In0.05Ni solders was localized next to the LTCC/solder interface in the more rigid assemblies with 1.2 mm thick LTCC modules and Arlon (Nousiainen et al , 2010a).…”

“…The as‐soldered microstructure of the Sn7In4.1Ag0.5Cu alloy in the pre‐tinned modules is shown in Figure 5(a). It consists of Cu 6 (Sn,In) 5 and Ag 3 (Sn,In) particles within the Sn matrix, in which a small amount (2‐4 at%) of indium exists as solute atoms, similarly to earlier studies with this alloy (Nousiainen et al , 2006, Nousiainen et al , 2009, 2010b). The only exception was the occurrence of small (<0.25 μ m) Au‐In‐Sn particles.…”

“…The difference is notable, but practical use of indium alloying in these assemblies with a small global thermal mismatch is marginal, since sufficient thermal fatigue endurance was also obtained using SAC387. However, the benefit of In alloying can be utilized in assemblies that are exposed to more severe thermal stresses (Suhling et al , 2004; Nousiainen et al , 2009). Therefore, LTCC modules with In‐containing pre‐tinned pads may be a potential option for thicker and/or larger LTCC applications with LGA joints.…”

“…Therefore, the above‐mentioned methods for improving the thermal fatigue endurance of solder joints in LTCC assemblies may not be feasible. In considering a suitable method for LTCC technology, the use of indium‐containing solders improved the thermal fatigue endurance of ceramic capacitor and LTCC modules with non‐collapsible ball grid arrays (BGA) joints tested in harsh test conditions when compared with SnAgCu alloys (Suhling et al , 2004; Nousiainen et al , 2009). Thus, the first aim of this study was to investigate the effect of indium alloying on the reliability and processability of the LGA joint structure in LTCC/printed wiring board (PWB) assemblies.…”

Effect of ENIG deposition on the failure mechanisms of thermomechanically loaded lead‐free 2nd level interconnections in LTCC/PWB assemblies

“…In other words, intergranular (creep) cracking was the primary failure mechanism in the TCT over a temperature range of 240 to 1258C (Nousiainen et al, 2008a). On the other hand, transgranular failure existed in the SAC-In joints in extremely harsh test conditions over a temperature range of 2 to 1508C, but the growth rate of the crack was significantly lower compared with SAC387 joints tested in the same conditions (Nousiainen et al, 2009). Thus, the failure mechanisms of the SAC-In joints depended on the temperature range and the magnitude of the global thermal mismatch, similar to the ternary SAC joints, but the occurrence of transgranular cracking in the SAC-In solder joint required higher stress/strain conditions compared with the ternary SAC joints (Nousiainen et al, 2007b(Nousiainen et al, , 2008a(Nousiainen et al, , b, 2009.…”