Issue link: https://iconnect007.uberflip.com/i/1050827
NOVEMBER 2018 I PCB007 MAGAZINE 47 At the recent SMTA International Confer- ence in Rosemont, Illinois, one notable top- ic seeing increased interest in the industry is the board reliability of low-temperature solder (LTS). LTS is gaining attention for multiple rea- sons, including lower power consumption, less warpage, higher assembly yields, and a pos- sible path to more complex assemblies. Pres- ently, much work is being performed to com- pare the reliability of LTS with the convention- al SAC (tin-silver-copper) alloy. Some of that work was presented at the conference. iNEMI's investigations of third-generation lead-free alloys presented by Richard Coyle showed that such alloys provide significantly better high-reliability accelerated thermal cy- cling (ATC) performance than traditional SAC alloy, in one case, doubling 0/100°C charac- teristic life. Among these alloys was an indi- um-containing SAC alloy that one might con- sider to be an LTS (202–206°C liquidus). A ho- mogenous assembly of this alloy in which the paste and solder ball alloys are the same had equal to or better characteristic life as a homo- geneous SAC alloy in 0/100°C, 40/125°C, and -55/125°C ATC; however, it had earlier first failure in -55/125°C ATC [1]. In a paper reporting the potential of what is considered to be true LTS for enterprise com- puting and automotive electronics, Paul Wang and his team at Mitac point out that back- ward compatibility will be important. Their ATC testing is not complete, but they reported that their mixed assembly using tin-bismuth LTS paste (130–140°C liquidus) and SAC sol- der balls (Figure 1) is exhibiting earlier failures in -40/100°C ATC than their homogenous LTS assembly [2] . Other work reported this year by Satyajit Walwadkar et al. at Intel investigates the use of a novel method to determine the mechanical fatigue performance of solder us- ing only a single solder joint corroborates what Wang reported. Joints made with tin-bismuth LTS pastes (140–150°C liquidus) and SAC405 solder balls did not perform as well as their ho- mogeneous SAC joint [3] . Wang expressed concern that tin-bismuth LTS alloys, being more brittle, would not per- form as well in mechanical shock testing. That concern was shown to be well-founded by re- sults reported in another iNEMI paper by Jag- adeesh Radhakrishnan et al. Tin-bismuth LTS (liquidus 125–139°C) exhibited a significant- ly lower drop in test performance compared to SAC305. The iNEMI team also looked at im- proving performance using solder pastes that create a polymeric reinforcement referred to by some as a polymer collar. The reinforcement improved drop performance, but it could not match the baseline performance of SAC [4] . In their paper, Wang suggested that an edgebond adhesive might be used to enhance drop test performance. Figure 2 illustrates how such an adhesive would be used. Using an edgebond adhesive to enhance me- chanical shock performance raises the concern over what effect such an adhesive would have on board level reliability. That matter was in- vestigated and reported on by Professor Tae- kyu Lee of Portland State University. A mixed assembly—in this case, SAC305 paste with an LTS ball (197°C liquidus)—exhibited poorer -40/125°C ATC performance than the homo- geneous SAC assembly. Lee's team looked into the use of a reworkable edgebond adhesive and found that the ATC performance of the mixed Low-temperature Solder Gaining Ground Figure 1: Mixed assembly with SAC ball and LTS paste.