Issue link: https://iconnect007.uberflip.com/i/1243344
30 SMT007 MAGAZINE I MAY 2020 responding degradation (if any) due to thermal cycling. Figure 5 shows a few examples of as-sol- dered hybrid HRL1/SAC305 solder joints (BGA432, BGA208, and BGA84) and HRL1 homogeneous solder joints (LGA256, MLF100, and chip resistors 1206, 0805 and 0201). For opti- mized assembly and reflow conditions, and consider- ing the size of these pack- ages, warpage or soldering defects were not observed. Figure 6 shows the cross-sections of (a) BGA84 solder joints after 1,500 thermal cycles and (b) chip resistors 1206 after 2,500 ther- mal cycles. Hybrid eutectic SnBi/SAC305 solder joints show higher degradation than hybrid HRL1/SAC305 solder joints, after 1,500 cycles. Cross-sections of chip resistors 1206, from a separate thermal cycling test, show that after 2,500 cycles, HRL1 homogeneous solder joints have little degradation. Compar- quickly accelerated for the eutectic Sn-Bi alloy, while HRL1 closely followed SAC305 behavior until 2,000 cycles. Solder Joint Evaluation While in-situ monitoring of solder joint elec- trical resistance provides quantitative infor- mation of what happens to the solder joint during thermal cycling, the analyses of cross- sections provide visual reference of the cor- Figure 4: Thermal cycling accumulated failures on hybrid LTS/SAC305 BGA84 solder joints. Figure 5: Examples of cross-sections showing (a–c) hybrid HRL1/SAC305 solder joints and (d–h) HRL1 homogeneous solder joints. Images taken at various magnifications, as indicated in the respective image.