Issue link: https://iconnect007.uberflip.com/i/1137649
JULY 2019 I SMT007 MAGAZINE 71 board land pad (shown as Type 5 failure mode in Figure 29) and many pulled some board material (similar to pad cra- tering) while the low-tempera- ture materials tended to break through the IMC (intermetal- lic) boundary in the bulk of the solder joint on the board side, shown as Type 4 failure mode in Figure 29. Interestingly, the strongest point has historically been the bond between the board land pad and the board. In this (lim- ited) test, the control mate- rial did not have the strongest response (Figure 30). This may indicate that the higher process temperatures of the SAC-305 reflow profile have an effect on the pad-to-board strength. Reliability: Accelerated Temperature Cycle Testing We followed JEDEC 9701 recommended pro- cedures to perform accelerated temperature cycle testing. We mounted multiple daisy- chained and 0-ohm components to a four-layer PCB. The board was 62 mils (1.6 mm) with 18–20 boards used for each solder material. The boards were subjected to 0–100°C 40-min- ute temperature cycles with 10-minute ramps and dwells. We measured electrical continu- Figure 30: Results of the dye and pry and cross-section post-strain (solder joint) fracture failure mode analysis. The control paste mostly failed between the pad and the board while the other materials mostly failed at the IMC. Figure 29: Failure mode descriptions. A Type 5 failure mode is between the board land pad and the board while a Type 4 failure is in the IMC on the board side.