The Challenge of Lower Temperature Soldering for Large Ball-Grid Array Board-Level Assembly Process

Abstract

Lower temperature soldering has been considered as one of the effective ways to reduce the risk of warpage on PCB assembly. Use of lower-temperature solders, including a BiSnAgCu eutectic and two In-containing, Bi-free solders DFLT (Durafuse® LT), and DFLT-2 has been attempted to reflow the 40mmx40mm plastic ball grid array (PBGA928). BiSnAg, under the combinations of reflow profiles P185 and P200 (~185°C and ~200°C peak temperature) and the paste-to-ball (P2B) volume ratio (0.13, 0.25, and 0.5), always formed defective joints. Under the P200 profile with P2B volume ratio of 0.5, SAC305/BiSnAg joints were dominated exclusively by hot-tearing and shrinkage voids despite forming the desired drum shape. Using P220 profile (~220°C peak temperature, right above the liquidus temperature of SAC305) with P2B ratio of 0.13, SAC/BiSnAg had presented the drum shape joint and free of hot-tearing and shrinkage voids. With the constant P2B volume ratio of 0.13, P200 also renders various malformed joints for both DFLT and DFLT-2. The major challenge will be the insufficient molten solder volume under low temperature soldering to accommodate the displacement caused by the warpage. Increasing reflow temperature improved the joint shape and greatly reduced the defects for both DFLT and DFLT-2. Under P220, both SAC/DFLT and SAC/DFLT-2 joints achieved the desired short-and-fat drum shape, comparable to those reflowed under P240. The formation of the optimal joint for all three pastes under the hot profiles is attributed to the sufficient liquid solder volume since both paste and SAC ball fully melt and merge, which compensates the displacement caused by the dynamic warpage. Both SAC/DFLT and SAC/DFLT-2 joints exhibited the limited hot-tearing under the P200 profile while SAC/BiSnAg joints were dominated by those defects. The different metallurgy of DFLT and DFLT -2 did not result in an enlarged pasty range after joining with SAC305 and thus led to the lower defect rate. Investigation of the joint reliability performance is still ongoing.