iNEMI PROJECT ON PROCESS DEVELOPMENT OF BiSn-BASED LOW TEMPERATURE SOLDER PASTES - PART VI: MECHANICAL SHOCK RESULTS ON RESIN REINFORCED MIXED SnAgCu-BiSn SOLDER JOINTS OF FCBGA COMPONENTS

摘要

Low temperature solder pastes in the Sn-Bi systems are being evaluated by the iNEMI Low Temperature Solder Process and Reliability (LTSPR) Project to attain economic, environmental, and manufacturing benefits for handheld, mobile computer and server board products. The Bi-Sn solder system is beset by the inherent brittleness of the bismuth phase, which affects the reliability of solder joints formed. Solder suppliers have developed ductile Bi-Sn solder pastes with ductile metallurgies and resin reinforced formulations to enhance the reliability of the Bi-Sn solder joints. Current lead free BGA components have SnAgCu (SAC) solder balls. When these are reflow soldered with BiSn solder paste, mixed SAC-BiSn BGA solder joints result, whose mechanical shock and thermo-mechanical fatigue reliability needs to be assessed. The iNEMI LTSPR Project team had assessed the mechanical shock reliability of mixed SAC-BiSn solder joints for a small Package on Package (POP) component assembled on a JEDEC Standard Shock testing board using 13 selected solder pastes. These pastes were divided in four different categories (SAC baseline, BiSn Baseline, BiSn Ductile and BiSn Joint Reinforced Pastes) and their results were reported at the 2018 SMTA International Conference. This work assesses the mechanical shock drop reliability of a much larger 42 × 24 mm body size flip chip BGA (FCBGA) package, having 1356 SAC405 balls on a mixed pitch ball array, assembled on a specifically designed shock test board. Both OSP and ENIG surface finished board lands were evaluated, for mixed SAC-BiSn solder joints formed by using four BiSn Joint Reinforced Pastes (JRP) containing resin. These were compared with mixed SAC-eutectic BiSn solder joints and homogenous SAC305 solder joints. The Characteristic Life (η) metric was extracted from Weibull plots of the shock test for each of these six legs and compared. Results showed that resin encapsulated mixed alloy SAC-BiSn solder joints on OSP surface finished boards exhibited better mechanical shock solder joint reliability than ENIG surface finished boards. The increase in η for OSP over ENIG was in the 17 to 78% range. The reliability of the solder joints formed with the four JRP resin solder pastes varied significantly, and is linearly correlated to the % height of the solder joint being encapsulated by it. In all cases however, the η was higher than that for the BiSn baseline leg, the increase being in the 1.6X to 4.0X range. This indicates that the addition of resin encapsulation around the FCBGA mixed SAC-BiSn solder joints significantly reinforces their drop shock reliability.