SOLDER JOINT RELIABILITY ON MIXED SAC-BISN BALL GRID ARRAY SOLDER JOINTS FORMED WITH RESIN REINFORCED BI-SN METALLURGY SOLDER PASTES

摘要

Due to the decreasing size of consumer electronic products such as smart phones, tablets, and personal computers, ultra-thin flip chip ball grid array (FCBGA) packages are needed to meet the demand of lower z-heights for slimmer form factors. However, packages used in consumer electronics are commonly assembled on printed circuit boards (PCB) with lead-free SnAgCu (SAC) solder paste at peak reflow temperatures in the 240C to 260C range. Assembly challenges are observed at these peak reflow temperatures due to dynamic warpage of the component substrates of ultra-thin FCBGAs, as well as the PCB. The Bi-Sn low temperature solder metallurgical system has been proposed as an alternative to the SAC metallurgical system to overcome these package substrate and PCB warpage induced assembly challenges. Besides improving solder joint yields on ultra-thin FCBGAs, the lower melting point of this Bi-Sn metallurgy also enables manufacturing cost savings and environmental benefits. However, based on previous literature studies, the presence of Bi in Sn-based low temperature solder has exhibited solder joint embrittlement and thereby decreased the mechanical shock resistance of such solder joints. In order to strengthen the solder joint, low temperture solder pastes have been developed containing resin, which by flowing around the solder joint and curing during the reflow process it provides polymeric encapsulation reinforcement at the solder joint level. In a previous study [16], this type of low temperature joint reinforced paste (JRP) had shown improved mechanical shock resistance on mixed BiSn+SnAgCu FCBGA solder joints when compared to those without the polymeric encapsulation. However, the effect of thermal fatigue on the resin reinfroced BiSn+SnAgCu BGA solder joint when subjected to thermal cycling needed to be better understood. In this paper, the investigation centered on both the mechanical shock and the thermal cycling performance of the mixed BiSnAg (BSA)+SAC solder joints assembled with JRP pastes on a Flip Chip Molded Package BGA (FCMB) and compared the data to those assembled with SAC305 paste. Reliability failure rate characterized with Weibull distributions, failure modes and locations within the solder joint stack-up were determined. Results indicated that the mixed SAC+BiSn solder joints with polymeric reinforcement when using two different JRPs were less shock resistant than the SAC solder joints for both JRPs. More development is therfore necessary with JRPs to enable low temperature Bi-Sn solder joints to become comparable with un-reinforced SAC BGA solder joints under mechanical shock. There was significant improvement in the thermal cycling performance for mixed SAC+BiSn solder joints formed using one of the JRPs, but this improvement was for the solder joints located at the package corners only. The level of encapsulation of the solder joints by the reinforcing resin was inconsistent across the FCMB array and this could have caused this inconsistent temperature cycling resistance.