iNEMI project on process development of BISN-based low temperature solder pastes — Part II: Characterization of mixed alloy BGA solder joints

In 2015, the International Electronics Manufacturing Initiative (iNEMI) initiated the Bismuth-Tin (Bi-Sn) based Low-Temperature Soldering Process and Reliability (LTSPR) Project. The goal of this project was to address the challenge of evaluating new formulations from multiple suppliers of low temperature melting solder pastes incorporating Bi-Sn solder metallurgy. These new formulations were developed to overcome the inherent brittleness of the Bi-Sn eutectic solder composition. Five solder pastes from the ductile Bi-Sn solder paste category and four solder pastes from the Joint Reinforced Paste (JRP) resin containing solder paste category were chosen for evaluation by the Project team members. Three Bi-Sn eutectic composition pastes, with Ag added in varying amounts to the solder alloy, and one SnAgCu (SAC) solder paste were also chosen to be evaluated for comparison as the baseline materials. Phase 1 of the LTSPR Project plan entailed determination of the surface mount processing ability of these solder pastes. Results from this assessment were published before at the 2017 SMTA International Conference [1]. The present paper is a continuation of the previously published work but with a focus on the characteristics of the mixed SAC-BiSn solder joints formed when high density Ball Grid Arrays (BGA) with SAC solder balls are assembled to printed circuit boards (PCB) using Bi-Sn solder pastes at peak reflow temperatures that are significantly below the liquidus temperatures of the SAC solder ball. The characteristics of the mixed SAC-BiSn alloy BGA solder joints analyzed and results obtained were as follows: (i) Solder Joint Stand-off Height (SJH) for mixed SAC-BiSn solder was significantly less than that for homogenous SAC solder joints, with, for example, the collapse during reflow being almost half for the Socket R4 (SAC balls) solder joints; (ii) The amount of Bismuth mixing in the solder joint quantified as a % of Solder Joint Height, varied widely from 10 to 100% across the 13 BiSn based solder pastes evaluated; hot tearing, an insidious solder joint at the package interface for FC BGA solder joints, noted in [1], was found to occur only at the high (>80%) and low (<;30%) Bi mixing levels; (iii) The amount of encapsulation of the solder joint by the cured resin, quantified as a % of SJH, for solder joints formed using JRP materials varied from 25 to 55% and the distribution was different for each paste tested; (iv) the voids content measured along three relevant slices of mixed SAC-BiSn solder joints was at or below the 5 % area of joint baseline for SAC solder joints.