A product feasibility study of assembling Pb-free BGAs in a eutectic Sn/Pb process

As the industry moves towards meeting the RoHS-6 requirements of eliminating the use of Pb in products, some BGA packages have become available only with Pb-free balls or available with Sn/Pb balls but only at a high cost premium. In many cases, companies are not yet ready to convert existing products to Pb-free. For this reason, the feasibility of using a backwards compatible (BGA balls with Pb-free balls mounted onto boards using eutectic Sn/Pb solder paste) assembly process must be evaluated. Accelerated thermal cycling, bend and shock tests have shown when there is full diffusion/mixing of the Pb in Sn/Pb solder paste with the Pb-free solder balls, there is improved solder joint reliability compared to the same packages assembled using reflow temperatures which did not achieve full mixing. This paper presents the experimental methodology and results of taking an actual ldquomedium complexityrdquo product with a broad size range of 1mm and 1.27mm pitch BGA packages and determining the feasibility of implementing a backward compatible assembly process. The two key considerations in making this determination were (a) the ability to find a process window large enough to repeatedly achieve full solder mixing of all Pb-free BGA packages and (b) achieved without potentially degrading other components or the PCB.