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

Abstract

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 [1] [2]. This paper presents the experimental methodology and results of taking an actual “medium complexity” 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.