Development Of Processing Parameters For Soldering Lead-Free Ball Grid Arrays Using Tin-Lead Solder
Authors: W. Fox, B. Gumpert, and L. Woody Company: Lockheed Martin Date Published: 10/14/2012
Abstract: The move to lead-free electronics by the commercial industry has resulted in an increasing number of ball grid array components (BGAs) which are only available with lead-free solder balls. The reliability of these devices is not well established when assembled using a standard tin-lead (SnPb) solder paste and reflow profile. Previous studies in processing mixed alloy solder joints have demonstrated the importance of using a reflow temperature high enough to achieve complete mixing of the SnPb solder paste with the lead-free solder ball. Research has indicated that complete mixing can occur below the melting point of the lead-free alloy and is dependent on a number of factors including solder ball composition, solder ball to solder paste ratio and peak reflow times and temperatures. Although increasing the lead content in the system allows the required peak reflow temperature to be reduced, previous research is conflicting regarding the effect that lead percentage in a mixed alloy solder joint has on solder joint reliability, and optimal reflow peak temperatures corresponding to lead content have not been established. This study uses a designed experiment with three output variables (degree of solder mixing and resultant solder ball pull and shear strength) to establish a protocol for soldering lead-free BGAs with SnPb solder paste based on solder ball size and target lead content in the final solder joint. Test units were built using those reflow profiles yielding high strength solder joints with a high degree of lead mixing. The units were thermal cycled between -55°C and 125°C and compared to SnPb assemblies to compare reliability. Failure analysis of the BGAs was then performed to determine the failure modes for the two solder joint types.