Mechanical Reliability of Lead-Free Solder Paste Alloys in Surface Mount Applications with Different Surface Finishes

Abstract

It has been a little more than 23 years since the European Parliament released the lead-free directives that piloted the electronics manufacturing industry to adopt lead-free solder alloys. During this time, there still doesn’t exist a clearly identified lead-free solder alloy to replace the eutectic Sn63Pb37. Even the dominate use of SAC305 (Sn-3Ag-0.5Cu) is still being evaluated and tested with different surface finishes while introducing doping elements for various applications where shear, fatigue and creep continue to plague electronic assemblies. This study takes the approach of utilizing different solder paste alloys in small quantities with SAC305 being used for the final assembly of components. The attempt of this approach is to provide data to support whether the use of a solder paste alloy with additional doping elements (Bi, Ni, and Sb) would improve the mechanical performance over a single SAC305 solder joint. The smaller quantities of the solder paste would provide cost savings over using a more expensive four element or five-element alloy for all soldering locations for a given product. The study will also consider the effects of two different surface finishes, Organic Solderability Preservative (OSP) and Electroless Nickle Immersion Gold (ENIG). Four different strain rates will be used to collect ultimate shear strength data while three different cyclic fatigue tests will be used to estimate reliability. These examinations will evaluate three different solder paste alloys with a SAC305 solder sphere on a single SMT mount: 1) SAC305, 2) SAC308-Bi-Ni, and 3) SAC347-Bi-Sb-Ni-x. These solder paste alloys are examined at 4 mil and 5 mil thicknesses over OSP and ENIG surface finishes.