Pb-free Assembly, Rework, and Reliability Analysis of IPC Class 2 Assemblies

Abstract

A team of NEMI companies collaborated for three years to develop Pb-free assembly and rework processes for double-sided, 14-layer, printed circuit boards (PCB) in two thicknesses (0.093 and 0.135) with electrolytic NiAu and immersion Ag surface finishes. This work followed the initial SMT manufacturing feasibility effort carried out by the first NEMI Pb-free development team (1999-2002). All SMT assembly, PTH wave assembly and component rework processes were carried out on production equipment. Various test vehicles including the reliability test board were used in a multiphase development project to develop Pb-free assembly and rework parameters and temperature profiles prior to a 100-board process technology verification build. Following the double-sided SMT and wave assembly build, half of the printed circuits assemblies were passed through a series of representative component rework protocols. Each build group was then subjected to a series of mechanical and thermal reliability stress tests, including 5700 cycles of 0 to 100degC, followed by failure analysis. A special test board was designed utilizing a high temperature laminate designed for Pb-free soldering. Approximately 30% of the assemblies were SnPb control samples. This paper will present the Pb-free SMT assembly and rework development process using the NEMI Sn3.9Ag0.6Cu solder, and results of the reliability stress tests. The rework of large, thick PCB's with Pb-free solder poses a significant challenge to the industry. The lesson's learned and recommendations for future work will be discussed