Industrial Backward Solution for Lead-Free Exempted AHP~* Electronic Products Part 1 User Context and Reliability Characterization

摘要

Since the European 2002/95/EC RoHS directive enforcement on 1st July 2006, a dominating part of the electronic industry suppressed the use of Pb in electronic equipment. As one of the consequences, exempted industries like avionics, military and telecom servers are facing increasing SnPb component obsolescence issues. Before transitioning to the qualification and deployment of full lead-free for AHP electronic products, reliable backward solutions are urgently needed to overcome SnPb package procurement difficulties. The main concerns lie in Pb-free BGAs and SnBi leaded packages assembled with a conventional SnPb reflow soldering process which affects the solder joint microstructure and can compromise the 2nd-level reliability. This paper deals with an extensive cooperation program between an EMS and an equipment maker engaged to evaluate the use of a specific "SnPb~+" assembly process as an option to solve the backward compatibility issues inherent to these kinds of components. The SnPb~+ process window as well as the incidence of reflow conditions on BGA mixed interconnects have been studied for a variety of generic components ranging from large plastic- and ceramic-BGAs down to 0.5mm pitch CSPs. The reliability has been assessed with statistics in comparison to full SnPb both under thermo-mechanical and mechanical stresses (vibrations, shocks) focusing on mission critical high-end applications in harsh environments. Numerous failure analyses have been conducted to evidence the failure modes and support the obtained reliability data. This paper presents the -55°C/+125°C ATC reliability results of mixed BGAs assembled under various reflow conditions on dedicated test vehicles reflecting typical functional boards with the PCB finish as a variable (ENIG and Immersion Sn). As a conclusion, results indicate that SnPb~+ is a very promising industrial solution for complex assemblies with long term mission in severe environments. Completion of the overall program including mechanical testing will permit to fully validate the use of the SnPb~+ process to secure the backward transition phase.