Understanding the Impact of Accelerated Temperature Profiles on Lead-Free Soldering

摘要

Traditional reflow profiles for lead-free soldering typically require longer processing times due to elevated peak temperatures and flux activation times defined by solder paste suppliers. These profiles become particularly challenging when a wide variety of packaging types are integrated within a single circuit design. Further difficulty is presented when product designs with high thermal mass, such as heat slugs and metal substrates, are processed. These designs create large thermal gradients throughout a circuit assembly and add further complexity to finding an "optimal" profile window. All of these issues create a significant increase in reflow processing times for lead-free soldering. This paper investigates these increased processing times required for high volume manufacturing of lead-free electronics. A study of typical process capacity and real throughput capacity is presented. The study evaluates high volume electronics manufacturing ranging from small circuit assemblies (e.g. cell phone) to large circuit assemblies (e.g. automotive and computers) and investigates a series of "best" reflow profiles to accelerate the standard lead-free process window to meet a targeted manufacturing capacity using an automated profiling system. A test vehicle is then fabricated using this defined process window and tested for quality (solder voiding and appearance) and solder joint reliability (accelerated life testing). The designed test vehicle includes components from a large physical distribution including: small and large BGAs, QFNs, and any type discrete components. During assembly, virtual profiling is used to document any deviations to the process profile window. The quality and reliability data are presented within this publication and failure analysis is included to determine the capability of this proposed profile. When employed, this profiling strategy allows many manufacturers to reduce the processing time for reflowing lead-free circuit assemblies without significant lost in manufacturing quality or reliability. Furthermore, this study provides a sound understanding and limitations for using accelerated profiling speeds for lead-free soldering applications.