Nicholas Herrick, Alpha Assembly Solutions
Solder interconnect degradation and eventual loss of contact is a key root cause of failure for LED light engines and systems. At the die packaging level, there is an increasing push to adopt solder based L1 interconnects for flip-chip and CSP dies. At the package on board level (Level 2), LED packages are getting bigger, being driven at higher currents (higher junction temperature) – especially for higher power automotive and outdoor applications. These trends have brought new challenges for assembly processing (especially flip-chip LED die attach) as well as reliability (like thermo-mechanical stresses for Level 2). As a result selection of solder alloy and application process is getting increased attention.
In this presentation we review lead-free alloy systems available for LED die bonding and assembly. Then we discuss the key drivers and considerations for making the selection for a particular LED assembly level and application. Process and reliability challenges like solder re-melting during secondary reflow, voiding, and thermo-mechanical stresses from CTE mismatch and solder feature size during application are addressed. Then it presents data from different application studies showing the effect of alloy systems in mitigating these challenges.
Finally, the presentation will provide recommendations for solder alloy selection based on die size, substrate material (metal-core PCBs vs FR4 vs flexible substrates), LED package design and application (LED operating power and temperature). This presentation is useful for anyone using solder for LED packaging and assembly.
Nicholas Herrick is a scientist at Alpha Assembly Solutions working on LED packaging and instrumentation. In his role he develops laboratory LED test methods to measure high thermal conductivity die attach materials. He also studies LED die attach failure mechanisms and the influence of solder alloy on LED reliability. Beyond his LED work, Nicholas also creates automatic test systems for Alpha including electronic impact bend and drop rigs, flux pin-testers, and various chemical and optical sensor systems. He previously worked for Duracell on rechargeable battery research. Nicholas is co-author on four high-intensity optics papers with pending LED papers and a patent.