SMTA International Conference Proceedings

High-Reliability Lead-Free Solder for Low-Cost Die Attach Applications

Authors: Niveditha Nagarajan, Sathish Kumar, Gyan Dutt, Ranjit Pandher, Carl Bilgrien
Company: Macdermid Alpha Assembly Solutions
Date Published: 9/22/2019   Conference: SMTA International

Abstract: The need for an appropriate lead-free, high reliability, low cost die attach solder arises due to increased reliability concerns and limited temperature hierarchy options for multi-level assemblies, using existing lead-free solder alloys, such as SAC305/405 and SnBi based alloys. This paper reports the development of a new SnCu based solder alloy with decreased fluidity than eutectic SnCu and SAC305, which helps to reduce the probability of issues such as die float and die tilt during secondary reflow. In addition, the solder has been designed to improve bulk mechanical strength and have a stable microstructure under exposure to high temperatures.

This paper reports the results of tests done to compare the reflow characteristics, solderability and thermomechanical reliability of the new solder alloy with industry standards such as eutectic SnCu and SAC305. The first set of tests compares the performance of the solder alloys with respect to secondary reflow problems such as die tilt and voids. The second test involves inducing thermomechanical stress on LED packages that have been assembled onto aluminum and FR4 substrates, by thermal cycling for up to 1833 cycles. LED performance metrics such as junction temperature, luminous efficiency and color stability etc. were monitored. In addition, solder joint strength and microstructure evolution were recorded.

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