SMTA International Conference Proceedings

Attachment Quality and Thermal Fatigue Reliability of a Surface Mount Chip Resistor Assembled with a Low Temperature Solder

Authors: Charmaine Johnson, Richard Coyle, Martin Anselm, Lenora Clark, Richard Popowich, Ajitesh Singh Parihar, Tayler Swanson, Jason Fullerton, and Chen Xu
Company: Nokia Bell Laboratories, Rochester Institute of Technology, MacDermid Alpha Electronic Solutions
Date Published: 9/22/2019   Conference: SMTA International

Abstract: Technical, economic, and environmental requirements are driving the development and usage of low temperature solders (LTS) for electronic assembly. The leading alloy candidates for achieving soldering below 200ºC are based on the Bi-Sn solder system that has a melting point of 138 ºC at its eutectic composition. One of the concerns with these alloys is low thermal fatigue life, which has been linked to their characteristically brittle, two-phase lamellar microstructure. The industry has been focused largely on the challenges of mixed metallurgy ball grid array (BGA) assembly (SAC BGA assembled with LTS solder paste), while a gap exists in assembly and reliability data for other components attached solely with a LTS paste.

In this study, a daisy chained printed circuit board (PCB) test vehicle was used to evaluate the thermal fatigue reliability of the solder joints of a 2512 chip resistor assembled with a commercial LTS compared to a SAC305 performance baseline. Two PCB surface finishes were included as variables, immersion Ag (IAg) and immersion Sn (ImmSn). ImmSn may have solderability limitations due to Cu-Sn intermetallic (IMC) growth and oxidation resulting from multiple reflow processes or exposure during long term storage. The potential for final finish degradation and loss of solderability were addressed using high temperature storage and reflow preconditioning as additional variables.

Thermal cycling was used to characterize and compare the resistor thermal fatigue reliability with LTS and SAC305, and to assess any effects of surface finish and preconditioning. The interconnect metallurgy and microstructural characterization were compared for all solder alloys and surface finishes and discussed in relation to the thermal cycling results.

Key Words: 

Low temperature solder, thermal fatigue reliability, surface finishes, thermal preconditioning

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