SMTA International Conference Proceedings

Microstructure Study of Lead-Free Solder Joints Reflowed Using Alternative Low Silver Alloy Solder Pastes

Authors: Elissa Grace McKay and Jennifer Nguyen
Company: Flextronics
Date Published: 9/28/2014   Conference: SMTA International

Abstract: Low-silver solder alloys for board-level interconnections provide a sustainable, reduced-cost alternative to existing lead-free alloys. Whilst reducing or eliminating the use of silver is known to produce solder joints with greater mechanical reliability, whether they possess sufficient thermal reliability for practical applications is still of concern.

There are limited publications on the study of alternative alloy solder pastes assembled with lead-free components. There is a lack of information on the microstructure of solder joints assembled using alternative low silver alloy solder pastes, and their failure modes are not well understood. In this paper, we will present our study on the microstructure of various lead-free alloy solder pastes after reflow and after thermal cycling test. The effect of the microstructure of various alternative lead-free alloys on the failure phenomenon after thermal cycle testing will be analysed and compared. Six different lead-free alloy solder pastes were investigated. Sn3.0Ag0.5Cu was used as the control. A low temperature SnBiAg solder paste was also included and compared in the study.

The study showed that there is no significant difference in the intermetallic layer of the solder joint assembled using SAC305 and other lead-free high temperature alloy solder pastes (SAC0307, SACX0307, SN100C, Sn992). The IMC thickness of the SnBiAg solder joints after the reflow process was typically thinner than that of the high temperature lead-free alloys. The IMC layer of SnBiAg solder joints grew and saturated during the thermal cycle testing. The thickness and composition of the intermetallic layers were not determined to affect the reliability of the solder joint during the thermal cycle testing. Most cracks occurred in the bulk of the solder joint at Sn grain boundaries. Coarsening of the Sn phase occurred during the thermal cycle test. Alloy compositions affected the microstructure of the solder joint and the Sn phase coarsening after thermal condition, which resulted in different failure mode mechanisms for solder joints assembled using different alloy solder paste materials.

Key Words: 

Pb-free, low-Ag solder paste, solder joint microstructure, SnAgCu, SnBiAg, SnCuBi, SnCuNi.

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