Authored By:
Watson Tseng, Chunyu Chang, Coti Chung, Keith Lee, Ken Lin
Shenmao America, Inc.
CA, USA
Summary
Low-temperature soldering (LTS) has gained prominence in the Surface Mount Technology (SMT) industry due to its energy efficiency and enhanced production yields. However, like the transition from leaded to lead-free solder, the increasing adoption of LTS presents compatibility challenges, particularly with Ball Grid Array (BGA) components. This study explores the reliability of different ball-paste combinations for LTS soldering.
Four combinations were investigated: 1) LTS ball with LTS paste, 2) SAC ball with LTS paste, 3) LTS ball with SAC paste, and 4) SAC ball with SAC paste. Testing was conducted using a wafer level package, which causes a large coefficient of thermal expansion (CTE) mismatch with epoxy glass fiber-based printed circuit board (PCB), as test vehicle. Soldering profiles were specified for LTS and SAC pastes. Microsectioning and thermal cycling from -40°C to 100°C were performed, with failure defined as a 20% resistance change. Findings from this investigation suggest that LTS soldering may enhance the reliability of joints in some combinations.
Conclusions
In this study, we conducted an investigation into the solder joint reliability across various combinations of SAC ball, SAC paste, LTS ball, and LTS paste to assess the compatibility of LTS soldering techniques. In the test vehicle characterized by a significant CTE mismatch, we made several key findings:- Homogeneous joint structures outperform hybrid joints: We observed that solder joints with a homogeneous composition demonstrated superior life performance compared to joints with a hybrid composition.
- Bismuth-containing joints exhibit better reliability: Solder joints containing bismuth showed improved life performance in comparison to bismuth-free joints. This is primarily attributed to the enhanced mechanical properties associated with bismuth-containing alloys.
- Lower bismuth content enhances reliability: Among joints with bismuth content, those with lower bismuth content exhibited better life performance. This trend aligns with the observation that as the bismuth content increases beyond 17 wt.% and approaches 58 wt.%, the mechanical properties of the solder tend to decline.
- Deformation in hybrid joints results from SAC tiny deformation accumulation: The deformation observed in hybrid joints is a consequence of the accumulation of tiny deformations within the SAC layer, leading to the extrusion of SAC. If the displacement is significant enough, it can induce cracks in the LTS portion of the joint.
These findings contribute to our understanding of solder joint reliability and the factors that influence it, particularly in the context of LTS soldering techniques.
Initially Published in the SMTA Proceedings
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