Authored By:
Francis M. Mutuku, Hongwen Zhang, Huaguang Wang, Jie Geng
Indium Corporation
NY, USA
Summary
This study reports a new near-eutectic Sn-Bi-based low-temperature solder alloy (Bi+) which can reflow between 165°C and 190°C. The Bi+ alloy showed improved thermal fatigue resistance under a thermo-cycling test (TCT) profile of -40/125°C and 10 min dwell time as compared to SnBi1Ag and SAC305 in a ball-grid array (BGA) assembly (SAC305 ball). The alloy is strengthened by the addition of elements like Ag and Ni, the choices of which target properties that include particle strengthening, grain boundary strengthening, intermetallic compound (IMC) toughening, solidification behavior changes, and microstructural changes. For example, Ni is known to reduce the undercooling of Sn-based alloys. This influences the formation of a near-eutectic microstructure, refining of the Sn grain size, and toughening of IMC at the interface. These properties jointly contribute to improved thermal fatigue resistance.
SAC305/Bi+ hybrid solder joints also showed significantly reduced solder joint shifting compared to SAC305/SnBi1Ag and other SAC305/low-temperature solder (LTS) solder joints when cycled at -40/125°C profile. Hybrid solder joints shifting is increased by an in-balance in the SAC/LTS regions that cause intra-joint phase CTE mismatch. The intra-joint phase CTE mismatch complicates the CTE mismatch between the board and the component, stretching the hybrid solder joints. The Bi+ alloy is designed to withstand this effect in a hybrid BGA/LTS assembly to maintain upright stable solder joints after thermal cycling for more than 6,900 cycles.
The SAC305/Bi+ hybrid solder joints have shown a higher percentage of Bismuth mixing at time zero (T0) and during thermal cycling as compared to SAC305/SnBi1Ag and other low-temperature solder alloys. The Bi+ alloy was designed so that the as-reflowed hybrid CABGA192 solder joints built with SAC305/Bi+ achieved an average Bi mixing of approximately 50%. After thermal cycling for about 6,900 cycles, the hybrid solder joints attained an average Bi mixing of more than 90%. An FR4 board of 1.54mm thickness with a glass transition temperature (Tg) of 130°C was used as the test vehicle (board) in this study.
Similarly, 0805 chip resistors that were reflowed on homogeneous Bi+ paste returned higher resistance to cracking as compared to SnBi1Ag and SAC305 after thermal cycling. When thermal cycled with -40/105°C for up to 5000 cycles, Bi+ solder joints had smaller percentage crack sizes and no cracks under the die as compared to both SnBi1Ag and SAC305.
The overall performance of the alloy is a combination of alloy design, process optimization, solidification behavior studies, and paste parameters tuning such as powder size, metal load and paste-to-ball volume ratio.
Conclusions
This study reports a new near-eutectic Sn-Bi-based low-temperature solder alloy (Bi+) which can reflow between 165°C and 190°C. Although reflowed at lower temperatures, this alloy in the hybrid SAC305/Bi+ BGA assembly configuration can be used in applications whose service temperature is higher than that of typical low temperature alloys. In a BGA assembly with SAC305, the Bi+ alloy showed improved thermal fatigue resistance under a TCT profile of -40/125°C and 10-minute dwell time as compared to SnBi1Ag and SAC305 in a BGA assembly (SAC305 ball). The performance of the SAC305/Bi+ BGA solder joints and indeed other LTS alloys is sensitive changes in composition and process parameters such as powder size, peak reflow temperature, paste to ball volume ratio, and thermal cycling profile.
When the design of the LTS alloy is right, the processing is right, the initial percent Bismuth mixing is optimized, percent Bismuth mixing is good, then thermal fatigue failure is delayed during thermal cycling as observed with hybrid SAC305/Bi+ BGA solder joints and not SAC305/SnBi1Ag BGA solder joints. It is observed that SAC305/Bi+ CABGA192 solder joints outperform SAC305/SnBi1Ag and the SAC305/SAC305 solder joints in thermal fatigue resistance. The BGA SAC305/LTS thermal fatigue failure is almost entirely caused by cracking on the component side. At early cycles, larger and non-catastrophic cracks on the board side dominate, but as thermal cycling continues, larger cracks are seen on the component side, which includes catastrophic cracks as well. This behavior is evident regardless of the thermal cycling profile and points to a shift in the stress concentration with thermal cycling.
Similarly, homogeneous Bi+ paste built with 0805 chip resistors returned higher shear strength and higher thermal fatigue resistance to cracking as compared to SnBi1Ag and SAC305 after thermal cycling.
Initially Published in the SMTA Proceedings
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