Hot tear cracks on the suppression of Sn–Bi alloy for low-temperature assembly
Authors: Qu Songtao , Shi Qingyu 
Published: May 2023

Purpose
In the electronic assembly industry, low-temperature soldering holds great potential to be used in surface mounting technology. Tin–bismuth (Sn–Bi) eutectic alloys are lead-free solders applied in consumer electronics because of their low melting point, high strength and low cost. This paper aims to investigate how to address the problem of hot tear crack formation during Sn–Bi low-temperature solder (LTS) in the mass production of consumer electronics.

Design/methodology/approach
This paper explored the development of hot tear cracks during Sn–Bi soldering in the fabrication of flip chip ball grid arrays. Experiments were designed to simulate various conditions encountered in Sn–Bi soldering. Quantitative analysis was conducted on the number of hot tear cracks observed in different alloy compositions and solder volumes to explore the primary cause of hot tear cracks and possible methods to suppress crack formation.

Findings
Hot tear cracks existed in Sn–Bi solders with different bismuth (Bi) contents, but increasing the solder volume reduced the number of hot tear cracks. Experiments were designed to test the degree of chip transient thermal warpage with temperature change, and, according to the results, glue was dispensed in specific areas to reduce chip warpage deformation. Finally, the results of combined process experiments pointed to an effective method of low-temperature soldering to suppress hot tear cracks.

Research limitations/implications
The study focuses on Sn–Bi solders only without other solder pastes such as SAC305 or Sn–Zn series.

Practical implications
With the growing popularity of smart electronics, especially in intelligent terminals, new energy vehicles electronics, solar photovoltaic and other field, there will be more and more demand for low- temperature, energy-saving, lead-free solders. Therefore, this study will help the industry to roll out LTS (Sn–Bi) solutions rapidly.

Social implications
In the long term, lean and green manufacturing is expected to be essential for maintaining an advanced manufacturing industry across the world. Developing new LTSs and soldering processes is the most effective, direct solution for energy conservation and emission mitigation. With the growing popularity of smart electronics, especially in intelligent terminals, new energy vehicles and solar photovoltaics, there would be an increased demand for low-temperature, energy-saving, lead-free techniques.

Originality/value
Although there are many methods that can be used to suppress hot tear cracks, there is little research on how to control the hot tear cracks caused by the low-temperature soldering of Sn–Bi in laptop applications. The authors studied the hot tear cracks that developed during the world’s first mass production of 50 million personal laptops based on low-temperature Sn–Bi alloy solder pastes. By controlling the Bi content, redesigning the solder paste printing process (e.g. through a printer’s stencil) and adding dispensing processes, the authors obtained reliable and stable experimental data and conclusions.