*Article* **Failure Analysis of SAC305 Ball Grid Array Solder Joint at Extremely Cryogenic Temperature**

**Yanruoyue Li 1, Guicui Fu 2, Bo Wan 2,\*, Maogong Jiang 2, Weifang Zhang <sup>2</sup> and Xiaojun Yan <sup>1</sup>**


Received: 11 February 2020; Accepted: 9 March 2020; Published: 12 March 2020

**Featured Application: It is an important trend that Pb-free materials in electronic devices**/**components are used in the aerospace field. Several reliability issues associated with this kind of material characteristic occurred and need in-depth studies. One of the issues is that Sn-rich material's characteristic changes under low-temperature. This change may cause components**/**device failure and has an influence on their reliability. This work of failure analysis provides an actual case of electronic components with failure when under harsh environments, such as extreme cryogenic temperature in space. It makes reliable use of Pb-free material under extremely cryogenic temperature conditions to be taken seriously, brings up an analysis process for this kind of failure, and suggestions for operating temperatures are put forward.**

**Abstract:** To verify the reliability of a typical Pb-free circuit board applied for space exploration, five circuits were put into low temperature and shock test. However, after the test, memories on all five circuits were out of function. To investigate the cause of the failure, a series of methods for failure analysis was carried out, including X-ray detection, cross-section analysis, Scanning Electron Microscope (SEM) analysis, and contrast test. Through failure analysis, the failure was located in the Pb-free (Sn-3.0Ag-0.5Cu) solder joint, and we confirmed that the failure occurred because of the low temperature and change of fracture characteristic of Sn-3.0Ag-0.5Cu (SAC305). A verification test was conducted to verify the failure mechanism. Through analyzing data and fracture surface morphology, the cause of failure was ascertained. At low temperature, the fracture characteristic of SAC305 changed from ductileness to brittleness. The crack occurred at solder joints because of stress loaded by shock test. When the crack reached a specific length, the failure occurred. The temperature of the material's characteristic change was −70–−80 ◦C. It could be a reference for Pb-free circuit board use in a space environment.

**Keywords:** SAC305; BGA; low temperature; fracture failure
