Early microstructural indicators of crack initiation in lead-free solder joints under thermal cycling

E. Ben Romdhane, P. Roumanille, A. Guédon-Gracia, S. Pin, P. Nguyen, H. Frémont
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引用次数: 3

Abstract

The microstructural analysis of as-reflowed SAC305 solder joints showed a highly textured microstructure. During thermal cycling tests, the as-solidified microstructure gradually transformed into a less textured structure with a high population of misoriented grain boundaries through a recrystallization process. Ag3Sn IMCs coalescence is also another strong phenomenon of lead-free solder microstructure evolution since the bigger and the more spaced they are, the less dislocation pinning can prevent recrystallization from occurring. The main observation is that recrystallization accompanied by Ag3Sn IMCs coalescence are preliminary leading to intergranular propagation in the high strain regions. This work presents the Sn3.0Ag0.5Cu (SAC305) solder joints microstructural evolution at different thermal cycling levels. Electron Back Scattered Diffraction (EBSD) analysis was conducted to assess the SAC305 microstructure corresponding to a specific number of thermal cycles. Microstructural properties as $\beta$-Sn grain size and crystallographic orientation, grain boundary angles and the size of Ag3Sn intermetallic compounds (IMC) are investigated to characterize the different stages of microstructural changes under thermal cycling. Results show that crack initiation in chip resistors solder joints starts very early and precedes recrystallization process. However, solder joint lifetime is controlled by recrystallization and Ag3Sn IMCs coalescence which are the most important phenomena leading to the intergranular crack propagation.
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热循环作用下无铅焊点裂纹萌生的早期显微组织指标
再流态SAC305焊点的显微组织分析表明,其组织具有高度织构性。在热循环试验中,凝固态组织通过再结晶过程逐渐转变为织构较少、取向错误晶界分布较多的组织。Ag3Sn IMCs聚结也是无铅焊料组织演变的另一个重要现象,因为它们越大,间距越大,位错钉住越少,从而防止再结晶的发生。主要观察结果是,在高应变区,再结晶伴随Ag3Sn IMCs聚结是导致晶间扩展的初步因素。研究了Sn3.0Ag0.5Cu (SAC305)焊点在不同热循环水平下的组织演变。利用电子背散射衍射(EBSD)分析了SAC305在特定热循环次数下的微观结构。研究了Ag3Sn金属间化合物(IMC)的晶粒尺寸、晶粒取向、晶界角和晶粒尺寸等显微组织性能,以表征热循环作用下不同阶段的显微组织变化。结果表明,片式电阻焊点裂纹萌生较早,且先于再结晶过程。而再结晶和Ag3Sn IMCs的结合是导致裂纹扩展的最重要因素。
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