倒装芯片PBGA封装中芯片边缘裂纹的评定探讨

Research of Evaluation of Die Edge Cracking in Flip-Chip PBGA Packages

在倒装芯片塑料球栅阵列封装(FCPBGA)中,增大芯片尺寸,大热膨胀系数的不匹配,已形成可靠性试验阶段芯片断裂这一主要的失效模式。以前观察到的多数芯片断裂,是芯片背部垂直方向的裂纹,是由于过度的封装扭曲和背部缺陷形成的。对于通过分离工艺和下填充材料诱发的芯片边缘缺陷,发现增加的芯片裂纹数量起源于芯片边缘,并沿着芯片横向传播。为了提高封装可靠性和性能,应消除芯片边缘裂纹现象。通过大量的有限元分析,弄清芯片边缘裂纹,并找出其解决方案。采用断裂力学方法,评定各类封装参数对芯片边缘初始断裂的影响。找出应变能释放率,对评定分离诱发裂纹的芯片边缘初始断裂来说,是一种有效的技术。探讨初始裂纹大小和各类封装参数的影响,与芯片底部裂纹现象不同,引起芯片边缘横向断裂的主要参数与局部效应关系更密切。

Increasing die size and large coefficient of thermal expansion （CTE） mismatch in flip-chip plastic ball grid array （FCPBGA） packages have made die fracture a major failure mode during reliability testing. Most die fracture observed before was die backside vertical cracking, which was caused by excessive package bending and backside defects. However, due to die edge defects induced by the singulation process and the choice of underfill material, an increasing number of die cracks were found to initiate from die edge and propagate horizontally across the die. In order to improve package reliability and performance, die edge cracking has to be eliminated. An extensive finite element analysis was completed to investigate die edge cracking and find itssolutions. A fracture mechanics approach was used to evaluate the effect of various package parameters on die edge initiated fractre. Strain energy release rate was found to be an effective technique for evaluating die edge initiated fracture from singulation-indueed flaws. The impact of initial flaw size and a variety of package parameters was investigated. Unlike in die backside cracking, the dominant parameters causing die edge horizontal fracture are more closely related to local effects.