Effect of CSP rework on surface intermetallic growth

Although a wide range of CSP rework techniques are available and component removal and replacement can be successfully achieved, there is still concern over the degradation in assembly reliability which may accompany the rework process. One main area of concern is the formation of secondary intermetallic compounds following CSP rework. These compounds are characterised by their brittleness, and their formation can adversely affect the mechanical properties of the joint, particularly when they make up a significant proportion of the joint thickness. For tin-lead solder and the conventional FR4 substrate with electroless nickel/immersion gold-coated copper pads, Ni₃Sn ₄ initially forms and grows. In this paper, we report on the investigation of different CSP rework techniques, and their effect on intermetallic layer formation. Two PCB pad-cleaning methods and three flux/paste deposition methods were investigated to assess their impact on intermetallic layer formation. The reworked joints were analysed using optical microscopy to determine the extent of intermetallic growth. The quality of the CSP joints was also assessed in terms of joint strength using shear strength testing, prior to, and after 300 hours thermal ageing at 100°C to accelerate solder grain growth, and intermetallic compound growth and oxidation