RELIABILITY ASSESSMENT OF REBALLED BGAs

The Printed Circuit Board (PCB) assembly domain has almost completed its transition to a lead-free environment. This shift has resulted in the obsolescence of tin-lead components. However, occasionally, on-going production or repair processes require SnPb components which are no longer available. Using a lead-free device in such cases could result in reliability concerns (due to the use of lead-free components on a SnPb PCB assembly). This issue is of particular concern with lead-free Ball Grid Array (BGA) devices due to the relatively high volume of lead-free alloy in the final solder joints. One possible solution is to manually attach lead-free components at a rework station. Due to the inherent manual nature of this process, it is time consuming and reproducibility is difficult to ensure. This process is typically prone to a relatively high defect rate. Therefore, a BGA reballing process was introduced as a fall back solution for a scenario in which lead-free components had to be populated on PCB assemblies that were initially manufactured through a SnPb process. In this study, SAC305 solder bumps from lead-free components were replaced with SnPb solder bumps. A series of tests and inspections were carried out to evaluate the reliability of these reballed components. After the reballing operation, the solder bump sizes were measured and they exhibited ‘good’ dimensional consistency. Voiding, which has been reported as a concern in the case of reballed components, was assessed via X-ray inspection and was determined to be a non-issue. Ball shear testing, which was used as a solder joint-level mechanical test to evaluate the strength of the re-balled solder joints, indicated adequate solder joint strength. Solder joint microstructure was studied via metallographic techniques. It was found that the thickness of the interfacial Intermetallic Compound (or IMC) was in the acceptable range. In addition, after assembly of the reballed BGA components, the packages were subjected to Environmental Stress Screening (ESS) tests, followed by in-circuit and functional tests. No failures were detected. It was therefore inferred that the re-balled components exhibited adequate performance without any degradation and can serve as a solution for such a ‘mixed’ system.