Reliability Assessment of Reballed BGAs

J. Li, S. Poranki, K. Srihari, Ph.D.
Watson Institute for Systems Excellence
State University of New York, Binghamton, NY, USA

M. Abtew, R. Kinyanjui, Ph.D.
Technology Development Center
Sanmina-SCI Corporation, San Jose, CA, USA

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.

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.

Because of the reliability concerns and the challenges in developing backward compatible process, BGA reballing was considered as an alternative option for the mixed system assembly process. The main focus of this study was to comprehensively evaluate the reliability of reballed BGAs. Several potential issues that could impact the use of reballed BGA components were evaluated. The sizes of reballed spheres were measured and the process Cpk values indicated a high consistency in both ball diameter and height. As reported, voiding could be a major concern for reballed components. However, in this study, this was not the case.

According to cross-section images, excessive growth in the IMC layer was not seen. Besides, solder joint strength did not degrade, as indicated by shear strength data. After populating the BGA components on PCBs, it was seen that all assemblies passed ESS testing, ICT and functional tests. All the evaluations mentioned above showed that the reballed components exhibited adequate performance and can be recommended as a solution for the mixed system assembly process.

Interesting and informative. This study will help to assure our Reliability Engineers that reballing is not a scarey process to be avoided.