BGA Re-balling from Lead-free to Tin-lead

S. J. Meschter, S.A. McKeown
R. Feathers and E. Arseneau
BAE Systems, Johnson City, NY, USA

As a result of a global movement away from using Lead (Pb) in electronic assemblies, component manufacturers are almost exclusively providing lead-free parts to satisfy the high volume consumer markets. Unfortunately, relatively little is known about the performance of lead-free solders in harsh vibration and shock environments.

These concerns are amplified because the consumer industry is currently evaluating another generation of lead-free solder alloys in an effort to improve reliability.

Tin whiskers not withstanding, nearly all the current lead-free electronic piece-part termination finishes are compatible with tin-lead assembly solder with the exception of ball grid arrays. Reprocessing lead-free BGAs with tin-lead ball metallurgy is one means of mitigating the risk of lead-free solder material failure modes such as tin whiskers, high cycle fatigue, printed circuit board pad cratering, and intermetallic fracture.

In addition, because qualification of a metallurgy change in a high reliability application can take years, BGA reballing allows original equipment manufacturers to maintain the certification and qualification status on existing configurations while managing the on-going lead-free alloy changes occurring on BGAs. In the present work, an assessment of the mechanical integrity of four different commercially available BGAs was evaluated after reballing using visual inspection, cross-section evaluation, scanning acoustic microscopy, moire interferometry, ball shear, ball pull and assembly level thermal cycling.

With careful attention to detail, BGA reballing remains a viable solution to manage the obsolescence of tin-lead ball metallurgy. The cross-section evaluation provided the key package construction details needed for modeling. The scanning acoustic microscopic examination was generally successful in assuring that the package structures did not delaminate during the reballing process, though it was not useful for the package with the lid.

Visual inspection of the package is an important part of the reballing quality verification. Warpage measurement appears to be more useful than coefficient of thermal expansion measurement in assessing BGA changes during reballing. However, warpage change acceptance limits are still need. The increase in warpage observed after reballing suggests that the polymer insulating materials may have increased in modulus (e.g. increased degree of cure).

The ball shear and ball pull tests verified that the pad-to-package interconnections were not compromised by reballing. The thermomechanical stress modeling effort needed the cross-sectioning, warpage and CTE measurements. In general, these preliminary modeling efforts suggest that increasing maximum temperature of the die (top center) during ball removal increases the maximum stresses within the part and that the location of the maximum stress is near the die corners and edges. It is hopeful that efforts such as this will contribute to the body of knowledge needed for industry standard development