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Predicting the Reliability of Package-On-Package Interconnections
Production Floor |
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Authored By:P. T. Vianco, Ph.D., J.A. Rejent, J.M Grazier, A.C. Kilgo, B.M. McKenzie, and M.K. Neilsen Sandia National Laboratories Albuquerque, NM, USA SummaryPackage-on-package (PoP) and package-on-package-onpackage (PoPoP) technologies are being considered to reduce the size, weight, and power (SWaP) of military, space, and satellite electronics. The long-term reliability of PoPoP solder joints was assessed under thermal mechanical fatigue (TMF). The study included accelerated aging experiments (-55 Celsius/125 Celsius temperature cycling) and development of a computational model. Data were analyzed using the two-parameter (2P) Weibull probability distribution (characteristic lifetime, and slope). Backwards compatible, test vehicles (bottom solder joints (SnPb paste/SAC305 solder balls) exhibited excellent TMF performance (characteristic lifetime = 2600 +/- 200 cycles; slope = 7.6 +/- 3.5) as did the middle (characteristic lifetime = 2500 +/- 300 cycles; slope = 6.6 +/- 3.4) and top joints (characteristic lifetime = 2600 +/- 200 cycles; slope = 8.7 +/- 4.0). The 100% SAC305 test vehicle showed comparable characteristic lifetime values: bottom joints, 2200 +/- 200 cycles; middle joints, 2600 +/- 500 cycles; and top joints, 2400 +/- 200 cycles. Underfill in the bottom gap improved the characteristic lifetime of the SnPb/SAC305 interconnections but reduced that of the 100% SAC305 solder joints. When underfilled, the 100% SAC305 joints exhibited two failure modes: a "low characteristic lifetime" mode that showed only TMF cracks and a "high characteristic lifetime" mode that had both TMF and tensile cracks. The computational model predicted that PoPoP warpage was not significantly changed by underfill placed in the bottom gap. Initial quantitative predictions of Nf were less than satisfactory, requiring further development of the model for the latter be a suitable design tool. Conclusions1. A long-term reliability program is underway to assess the feasibility of stacked packages for use in high reliability military, space, and satellite electronics. The present study examined the TMF performance of PoPoP assemblies tested under accelerated aging experiments (-55 Celsius/125 Celsius temperature cycling). The development of a computational model is included in this effort to predict the TMF of all three levels of solder interconnections. 2. Empirical failure event data were analyzed by the two parameter (2P) Weibull probability distribution(characteristic lifetime and slope). A detailed, failure mode analysis accompanied the quantitative evaluations. 3. The test vehicles included variants of bottom solder joints - backwards compatible mixed SnPb/SAC305 versus 100% SAC305 Pb-free joints - as well as the placement of underfill in the gaps - bottom, only; bottom and middle gaps; as well as bottom, middle, and top gaps. 4. Aside from a single test vehicle, all of the other SnPb/SAC305 solder joints exhibited the homogeneous microstructure that results from complete mixing of the two solders. 5. The test vehicles having the backwards compatible, bottom solder joints exhibited excellent TMF performance for, not only those interconnections (characteristic lifetime = 2600 +/- 200 cycles; slope = 7.6 +/- 3.5), but also for the middle joints (characteristic lifetime = 2500 +/- 300 cycles; slope = 6.6 +/- 3.4) and top interconnections (characteristic lifetime = 2600 +/- 200 cycles; slope = 8.7 +/- 4.0). 6. The test vehicles having 100% SAC305 bottom solder joints exhibited Weibull parameters that were comparable to those of the mixed solder joints to within experimental error. Characteristic lifetimes were: bottom, 2200 +/- 200 cycles; middle, 2600 +/- 500 cycles; and top, 2400 +/- 200 cycles. 7. The introduction of underfill to the bottom solder joints improved the characteristic lifetimes of all interconnections on the SnPb/SAC305 test vehicle. The failure data exhibited slightly lower slope parameters. 8. The test vehicles having 100% SAC305 bottom solder joints exhibited a degradation to those interconnections with the addition of underfill in the corresponding gap. Moreover, the empirical statistics indicated two failure modes: a "low characteristic lifetime" sub-set that exhibited the typical TMF crack development and a "high characteristic lifetime" sub-set that exhibited both TMF and tensile cracks in the solder joint. 9. The tensile cracks, which improved reliability against TMF, were generated by the thermal expansion of the underfill. This hypothesis was corroborated by the temperature-at-failure data. 10. The computational model, when used to predict static mechanical behavior, showed that PoPoP warpage was not significantly changed by the presence of underfill in the bottom gap. Z-axis thermal expansion displacements were of a comparable magnitude to the warpage magnitudes. A validation of the initial, quantitative predictions of Nf indicated the for further development the PoPoP model to realize the fidelity required for the latter to serve as a design tool of this technology. 10. The computational model, when used to predict static mechanical behavior, showed that PoPoP warpage was not significantly changed by the presence of underfill in the bottom gap. Z-axis thermal expansion displacements were of a comparable magnitude to the warpage magnitudes. A validation of the initial, quantitative predictions of Nf indicated the for further development the PoPoP model to realize the fidelity required for the latter to serve as a design tool of this technology. Initially Published in the SMTA Proceedings |
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