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Thermal Mechanical Fatigue of a 56 I/O Quad-Flat No Lead Package
Thermal Mechanical Fatigue of a 56 I/O Quad-Flat No Lead Package
A fatigue computational model was used to predict the effects of conformal coating and underfill on solder joints between a 56 I/O plastic quad-flat and a polyimide PCB.
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Authored By:
Paul Vianco, Ph.D., and Michael K Neilsen, Ph.D.
Sandia National Laboratories
Albuquerque, NM, USA

Summary
High reliability systems are using the commercial electronics supply chain for providing advanced packages that meet reduced size, weight, and power (SWaP) goals as well as provide adequate, second-level interconnection reliability. The Sandia solder fatigue computational model was used to predict the effects of conformal coating and underfill on the thermal mechanical fatigue (TMF) reliability of 63Sn-37Pb (wt.%) solder joints formed between a 56 I/O plastic quad-flat (package), no-lead (PQFN) component and polyimide glass printed circuit board (PCB). The analysis included a 24 I/O leadless ceramic chip carrier (LCCC) package of similar footprint size. An accelerated aging thermal cycle was used having temperature limits of -55 degrees C and 85 degrees C.

Two failure criteria were employed: (a) cycles to crack initiation and (b) cycles to cause 100% cracking (electrical open) of a solder joint. As expected, the LCCC experienced a considerably shorter baseline TMF lifetime than did the PQFN. Placing a conformal coating over the PWA reduced the TMF lifetime of both components in similar proportions to their baseline values. Having the conformal coating flow into the package/PCB gap caused a precipitous drop of TMF lifetime for both package types. The lower lifetimes reflected the significant conformal coat pressure pushing up on the packages. The introduction of the underfill material not only prevented the incursion of conformal coating under the packages, but also enhanced their TMF lifetimes above their respective baseline values. The presence of 30% voids in the paddle joint of the PQFN had negligible effect on the TMF lifetimes of it and the peripheral solder interconnections.

Conclusions
The numerical predictions have been summarized in Table 4 to assist the reader with understanding the important findings described below:
  1. High reliability systems are looking to the commercial electronics supply chain to provide advanced packages that meet the goals of reducing size, weight, and power (SWaP) requirements.
  2. It is necessary to assure that these new package concepts also provide for second-level interconnections that meet stringent reliability specifications.
  3. In light of program constraints on budget and schedule, design engineers are relying heavily on computational modeling to predict the reliability of solder joints as an alternative to expensive, time-consuming experimental programs.
  4. The Sandia solder fatigue computational model was used to predict the effects of conformal coating and underfill on the thermal mechanical fatigue (TMF) reliability of 63Sn-37Pb (wt.%) solder joints formed between a 56 I/O plastic quad-flat, no-lead (PQFN) package and a 2.36 mm thick, polyimide glass printed circuit board (PCB). The analysis included the traditional 24 I/O leadless ceramic chip carrier (LCCC) package of similar footprint size.
  5. Model predictions were made for an accelerated aging thermal cycle of -55 degrees C/85 degrees C. Hold times were 90 min at the temperature extremes. Two failure criteria were used: (a) cycles to crack initiation and (b) cycles to cause 100% cracking (electrical open) of a solder joint.
  6. As expected, the LCCC experienced a considerably shorter baseline, TMF lifetime than did the PQFN.
  7. Placing a conformal coating over the PWA reduced the TMF lifetime of both components in similar proportions to their baseline values because the coating caused a greater, effective CTE to the PCB that enhanced the shear loads introduced to the solder joints.
  8. Having the conformal coating flow into the package/PCB gap caused a significant drop of TMF lifetime (crack initiation or 100% crack) for the LCCC (12 cycles, 270 cycles) and PQFN (11 cycles, 160 cycles). The lower lifetimes reflected the similar magnitude of pressure applied to package bottom by the conformal coating.
  9. The introduction of the underfill material not only prevented the deleterious effects on TMF caused by incursion of conformal coating under the package, but also enhanced the TMF lifetimes of both packages above their baseline values.
  10. The presence of 30% voids in the paddle joint of the PQFN had negligible effect on the TMF lifetimes of the peripheral solder interconnections. However, there was a slight enhancement of crack propagation in the paddle solder joint.


Initially Published in the SMTA Proceedings

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