Thermal Mechanical Fatigue of a 56 I/O Plastic Quad-Flat Nolead (PQFN) Package
Authors: Paul Vianco, Ph.D., and Michael K Neilsen, Ph.D. Company: Sandia National Laboratories Date Published: 9/27/2015
Abstract: 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°C and 85°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.