MIL-HDBK-217F, Notice 2 provides two models for a PCB failure rate – a Plated Through Hole (PTH) model and a Surface Mount Technology (SMT) model. In order to estimate the failure rate using the PTH model, the number of plated through holes, number of circuit planes and the board technology is required. In order to use the SMT model, the average number of thermal cycles to failure is calculated for each component using the component topology, solder joint height, circuit board substrate thermal coefficient of expansion (TCE), temperature range, package material TCE, and the lead configuration. The SMT failure rate is then estimated from the average number of cycles to failures for the worst case component and the cycling rate. The average failure rate is then calculated from the number of cycles to failure and the design life.
Another alternative is to use the models defined in 217Plus. In 217Plus, the solder joint failure rate is included at the component level. Specifically, the failure rate model at the component level contains a solder joint term consisting of a base failure rate and an acceleration factor based upon the temperature excursion between the non-operating and operating states. This is rolled up along with the operational, non-operational (environmental), temperature cycling and electrical overstress failure rates to yield the overall failure rate for a component. Therefore, it is not necessary to include a separate PCB failure rate in the manner of MIL-HDBK-217, unless one wants to include the raw (unpopulated) board. In this case data from NPRD/EPRD publications should be used. Note, the solder joint failures described here are those related to “random” causes, that is, due to the occasional defect that might be expected to be present in a population. This term does not apply to solder joints which have failed due to wear-out. The model is only valid when the equipment is operating within its solder joint life.
Finally, if you have test or analysis data pertaining to the solder joint life that specific to your application, that information can be used in lieu of the models described above.
Richard Wisniewski,
Senior Reliability Engineer
Defense Systems Information Analysis Center (DSIAC)
Please note that the RIAC has been transitioned to the newly-formed Defense Systems Information Analysis Center (DSIAC). Inquiries related to the technical areas of reliability, maintainability and quality (RMQ) can continue to be made to the toll-free number at (877) 363-7422.
