Issue link: https://iconnect007.uberflip.com/i/1509873
46 PCB007 MAGAZINE I OCTOBER 2023 results, a new acceleration model must be developed for this failure mode. e new failure mode may be promoted more or less effectively by the test conditions than the mode originally assumed. 3) ere may be several failure modes. In this case, the two failure distributions should be considered separately, so that life predictions will be meaningful. e dif- ficulty in determining which of the above scenarios holds is that for genuinely new technologies or service environments, the failure mode in service may not be known. In these situations, it is desirable to con- duct a parallel test with less aggressive acceleration for comparison. 7. Determine life distribution from acceler- ated life distribution. e accelerated life dis- tribution should be determined by fitting the data with the appropriate statistical distribu- tion, such as the Weibull or log-normal dis- tribution. e life distribution in service can be determined by transforming the time axis of the life distribution using the acceleration model. is predicted life distribution in ser- vice can then be used to estimate the number of failures in the specified service life. e following discussion of testing for some specific failures will provide examples of this methodology. Printed Circuit Board Reliability Tests ermal: PTH failures are the predominant source of PCB failures in service, and predict- ing them is the primary goal of PCB testing at elevated temperatures. PTH reliability test- ing should simulate the thermal excursions of a PTH throughout its life. Generally, the most severe thermal cycles are experienced during assembly and rework. One of the older accept- able thermal stress test is MIL-P-55110 (also found in IPC-TM-650). Following baking at 120–150°C, the specimens are immersed in an RMA flux and floated in a eutectic (or near- eutectic) Sn-Pb solder bath at 288°C for 10s. For lead-free, the bath temperature is at 260°C or higher. Following the test, the samples are cross-sectioned, and the PTHs are examined for cracks. is is a severe test that ensures that the sample will survive a single wave-soldering or solder pot rework cycle. Most thermal cycling tests for PCBs cycle the PCB repeatedly over a wide temperature range; many are actually thermal shock tests using liquid-liquid cycling. e results of five accelerated tests with different temperature extremes, ramp rates, and dwell times have been compared by the IPC, which also pro- vides a simplified analytical model to estimate PTH life. 11 e results of all tests suggest the same approaches for maximizing PTH reliabil- ity, but they do not all correlate well quantita- tively. Figure 1 shows a number of suitable test cou- pons for PCB PTH/microvia evaluation. ese include: 1) A test coupon that contains 3,000 PTHs and varying annular ring sizes 13 2) An IST (interconnect stress testing) coupon with hundreds of PTH and microvias with direct current induced thermal cycling 16 3) A HATS (highly accelerated thermal shock) coupon with four daisy-chain nets and up to 36 coupons per chamber and air-to-air cycling 17 4) PCQR 2 (process capability, quality, and relative reliability) test panel for standard comparison of printed circuit board manufacturing processes 18 Mechanical: PCBs are rarely subjected to mechanical tests that could cause electrical failures; however, adhesion of both Cu and sol- der mask to the laminate is critical and is oen tested. Loss of solder mask adhesion can pro- vide a place for corrodents and moisture to accumulate, which can be the cause of electri- cal failures when the board is exposed to tem- perature and humidity.