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38 The PCB Magazine • March 2015 The goal of this paper is not to determine if a PWB design and material selection is ap- propriate for end use conditions as there are adequate models for this [18, 21] , coupled with em- pirical data. The goal is to equate accelerated testing conditions to field use conditions. Look- ing at the Freda and Baker data [7] , the worst case estimate of the T1% (1% cumulative failure) with 18 cycles at 245°C is 3,500 cycles to failure (CTF). Using the Coffin-Manson model with18 cycles at 245°C, and m ≈ 4.6 matches the cycles. Caution is needed, as the authors point out, be- cause a difference of 1 or 2 cycles to failure is equivalent to 170 to 3,000 at thermal cycle tem- perature [7] . Based on this work, coupled with the m constant data compiled by Blish [19] , m = 5 is a reasonable starting point for plated through hole modeling with the Coffin-Manson model. Testing to failure was done at several differ- ent temperatures above laminate T g and the data were used, along with research data, to generate an algorithm for estimating the m constant re- quired for the Coffin-Manson model. Caution must be exercised when thermal testing exceeds the laminates T g as failures modes can shift be- tween interconnect defects (ICDs), electroplat- ed barrel cracks, and laminate decomposition (carbonization), which can significantly un- derestimate the m constant. These defects are shown in Figure 6. It's then relatively easy to input all param- eters of the Coffin-Manson equation into a spreadsheet to simplify calculations (Figure 7). Let's look at an example for a military avion- ics printed circuit board that has the following assembly, and anticipated harsh use field condi- tions [5] : Assembly: 4x @ 245°C Life expectancy: 10 years Ambient temp: 20°C Peak temp: 100°C Power cycles: 1 per day Total cycles: 4,467 Thermal cycling options: 1. 6x preconditions @ 260°C, 250 cycles at 150°C: Simulated cycles: 4,510 2. 6x preconditions @ 245°C, 300 cycles at 150°C: Simulated cycles: 4,571 RELIABILITy TESTING AND STATISTICS continues figure 6: Testing to failure at 260°C. note laminate T g is 200°C. a: ICD failure 7. b: Electroplated copper crack at the knee. c: Laminate thermal decomposition (carbonization). Feature