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June 2015 • The PCB Magazine 15 A HIGH-RELIABILITY, STRESS-FREE COPPER DEPOSIT FOR FPC, POLYIMIDE AND RIGID-FLEx continues FeAtuRe these properties can affect the reliability of the copper deposit. ICD solder shock test coupons were pro- cessed through these solutions at various spe- cific gravity levels and evaluated accordingly. In all situations there were no ICDs encountered. Each data point in Figure 5 represents 12 inter- connects evaluated in seven PTHs for a total of 84 opportunities per data point. In total, ap- proximately 4500 interconnects were evaluated. IST coupons were also evaluated after pro- cessing through the electroless copper solutions with stress reducing additives. Table 1 shows the results of each test according to specific gravity and deposit thickness. Test coupon outcomes are reported as cycles to failure and results listed as power, sense or accept. Coupons that reached 1000 cycles received an accept result. Failure be- fore 1000 cycles was initially determined by the IST system to be on the power or sense circuit. In general, a failure in the power circuit can in- dicate a failure at the copper barrel/interlayer interface which is of primary interest when evaluating an electroless copper deposit inter- connect reliability. A failure in the sense circuit is typically attributed to a failure in the PTH copper plating (i.e., a barrel crack). However, in both cases cross section evaluation is necessary to confirm failure mode. As mentioned previously, all IST coupons that failed to reach 1000 cycles were evaluated by cross section to determine the root cause of failure. In all instances, the failure was attrib- uted to cracking of the copper plating, or bar- rel cracks, as shown in Figure 6. There were no failures attributable to the electroless copper de- posit or ICDs. Next, life data regression analysis was per- formed with the two predictors, specific gravity and deposit thickness, to determine their effect on IST cycles to failure. The data was fit using a Weibull distribution. In both cases, the two factors were not statistically significant at α= 0.05. Since the two factors were not significant, the results were pooled together and fitted to a Weibull distribution shown in Figure 7. The results were then compared to historical data of electroless copper without stress reducing ad- Figure 5: 10x solder shock test from a bath specific gravity of 1.02–1.10.