Issue link: https://iconnect007.uberflip.com/i/1313710
78 SMT007 MAGAZINE I DECEMBER 2020 ECSS-Q-ST-70-60C specifies the following protocol for CAF testing: • Ambient phase using the parameters: 24h, 25°C, 50% RH, 0V • Preconditioning phase using the parameters: 96h, 85°C, 75% RH, 0V • CAF phase using the parameters: 500h, 85°C, 75% RH, 50V • Ambient phase using the parameters: 24h, 25°C, 50% RH, 0V The overall approach is in line with IPC- TM-650 2.6.25B, with two noticeable excep- tions. The test voltage of 50 V is chosen to fol- low the guideline of two times the maximum voltage for HDI technology (in fact, the maxi- mum voltage is 30 V, but 50 V is deemed a more standardized test voltage). The test applies a relative humidity of 75% RH, which is less than the 85% RH specified in 2.6.25B. But this still provides some margin and acceleration of test compared to the maximum relative humid- ity of 65% in cleanrooms for ground testing and assembly. Sample preparation before test- ing consists of electrical registration measure- ment and baking, followed by six times vapor phase reflow at 215°C, ultrasonic cleaning, and again baking for eight hours at 120°C. Test Results Three panels were selected from the QTV manufacturing batch and subjected to the tests described previously. Each panel contains three IST coupons (one TVX and two SLX cou- pons), two BGA coupons, and two times the dedicated test structures for HDI qualification test flow (coupons A/B, Bn, E, H, and P). For CAF testing, 10 samples of the HDI CAF test vehicle were manufactured. All manufacturing was performed by ACB (Dendermonde, Bel- gium). Nine coupons from three different panels are subjected to IST. Plated through-holes and through-vias are evaluated on coupons TVX3A, 6A, and 9A. Buried vias are tested on coupons SLX3A, 6A, and 9A, and coupons SLX3B, 6B, and 9B are used for microvia evaluation. Prior to testing the microvias, the buried vias on coupons SLX3B, 6B, and 9B are subjected to 500 cycles to 170°C (with preconditioning) to reflect the test conditions for procurement. The IST results are shown in Tables 2 through 5. The through-vias on the TVX coupon reach between 596 and 741 cycles. This is well above the requirement of 400 cycles, although a 500 µm via in a 2.8-mm polyimide board could be expected to sustain longer testing. The pitch ("grid") of the vias in the test coupon was 1.27 mm to represent the high-density connector layout, which could explain the lower number of cycles that is reached compared to the pre- vious testing at a larger grid. The SLX coupons contain three sense cir- cuits. Sense 1 (buried vias only) and Sense 2 (buried vias and microvias) were monitored during testing of the buried vias. Testing is stopped when both sense circuits have reached the acceptance criteria of a 5% resistance increase. Sense 3 (microvias only) is used for Table 2: Results for through-vias tested at 170°C after six times preconditioning to 230°C. Table 3: Results for buried vias tested at 170°C after six times preconditioning to 230°C. Table 5: Results for microvias tested at 210°C after six times preconditioning to 230°C and 500 cycles at 170°C. Table 4: Results for pre-cycling of buried vias after six times preconditioning to 230°C.