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76 SMT007 MAGAZINE I DECEMBER 2020 verification, impedance test, cleanliness, and electrical testing. Peel testing is performed in Group 2 (miscellaneous testing), and Group 3 (thermal stress) covers microsectioning before and after solder bath float and rework simu- lation, and IST. Group 4 (assembly and life test) combines reflow simulation, rework sim- ulation, and thermal cycling. ECM testing is part of Group 5. A selection of test methods is described in more detail later. Within the ECSS-Q-ST-70-60C standard, IST is an important test method to evaluate the quality of a via (PTH, blind, buried, and microvia). The test method is applied for pro- cess monitoring, procurement, and qualifica- tion. IST is a form of current-induced thermal cycling and is described in IPC-TM-650-2.6.26 (Method A). The coupon is heated using a power circuit, and the resistance of the struc- tures under test is monitored continuously via the sense circuit(s) of the coupon. The coupon design should represent the PCB technology of the highest complexity and cover all aspects of a given design or technology that are expected to affect thermal endurance. Before cycling, the coupons are subjected to six times preconditioning to 230°C using a "superheat" circuit. This meandering circuit on the outer layers of the coupon is used to mimic the assembly process. The temperature of 230°C is chosen to represent SnPb assem- bly, and the six preconditioning cycles should cover the worst-case number of assembly, repair, and rework operations. Blind, buried, and through-vias are cycled from room tem- perature to 150°C (epoxy-based materials) or 170°C (polyimide) in a three-minute heating and two-minute cooling cycle. The testing is stopped as soon as one of the sensing circuits reaches a 5% increase in resistance at a high temperature compared to the first cycle. Based on an acceleration study, it was determined that an IST endurance of ≥400 cycles should be reached for blind, buried, and through-vias. Microvia testing using IST follows a differ- ent approach. As microvias experience lower stress levels during cycling, the test tempera- ture needs to be increased to apply sufficient stress to the microvia. For epoxy-based materi- als, a test temperature of 190°C is used, while for polyimide testing, it is performed to 210°C. This stress level is no longer related to the mis- sion profile but applied to determine if the manufacturing quality of the microvia is ade- quate. Until recently, it was believed that the microvia would fail either early (less than 100 cycles) or not at all. An IST endurance of ≥100 cycles was specified with a maximum increase in resistance of 4%. After encountering fail- ures on microvias that successfully passed the 100 cycles and observing wear-out type failure mechanisms in microvias after a few hundred cycles, it was decided to increase the threshold to 400 cycles, in line with the standard vias. This is further motivated by the microvia reli- ability concerns described in IPC-WP-023 [10] . An additional caveat with IST of microvias is that the standard power circuit cannot be used to heat the coupon. The (buried) vias within the power circuit would obviously fail before the microvias. For microvia testing, the coupon can be heated using the superheat cir- cuit, or the power can be connected directly to a sense circuit containing the microvias. The drawback of the latter is that only one circuit can be tested at a time, but the temperature of the microvia can be better controlled. ECSS- Q-ST-70-60C specifies power-on-sense as the preferred method, based on the recommen- dation from PWB Interconnect Solutions Inc. (Ottawa, Canada). The reference test method for evaluating thermal reliability remains thermal cycling. This is performed in a single chamber system at ambient pressure. The test samples are sub- jected to reflow simulation (two times vapor phase reflow at 215°C) and rework simulation (ten times manual soldering) after baking for eight hours at 120°C. Following the reflow and rework simulation, the coupons are submitted to 500 cycles from -55°C to +100°C (10°/min and 15 min dwell time). As this is commonly applied for project qualification, a shorter test consisting of 200 cycles from -60°C to +140°C is also applied. Electrical monitoring of daisy chains can be performed during testing in addition to the required evaluation method of microsectioning afterward.