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44 The PCB Magazine • October 2014 level was carried out with a designated daisy chain coupon for all three test vehicles: TV1, TV2 and TV3. Resistivity of the daisy chain structure at each sample was checked at defined checkpoints during the test duration. For the bareboard configuration in total eight coupons of each test vehicle type have been tested. The test is considered as passed if after 1000 temper- ature cycles the change of resistivity of the daisy chain structure is within the limits of 10%. Additionally, four populated samples each were tested for TV2 and TV3. The populated sample was assembled with 12 CTBGA288 dum- my components (Figure 6). Each of the compo- nents during the test was monitored online by an event detector device. Test is considered as passed if no failure at a monitored structure is identified. Highly Accelerated Stress Test (HAST) HAST testing was done in accordance with JEDEC JESD 22-A110C [7] . The HAST test provides four process parameters: temperature, humidity level, applied bias and duration. For the current test the following parameter set was applied. For pre-treatment, the samples have been thermo cycled five times between -40°C to +125°C, then stored for 48 hours at 60ºC and 60% humidity level. Last step of preconditioning for the HAST test have been five lead-free reflow cycles with +260°C peak temperature. For the actual HAST test then the parameter set given in Table 5 has been applied. The used test coupons provide structures for via-to-via testing (one structure per coupon), via- to-plane testing (one structure per coupon) and comb structures (one comb structure per layer) as shown in Figure 7 and Figure 8, respectively. In total, eight coupons per part number were tested. All test structures have been monitored continuously during the test period, and the re- sults have been recorded. As failure criterion at each of the tested structures a drop of resistivity below a value of 106 ohms was considered. Results Test vehicles and board thickness After pressing, the test vehicles were singular- ized from the panel and the final thickness was evaluated. Table 6 shows the measured thickness THIN PCBS FOR SMARTPHONES continues table 4: temperature cycling test parameter setting. figure 5: bareboard daisy chain coupon for ther- mo cycling. figure 6: assembled coupons (tV1) for thermo cycle testing.