Issue link: https://iconnect007.uberflip.com/i/1305670
66 SMT007 MAGAZINE I NOVEMBER 2020 internal layer connections of a specific group of holes—usually, 200 daisy-chained vias— interconnecting through two adjacent layers, called the power circuit (Figure 6c). Switching the current on for three minutes creates heat to take the connections from room tempera- ture to a designated higher temperature. Stop- ping the current and with forced-air cooling, the connections cool in two to three minutes (Figure 6f). Another group of interconnects involves two independent daisy chains interconnecting 500 vias through any two inner layers at vari- ous levels. The ones under test are the sense circuits (Figure 6b). An isometric view of the two sets of interconnects running parallel to and sequentially overlapping is shown in Fig- ure 6d. The equipment providing the cou- pon fixturing, current, cooling, and resistance measurement is shown in Figure 6e. An accelerated failure will occur because of the differential thermal expansion of the interconnect structure. Failure can occur in a number of locations (Figure 6g)—either a via crack, post separation, connection crack, or material delamination in a specific region within one or multiple areas. Cycling con- tinues until the specific rejection criterion is achieved or the required numbers of cycles are passed. The IST site (pwbcorp.com), or its European licensee Polar Instruments (polarinstruments. com), has extensive application data available, including standard designs of 5- and 6-inch coupons for any number of layers covering these structures: • GB400000A: Any two-layer sequentially laminated blind vias • GB400000B: Any two-layer drilled blind vias • GC400000B: Any layer sequentially laminated buried vias • GM400000B: Any two-layer blind microvia • GP400000A: Any-layer through-hole on a PCB >0.125 inches Figure 5: HATS is accomplished by the unit in (a) generating refrigerated and heated air passing over the test fixture (b) that contains 36 test coupons as shown in (c) the 1.0" x 1.0" or (d) the 2.0" x 1.0" or (e) the 0.5" x 1.0" coupons (hats-tester.com) [4] .