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82 The PCB Magazine • July 2016 ditions. According to Bill Birch of PWB Inter- connect Solutions, "The failure hierarchy of the interconnect plays an important role in estab- lishing whether the foil (or post interconnect) becomes a dominant or latent failure mecha- nism. The basis for the hierarchy is determined by the reliability of the PTH barrel." [1] Thus the stress was shifted through the bar- rel of the PTH, increasing the opportunity for failure. Turning the tables a bit, Figure 3 shows a second failure mode, namely ICD. In this case the adhesion of the plated copper to the inter- connect is less than robust. Here the weakest link appears to be the copper-to-copper bond. There are several possible reasons for this in- cluding: • Excessive electroless copper thickness on the post interconnect • Drill debris at post interconnect surface • Excessive dwell time in the palladium catalyst • Poor grain structure of the copper deposit leading to stress • Poor overall plating practice • Electroless copper solution out of specification Stress and Strain in the PTH Most organic resin matrix substrate materi- als are highly anisotropic, with a much higher CTE above the glass transition temperature Tg in the through-thickness (z) direction than in the plane of the woven matrix cloth (the x-y plane of the board). Since above Tg the CTE climbs sharply, aggressive thermal cycles can result in large strains in the z direction and, consequent- ly, on the PTHs. The PTH acts like a rivet, which resists this expansion, but the copper barrel is stressed and may crack, causing electrical fail- ure. There is increasing strain on the barrel as- sociated with a high temperature excursion. Failure may occur in a single cycle or may take place by initiation and growth of a fatigue crack over the course of a number of cycles. For high- aspect-ratio through-holes subject to repeated thermal shocks from room temperature to sol- der reflow temperatures (220–250°C) during board fabrication (e.g., hot-air solder leveling) and assembly (reflow, wave soldering, rework), it is not unexpected to experience failures after 10 or fewer of these thermal cycles. This raises even more concerns with respect to lead-free assembly. Multiple lead-free assem- bly cycles with higher peak reflow temperatures (245–260°C) places even greater thermal strain on the PTH when compared to conventional tin-lead assembly. In addition, the laminate res- in material chosen for the fabrication may con- tribute to PTH failure. Why? Consider the prop- erty of Td (temperature of decomposition). Td is not the same as Tg. Temperature of decomposi- BUILDING RELIABILITY INTO THE PCB, PART 1 Figure 2: Barrel cracks in PTH subjected to thermal cycling. Note that the copper-to-inner- layer connections remain intact. Figure 3: Interconnect defect (Source: IPC 9121).