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70 PCB007 MAGAZINE I AUGUST 2018 qualifies as the most severe of environments. Reliability testing should simulate the thermal excursions of a PTH throughout its life. This would include thermal cycling at temperature extremes of -40°C to +125°C. In addition, the most severe thermal cycles are experienced during assembly and rework. Thus, simulat- ing assembly reflow and rework cycles will provide a more in-depth understanding of the board's long-term reliability. More to come on this topic in a future column. As previously mentioned, the concern is that the protruding zinc tail, if not removed prior to metalization, will lead to stress points within the electrodeposited copper at a minimum. If one clearly studies the failure hierarchy within the PTH, one surmises that the plated-through via expands quite rapidly in the Z-axis at a temperature above the Tg of the resin mate- rial. During these temperature excursions, the PTH acts as a rivet working to resist expan- sion. However, when there are various anom- alies in the PTH, including protruding glass fi- ber bundles and zinc tails, stress points are in- troduced. Remember, the weakest link in the In Figure 1, the protruding foil treatment (tail) extends from the foil into the electrolyt- ic copper. Several fabricators have documented this condition and the subsequent degradation of PTH reliability after thermal excursions. It is easy to see why the zinc tail left unremoved would act as a fulcrum causing weakening in the plated through-hole (PTH). Another issue is that the tail may be misinterpreted as poor adhesion of the electroless copper deposit. Figure 2 shows the results of the SEM/EDAX of the protruding tail. One can easily see the high level of zinc present. Figure 3 shows another example of the zinc tail as it protrudes into the via. Subsequent metalization is forced to plate around the tail. Zinc Tail as a Possible Cause of PTH Failures Thermal cycling leads to the majority of fail- ures in the plated through-hole. It is also crit- ical to put in perspective the environment in which the circuit board will be used. A harsh- use environment (HUE), such as automotive applications under the hood or on an engine, ELEMENT NORMAL WEIGHT % C O Cu Zn As Total 2.01 0.62 80.77 15.45 1.15 100.00 Figure 2: SEM/EDAX results of protruding area and foil treatment shown in Figure 1.