Investigating the peel-strength of printed wiring boards copper layer

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Dr. Krammer Olivér
Department of Electronics Technology

The dissertation deals with the investigation of the peel strength of printed cicuit boards copper foil. The peel strength value of the copper is a relevant indicator of the reliability of the PCBs. To get a more accurate impression about the robustness of PCBs, before the copper peel strength investigation, the PCBs can be exposed to some impacts, which the PCB can meet during its life cycle (for example reflow soldering with convection or vapour phase technology and HAST). The vapour phase soldering nowadays became a more and more common technology. But the mechanical effects of it are less known, than by the convection reflow soldering.

Based on the literature i reviewed the types, the different manufacturing technologies, base materials and material- and mechanical properties of the printed circuit boards and the relevant international standards. Based on the standards of the peel strength measurement i designed a test PCB, and a specimen holder, what can use with a standard tensile strength tester. Than i compared the peel strength values of test PCBs, which were exposed to convection reflow soldering, vapour phase soldering and HAST.

Based on the tests results after one cycle reflow soldering, the VPS test PCBs show bigger addhesion between the copper foil and FR4 (7,59 N/mm, in case of convection reflow soldering: 6,94 N/m). After one more soldering cycle the PCBs with convection reflow soldering have higher average peel strength value (7,7 N/mm instead of 7,0 N/m). Compared with the one cycle soldering boards the results are almost the same. But after the highly accelerated stress test (HAST) the peel strength values decreased extremely (convection reflow soldering comibed with HAST: 3,46 N/mm, and vapour phase soldering combined with HAST: 3,5 N/mm). Furthermore i changed the test speed of the peel strength measurement, and based on the results with increasing the speed the peel strength values are increasing too (5mm/min: 7,04 N/mm, 500 mm/min: 8,16 N).

In conclusion the effect of the soldering processes on the peel strength values are small, the changing of the addhesion between the copper and FR4 is not significant (-3,4 % and 5,54%). But soldering combined with HAST caused a remarkable decreasing in the peel strength values (convection reflow soldering combined with HAST: 3,46 N/mm and 3,58 N/mm, vapour phase soldering combined with HAST: 3,5 n/mm and 3,2 N/mm)


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