Evaluation of smear and its effect on the mechanical integrity of plated through hole-inner plane interface in thick printed wiring boards

In multi-layer printed wiring boards (PWBs), electrical connections between different layers are accomplished with plated through holes (PTHs). The re liability of the PTH barrel and the PTH-inner plane (IP) connection depends not only on the design but also on the conditions of manufacturing and ass embly processes of the board. The concerns associated with manufacturing ar ise from drilling which heats lip and smears the surrounding epoxy onto the copper inner plane surfaces and also from subsequent chemical hole-clean o perations which desmear the drilled holes. Good adhesion of the PTH copper to the desmeared PTH wall and to the copper inner planes is important for t he reliability of the PTHs during assembly and field service. PWB coupons c onsisting Of resin-glass bundle areas as well as resin filled clearance are as surrounding the PTH have been considered for a series of experiments and tests. On these coupons, accelerated stress tests and failure analysis of the PTNs at the end of these tests have been conducted: An elasto-plastic f inite element analysis of the PTH strains for a temperature excursion of 10 2 degreesC has been carried out for signal PTHs without and with IP connect ions. A peel test, using a micro-mechanical tester, has also been carried o ut to assess the adhesion of PTH copper to different regions along the dril led hole. All of the testing techniques have been supplemented by suitable analytical techniques for studying the distribution of smear on the copper inner planes. A birefringence technique to estimate the temperature of the hole wall during drilling has been described. All of the fails observed dur ing stress testing are in the form of cracks in the PTH barrel, the plane o f the crack being perpendicular to the barrel axis. The cracks are localize d near the glass bundle-resin rich interface. It has also been observed in the failure analysis that the PTH/IP connections are not susceptible to fai lure during the accelerated stress testing conditions considered. These obs ervations are also supported by the results of stress analysis. The results of stress analysis show that the interior clearance holes show higher stra ins than those closer to board surfaces, suggesting that the PTH barrel fai lure is likely to occur at the clearance hole and that the; likelihood of f ailure at the interior clearance holes is higher thar? those closer to the PWB surfaces. The results of the peel test reveal that in the glass bundle- resin regions, the adhesion along the PTH wall is determined by the mechani cal interlocking between the plated copper and the glass bundles and is rel atively insensitive to desmear operations. However, adhesion in the resin-r ich areas is a strong function of the desmear operation, which enhances the adhesion of the PTH to the resin. Finally it is shown that the birefringen ce technique may be used effectively to estimate the drilling temperature a nd, hence, the degree of smearing and PTH quality. It is concluded that whi le smear may play an important role in the failure of PTHs in PWBs, for thi ck packaging boards with high aspect ratio PTHs, the predominant failure me chanism is less likely to be smear related. Such a failure mechanism would be preceded by failure irt the PTH barrels as a result of the large strain concentrations imposed at specific locations within the cross-section.