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28 The PCB Magazine • May 2015 outer parts of the through-hole walls are mass transfer dependent. Furthermore, both electro- chemical and plating studies indicate that the in- hibiting strength of levelers are also dependant on the leveler concentrations, as shown in Table 1. After extensive research and development efforts, a well balanced additive package was developed to achieve the desired through-hole filling performance. This process was developed to work with solution jet impingement and insoluble an- odes in both vertical batch mode and vertical in-line and horizontal conveyorized plating equipment. A wide variety of equipment de- sign features that further enhance though-hole fill plating performance may be incorporated. These include the use of engineered fluid deliv- ery devices such as eductors or nozzles designed to create optimized flow impingement on panel surfaces. Through-Hole Fill Performance Matrix Through-hole fill plating performance, like blind microvia filling, may be character- ized by the calculation of the dimple depth or bump height. Dimple depth is the most commonly used metric to quantify both through-hole and via plating performance. Depending on customer and application, void area (Figure 9) may also need to be deter- mined. Customer specifications for void area are typically expressed as % void, that is, void area/hole area. ELECTrOPLATING THrOUGH-HOLES wITH DIFFErENT GEOMETry continues FEaturE Table 1: Filling performance as a function of additive concentration. Figure 7: optimized THF copper chemistry. Figure 8: side view of jet impingement plating cell.