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30 The PCB Magazine • May 2015 Impact of Substrate Condition and Pretreatment The thickness, coverage and sur- face condition of the electroless cop- per metallization layer applied to the panel prior to the through-hole fill- ing process has a profound impact on through-hole filling and plating capa- bility. Discontinuous or overly thin electroless copper deposits will not electroplate as reliably as thicker, more uniform deposits. Oxidation of elec- troless copper can also adversely affect filling performance. Internal data has shown that, in particular, aged electro- less copper has an unfavorable impact on process consistency. Proper control of pre- treatment processes also plays an important role in achieving good hole filling. A typical process sequence uses acid cleaner and acid dip steps to ensure that copper surfaces are completely wetted and free of contamination prior to the subsequent copper plating step. Impact of Process Agitation and Current Density on Hole Filling Fluid delivery and flow agitation have a sig- nificant impact on hole filling performance, particularly in the presence of a leveler compo- nents. To understand the effect of agitation on hole filling, solution flow rate was varied in a ELECTrOPLATING THrOUGH-HOLES wITH DIFFErENT GEOMETry continues FEaturE Figure 9: Dimple and void measurements. Figure 10: Plating performance as a function of electroless type and quality.