PCB007 Magazine

PCB007-Nov2018

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80 PCB007 MAGAZINE I NOVEMBER 2018 ty interconnect (HDI). Maximizing via filling, while minimizing the surface copper thickness and deposit thickness variation, presents diffi- culties for the manufacturers, especially when the PCBs contain both THs and blind micro vias (BMVs). It is most desirable to obtain a good throwing power in electrodeposition pro- cesses. Particularly in the TH plating of PCBs, a uniform distribution of deposited copper is demanded including inside the holes on hole walls. In general, copper plating processes that provide good via fill and better leveling of the deposit across the substrate surface tend to worsen the throwing power of the electroplat- ing bath. In the fabrication of reliable PCBs, via filling, and plated through-holes (PTH) with various aspect ratios (ARs) at the same time, in the same electrolyte is highly challeng- ing for the manufacturers. The purpose of this work was to optimize a recently developed innovative copper process for simultaneously filling vias and PTHs with a minimal surface thickness. The process was evaluated in a wide variety of conditions to col- lect information on its capabilities. The effect of inorganic components and organic additives concentrations and the influence of the plat- ing parameters on the plating process perfor- mance in terms of throwing power and via fill- ing were determined. A series of copper elec- troplating solutions were evaluated. THs with various diameters/ARs in boards up to 1.6 mm were measured. Both insoluble and soluble anode applica- tions were considered. Filling of through-via- holes in core layers of HDI and integrated-cir- cuit (IC) substrates in a one-step DC process was also studied. Panel plating as well as pat- tern plating for vertical plating applications, including vertical continuous plating (VCP) equipment, were included in the experiments. The mechanical properties of plated copper de- posits, tensile stress, and elongation were mea- sured, and the thermal characteristics were evaluated. Acid Copper Plating Process A typical copper plating solution contains copper sulfate, sulfuric acid, chloride ions, and organic additives that control the deposi- tion process and the quality of the plated coat- ings [3–8] . Solutions that provide good via filling and leveling of the copper deposits usually are characterized by low polarization and presence of leveling agents. High-copper, low-acid base electrolytes in a virgin makeup solution (VMS) are used. The brighteners are adsorbed at low- current-density areas of the cathode surface, accelerating the process, while the levelers are adsorbed at the most negatively charged areas, thus slowing the deposition rate there. Mean- while, high throwing power (TP) is achieved in low-copper, high-acid baths. The throwing power of an electroplating bath depends on so- lution conductivity, electrodeposition kinetics (the slope of the polarization curve, the high- er and better the TP min), cell geometry, and temperature. The innovative plating process is an ad- vanced, direct current acid copper system of- fering simultaneously via filling and TH plat- ing [9] . The plating is uniform over the cathode surface, and no TH thin knee or slope is ob- served around the THs. Key features include filling BMV up to 5 x 4 mils with a low dim- ple size, and any layer build-up applications (Figure 1). Figure 2 shows a typical plating in the bath at 20 amperes per square foot (ASF). Surface copper thickness is 18–20 microns and the surface appearance is bright. The process is compatible with direct metallization or elec- troless copper. The low total organic carbon (TOC) system has an extremely long life, and it is easily maintained; all organic additives can be analyzed by cyclic voltammetric stripping (CVS). Process Optimization The capabilities of the innovative plating copper process for via filling (VF) and PTHs were studied as a wide range of plating con- ditions were tested. Factors included organic additive concentrations, inorganic components concentrations, and plating parameters. Large ranges of the additive concentrations were test- ed (i.e., wetter: 5–15 ml/L, brightener 0.5–4.5 ml/L, and leveler 5-25 ml/L). Current densities applied were 10, 20, and 30 ASF.

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