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78 PCB007 MAGAZINE I NOVEMBER 2020 Vias are an integral part of PWB design and manufacturing. They are the means by which different layers of a board are connected. There are three main types of vias: through-hole vias, buried vias, and blind vias. Through-hole vias are drilled from the top layer through the bottom layer. In a double- sided PWB, the through-hole via connects the top and bottom layers. In multilayer boards (MLBs), the through-hole via is the means of connecting all or any of the layers to meet the design requirements. The desired connectivity occurs when the via is electroplated. Before electroplating, drilled vias are desmeared and rendered conductive. Desmear– using chemical means or plasma–removes all traces of dielectric residues from the intercon- nect surfaces to ensure intimate connectivity. Electrical continuity is achieved by rendering the dielectric conductive using metallization like electroless copper or other means like car- bon or a conductive polymer. The electroplat- ed copper is the conduit that all signals travel through. This column will address the electro- plating of vias (Figure 1). Through-Hole Vias Through-hole plating remains the backbone of PWB connectivity. The objective is to plate a uniform copper layer that connects the dif- ferent layers. Connectivity is the key attribute here, whether it is only connecting two layers (top and bottom) or connecting 40 layers in a multilayer board. The plated copper should be able to with- stand 6X thermal shock simulating the stress- es that the board may encounter during as- sembly and through its normal useful life. It should also be able to withstand IST thermal cycling test where stress is induced by the ex- pansion and contraction of the dielectric along its Z-axis. Thickness uniformity of the plated copper must be controlled to ensure the current-carry- ing capacity of the conduit and to meet its impedance requirements. For electroplated copper to meet these requirements, the plating process must be optimized. Areas for optimi- zation include: 1. Pretreatment 2. Pattern vs. panel plate 3. Plating chemistry 4. Plating cell setup 5. Agitation 6. Rectification Via Plating for PWBs The Plating Forum Feature Column by George Milad, UYEMURA Figure 1: Via types used in PWBs.

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