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90 PCB007 MAGAZINE I JANUARY 2019 cannot overemphasize the importance of having a clean capture pad devoid of residue. With the situation as depicted in Figure 3, the plated copper will at best form a very weak bond with the capture pad. Some fabricators institute a plasma cleaning cycle to ensure a clean capture pad. At the very least, alkaline permanganate should be utilized to not only remove smear but also to microroughen the resin material. In Figure 4, you can see that the resin after laser ablation is quite smooth and not conducive to plating adhesion. Depending on the resin system and its Tg, the engineer may need to investigate different desmear parameters. This should include different solvent swell chemistries as well as variations in alkaline permanganate concentrations and dwell times. A less than optimum topography on any resin system as shown in Figure 4 presents problems with respect to the subsequent metalization process. Smooth topography has less surface area to attract both the catalyst from the electroless copper process or the conductive coatings used in the direct metalization processes. High-Tg resins are more chemically resistant to certain desmear systems. The solvent conditioner must have the ability to penetrate the polymer resin matrix and weaken those polymer-polymer bonds. This interaction is sometimes referred to as a swelling action. Once weakened or swelled, the bonds are more easily attacked (oxidized) by the alkaline permanganate solution. Certain resin systems will respond more profoundly to pure solvent systems, while others will swell more effectively with butyl-caustic combinations (Tables 1 and 2). In Table 1, note that the weight loss with a butyl/caustic system on high-Tg material is less than optimum. In Table 2, notice the modified Figure 3: Inadequate desmear after laser ablation. Note the separation of the plated copper from the capture pad due to resin remaining. Figure 4: Resin-coated foil after laser via formation with no desmear. Table 1: High-Tg FR-4 (170 Tg), standard solvent system. Table 2: High-Tg FR-4 (170 Tg), modified solvent swell system.

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