Printed Circuit Design & Fab - December 2007 - (Page 24)

Plating Voids, Part 2 Careful control of the drilling can lead to improved plating quality. MICHAEL CARANO IN PART 1, we began our exploration of PTH (plated through hole) voids. In this month’s column, we will discuss many of the other causes of PTH voids. Pre-Electroless Causes of Voids It is often said by seasoned engineers that there are so many process steps and variables that could have an influence on the success or failure of getting a continuous, void-free copper deposit in the hole. One of the obvious causes of voids or at least poor copper coverage is the result of poor drilling. FIGURE 4 shows voids on the glass bundle fibers. A closer look as this section will show a very poorly drilled hole wall, with glass bundles protruding from the resin. Astonishingly, many feel the plating process should always compensate for such a travesty. A surface such as the one shown in Figure 4 makes it very difficult to catalyze with the palladium-based activator from the electroless copper process. One should immediately investigate the drilling operation, looking at the following: ■ Drill bit quality: the number of hits and the overall condition of the drill tool. Spindle feeds and speeds: is the drill tool punching its way through the stack? ■ Slow up-feed – causing the torn out glass bundles – should be 2X the infeed rate. ■ Check chip loads: experiment with different chip loads to improve quality. ■ Stack height: the number of PCBs in the drill stack; consider reducing the number to improve quality. FIGURE 5 shows a horizontal section of a PTH. Note again the poor quality in the hole wall and its effect on the plating process. The desmear operation is another potential cause of PTH voids. The most effective desmear process available today is based on alkaline permanganate. As a matter of fact, the permanganate system is considered an industry standard. The use of alkaline permanganate for multiplayer PCBs has had a dramatic effect on copper coverage and adhesion in the plated through hole. However, this process, if not controlled, can be a major cause of voids. First, the process consists of three or four steps, as listed below that comprise the main chemical process for desmear. 1. A solvent conditioner designed to penetrate the polymer matrix of the resin system and weaken the polymer-polymer bonds of the cross linked chain. ■ 2. The alkaline permanganate solution consisting of sodium or potassium permanganate and its corresponding hydroxide salt. 3. A neutralizer (possibly in combination with glass etch) for removing manganese residues. 4. Glass etch as a separate step to lightly roughen (frost) or more aggressively remove glass fibers that may be protruding into the hole due to etch back. Any of these four processes if not controlled can lead to voids. If this is the case, even the most robust of electroless copper processes will not be able to compensate. For example, the effective alkaline permanganate process not only removes drill smear, but will microroughen the resin as shown in FIGURE 6. This texturing is often referred to as the honeycombed appearance. This is desirable for two reasons: ■ The textured surface promotes palladium catalyst adsorption, which in turn promotes electroless copper deposition and coverage. ■ Surface improves copper adhesion. This, in turn, promotes improved solderability by minimizing or eliminating the occurrence of blow holes seen in the wave-soldering operation. More on blow holes in a future column. Regardless, if the texturing is FIGURE 4. Glass fiber bundles are torn out exposing a deep gouge in the resin. 24 FIGURE 5. Poor hole wall quality created in the drilling process can lead to voids and other plating difficulties. FIGURE 6. Micro-roughened dielectric surfaces improve copper adhesion to the hole wall and enhance catalyst adsorption to accelerate plating coverage. DECEMBER 2007 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - December 2007

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