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54 PCB007 MAGAZINE I OCTOBER 2019 In my previous column, I presented sev- eral options with which to accomplish blind and through-hole via filling. In this edition of "Trouble in Your Tank," I will discuss filling blind vias and through-holes with polymeric pastes. Via Fill Paste Often, the phrase "plugging paste" is used to describe the method and material of completely filling blind vias and through-holes. In general, paste filling material selection is at the request of the end user and indicated for a number of reasons. It has been my experience that major OEMs are driving th e industry to migrate to the high-Tg/low-CTE plugging paste formulations for high-density applications. In addition, these formulations are of a non-conductive nature that provides a high-quality plugged via that is cost-effective (Figure 1). L imitations abound depending on PCB thickness, via diameter, and paste properties. Properties of Via Fill Materials What attributes are needed for a high per- formance via fill material? There are specific requirements for the plugging paste material, such as: • Good adhesion between copper and paste even under temperature influences • Good adhesion of copper, dielectrics, or photoresist • Solvent-free, one-pack system • No air inclusions in the paste • Tg>140°C • CTE<40 ppm (below Tg) • No shrinkage during curing • Easily planarized Additionally, the plugging paste material must maintain a reasonable shelf life at room temperatures. Keep in mind that these materi- als are thermally reactive. It is highly recom- mended that the fabricator use a 100% solids content of the paste material with the ther- mally cross-linkable epoxy resin and specially designed ceramic fillers. The ceramic filling material restricts Z-axis expansion when the filled vias are subjected to a thermal load. In - terestingly, the coefficient of thermal expan- sion must remain in the 40–60 ppm range to ensure that via cracks do not occur in the filled via. In addition, it is critical that Z-axis Via Hole Filling and Plugging, Part 2 Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Example of a paste-filled through-hole.