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64 PCB007 MAGAZINE I FEBRUARY 2020 In Part 1 and Part 2, I presented several op- tions with which to accomplish blind and through-hole via filling. In this edition of "Trou- ble in Your Tank," I will address filling blind and through-holes with polymeric pastes. Via Fill Paste Often the term "plugging paste" is used to de- scribe the method and material for completely filling blind vias and through-holes. In general, paste-filling material selection is at the request of the end-user and is indicated for a num- ber of reasons. It has been my experience that major OEMs are driving the industry to migrate to the high Tg/low CTE plugging paste formula- tions for high-density applications. In addition, these formulations are of a non-conductive nature that provides a high-quality plugged via and is also cost-effective (Figure 1). Limita- tions abound, depending on PCB thickness, via diameter, and paste properties. Properties of Via Fill Materials What attributes are needed for a high-perfor- mance via fill material? There are specific requirements for the plugging paste material. • 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 materials are ther- mally reactive. It is highly recommended 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 expansion must remain in the 40–60 ppm range to ensure that via cracks do not occur in the filled via. In addi- Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Via Hole Filling and Plugging, Part 3 Figure 1: Example of a paste filled through-hole.

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