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56 PCB007 MAGAZINE I OCTOBER 2021 Introduction In a previous column, I presented several options with which to accomplish blind and through-hole via filling. In this edition of Trou- ble in Your Tank, filling blind vias and through- holes with polymeric pastes will be presented. Via Fill Paste Oen the term "plugging paste" is used to describe the method and material of com- pletely filling blind vias and through-holes. In general, paste filling material selection is at the request of the end user and is indicated for sev- eral reasons. It has been my experience that major OEMs are driving the industry to migrate to the high Tg/low CTE plugging paste formu- lations for high density applications. In addi- tion, these formulations are of a non-conduc- tive nature that provide a high quality plugged Via Filling—Continued via, and are also cost effective (Figure 1). Lim- itations abound depending on PWB thickness, via diameter, and paste properties. Properties of Via Fill Materials What attributes are needed for a high per- formance via fill material? ere are specific requirements for the plugging paste material. ese are: • 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 temperature. Keep in mind that these materials are ther- mally reactive. It is highly recommended that the fabricator uses a 100% solids content of the paste material with the ther- mally cross-linkable epoxy resin and specially designed ceramic fillers. e ceramic filling material restricts Z-axis expansion when the filled vias are subjected to a thermal load. Inter- estingly, 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 Figure 1: Example of a paste-filled through-hole.