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58 PCB007 MAGAZINE I OCTOBER 2021 tion, it is critical that Z-axis expansion be min- imized to prevent the plated cap from liing (Figure 2). As noted previously, a properly formulated plugging paste for via fill must maintain a low CTE at and above 140⁰C. e ceramic par- ticles that are formulated in the resin system function to restrain Z-axis expansion under thermal loading. e ceramic fillers can be seen in Figure 3 under high magnification of the fully cured polymeric paste. ere is no disputing the fact that the vias must be filled void-free and maintain integrity throughout various thermal excursions. Z-axis expansion notwithstanding, the second criti- cal thermal characteristic is the glass transition temperature of the cured paste material. Typ- ically, a Tg of 140⁰C is ideal. However, the Tg can be increased by prolonging the final cur- ing time and increasing curing temperature from 140⁰C to approximately 175-180⁰C. It is desired to have the highest possible Tg with- out impacting the flow and metallization prop- erties 1 . With increased densification leading to higher I/Os, smaller components, higher assembly temperatures, and smaller vias, the CTE gains increased importance. us, the CTE values of the paste must be minimized to relieve stress that will cause the plug to over expand, causing the over-metallized copper deposit to li 2 . It is critical that to attain long term stability within the filled via under load conditions, load amplitudes must be mini- mized as much as possible. is means that the CTE must be as low as possible over the tem- perature ranges 2 . Regardless of the method of via filling cho- sen, this is a process that is here to stay. Via filling technology is a critical aspect of HDI printed circuit board fabrication and the never- ending quest for miniaturization. PCB007 References 1. Karsten Andra, "Hole Plugging Technology for Multilayers and HDI Packages," EPC PCB Conven- tion, 1999. 2. Internal communication with Lackwerke Peters. Michael Carano is VP of tech- nology and business develop- ment for RBP Chemical Tech- nology. To read past columns or contact Carano, click here. Figure 2: Plated copper separating from filled via due to excessive Z-axis expansion. Figure 3: Top view of filled via with plugging paste. Ceramic particles embedded in the matrix are clearly visible.