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56 PCB007 MAGAZINE I OCTOBER 2019 expansion is minimized to prevent the plated cap from lifting (Figure 2). As noted previously, a properly formulated plugging paste for via fill must maintain a low CTE at and above 140°C. The ceramic particles that are formulated in the resin system func- tion to restrain Z-axis expansion under ther- mal loading. The ceramic fillers can be seen in Figure 3 under high magnification of the fully cured polymeric paste. There is no disputing the fact that the vias must be filled void-free and maintain integrity throughout various thermal ex- cursions. Z-axis expansion not- withstanding, the second criti- cal thermal characteristic is the glass transition temperature of the cured paste material. Typical- ly, a Tg of 140°C is ideal. How- ever, the Tg can be increased by prolonging the final curing time and increasing curing tempera- ture from 140°C to approximately 175–180°C. It is desired to have the highest possible Tg without impacting the flow and metalliza- tion properties [1] . With increased densification leading to higher I/Os, smaller components, higher assembly temperatures and smaller vias, the CTE gains increased impor - tance. Thus, the CTE values of the paste must be minimized to relieve stress that will cause the plug to over-expand, allowing the over- metalized copper deposit to lift [2] . It is criti- cal that to attain long-term stability within the filled via under load conditions, load ampli- tudes must be minimized as much as possi- ble. This means that the CTE must be as low as possible over the temperature ranges [2] . Conclusion Regardless of the method of via filling cho- sen, this is a process that is here to stay. Via- filing technology is a critical aspect of HDI PCB fabrication and the never-ending quest for miniaturization. PCB007 References 1. Karsten Andra, "Hole Plugging Technology for Multi- layers and HDI Packages," EPC PCB Convention, 1999. 2. Internal communication with Lackwerke Peters. Michael Carano is VP of technol- ogy and business development for RBP Chemical Technology. 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.