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66 PCB007 MAGAZINE I FEBRUARY 2020 tion, it is critical that Z-axis expansion is min- imized 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 thermal 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 through-out various thermal ex- cursions. Z-axis expansion not- withstanding, the second critical thermal characteristic is the glass transition temperature of the cured paste material. Typically a Tg of 140°C is ideal. However, the Tg can be increased by pro- longing the final curing time and increasing curing temperature from 140°C to approximately 175–180°C. It is desired to have the highest possible Tg without impacting the flow and metalli- zation properties [1] . With increased densification leading to higher I/Os, smaller components, higher assembly temperatures, and smaller vias, the CTE gains increased importance. Thus, the CTE values of the paste must be minimized to relieve stress that will cause the plug to over- expand, allowing the overmetalized copper de- posit to lift [2] . It is critical that to attain long- term stability within the filled via under load conditions, load amplitudes must be minimized as much as possible. This means that the CTE must be as low as possible over the temperature ranges [2] . 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 technology 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 clearly visible.

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