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22 I-CONNECT007 MAGAZINE I JUNE 2026 But note the differences among the three mate- rials, as illustrated in Figure 3b. Products B and C have the same Tg value, but Product C obviously expands much faster vs. temperature above the Tg than Product B, resulting in significantly more total thermal expansion. On the other hand, Product A has the lowest Tg, but exhibits less total expansion than Product C, because of a lower post-Tg expan- sion rate. Figure 4 is an actual TMA scan, showing the Tg at 154°C and pre- and post-Tg CTEs of 45 ppm/°C and 220 ppm/°C, respectively. Modulus The modulus is usually thought of as a measure of a material's stiffness, or how much it resists deforma- tion under an applied stress. Specifically, Young's Modulus is the ratio of tensile stress to tensile strain. So, a high-modulus material is very stiff, and a low-modulus material is more flexible or rubbery. "Storage Modulus" values determined through dy- namic mechanical analysis (DMA) are often report- ed for base materials. For polymers below the Tg, Young's modulus and storage modulus values are almost the same. Note, however, that as materials exceed the Tg, modulus values will decrease dra- matically. Figure 5 illustrates how Young's modulus is determined but note that different materials can have very different-shaped curves. Considerations for Microvia Reliability In PCBs with multiple layers of microvias the prima- ry failure mechanism is thermo-mechanical stress from CTE mismatch. Copper expands at approxi- mately 17 ppm/°C while the dielectric layer expands at much higher rates. This exerts tensile and shear forces on via walls, pads, and interfaces. Cooling reverses this but potentially leads to fatigue crack- Figure 3: a) TMA measurement of Tg; b) TMA Tg comparisons of materials. a b Figure 4: Example of an actual TMA scan. Figure 5: How Young's modulus is determined.