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20 I-CONNECT007 MAGAZINE I JUNE 2026 higher reliability risks under thermal cycling. Staggered designs offer better durability at the cost of board real estate, whereas stacked microvias, with an increasing number of layers, are required for high-performance applications, including AI, high- performance computing, and networking equip- ment. While PCB drilling and plating processes are critical to microvia reliability, base material proper- ties will also determine whether these structures survive assembly processes and field operation for the expected service lifetime. As a PCB undergoes thermal cycling, first with exposure to high temperatures in reflow assembly processes, and then in the operating environment, these microvias are subjected to various ther- momechanical stresses. The actual level of these stresses will be affected by the specific PCB design features, including the number of layers of micro- vias, the individual dielectric thicknesses and resin contents, as well as the geometries of the micro- vias, including whether staggered or stacked. A few types of microvia defects related to material proper- ties are shown in Figure 2. From a base material perspective, there are a few critical properties to consider. Thermal Expansion Properties Glass-reinforced thermoset laminate and prepreg materials will have different CTEs in the X-Y and Z directions. While X-Y CTEs are important, especially when considering large-format component packag- es, for microvia reliability, Z-axis thermal expansion is most critical. Glass Transition Temperature (Tg) The Tg of a material represents the temperature at which it transforms from a rigid, "glassy" state to a softer, more flexible state. While Tg is usually report- ed as a specific temperature, there is a tempera- ture range over which this reversible process takes place (Figure 3a). The key point regarding microvia reliability is that CTE values above Tg will be much higher than those below Tg. This is why higher-Tg materials are generally preferred for the most com- plex designs. However, it is important to understand how the Tg and pre- and post-Tg CTEs combine to result in a total expansion over the temperature range of interest. Figure 3a illustrates how Tg is determined by thermomechanical analysis (TMA). The benefit of using TMA to determine Tg is that it also provides pre- and post-Tg CTE values for a material. Figure 4 shows an actual TMA scan, including the Tg and pre- and post-Tg CTE values. Figure 1: a) Illustration of staggered vs. stacked microvias; b) Seven stacked microvias on a buried multilayer sub-PCB. a b Figure 2: Examples of microvia defects.