Laminate Z Axis Thermal Expansion Limits in Stacked Microvia Stackups
Low z-CTE silica-filled laminates (IPC-4101/129) and staggered microvia topologies prevent target pad separation during 260°C lead-free reflow excursions.
Alpha 2 CTE
Thermal Expansion
Thermal expansion coefficients dictate the rate of dimensional change in a dielectric base material as it undergoes temperature transitions. The alpha 2 cte measures this rate of expansion along the vertical axis once the laminate has exceeded its glass transition temperature where the resin matrix softens. This value establishes the level of stress imposed on copper plated through holes during high temperature processes.
At temperatures above this threshold, the polymer chains gain mobility and expand at a rate that is typically three to four times greater than their low temperature state.
Out-of-plane Stress
Relieving strain on plated copper relies on selecting materials with controlled expansion parameters. When boards are subjected to reflow profiles, a high alpha 2 cte can lead to corner cracks and barrel failures in vias. The thickness of the substrate and the aspect ratio of the holes determine the severity of this risk.
High Tg materials generally limit the total expansion by delaying the transition to this high rate region.
Reliability Boundary
Material sheets behave isotropic or anisotropic depending on reinforcement styles. Because woven glass fabric restrains expansion in the horizontal plane, the bulk of the thermal volume change is forced into the vertical direction. Designers must evaluate resin content and laminate composition together to ensure that thermal fatigue does not compromise the copper structures.
This ensures the board withstands multiple assembly cycles.