Fluid Action
Capillary penetration is the mechanism by which liquefied dielectric material fills laser-drilled blind holes during the lamination process. During this thermal cycle, microvia resin flow determines whether the semi-cured prepreg can completely occupy the small cavity without leaving air pockets. The behavior of the resin depends on its dynamic viscosity and the heating rate of the lamination press.
Inadequate filling creates internal voids that fail under the thermal stress of subsequent assembly processes.
Heat Management
Thermal profiles in the lamination press must be controlled to maintain the resin in a low-viscosity state for a duration sufficient to fill the cavities. If the temperature rises too quickly, the resin cures prematurely and loses its ability to flow into the sub-mil structures. Slow heating rates present a different challenge, as the resin may drain away from the surrounding glass fibers and create laminate voids.
Mechanical pressure must also be applied at the precise moment when the viscosity reaches its minimum value. Board fabricators balance these variables by adjusting the lamination stackup and dummy copper features to distribute the pressure evenly.
Reliability Risk
Trapped air in an unfilled microvia expands rapidly during lead-free reflow where temperatures exceed two hundred and sixty degrees Celsius. This pressure can delaminate the copper cap or fracture the internal connection.
