Calculating Microvia Capture Pad Tolerances across Sequential Lamination Cycles
Capture pad size equals laser diameter plus twice the root-sum-square of tool, drill, and lamination movement tolerances.
Substrate Expansion
Thermal Distortion
Change in the physical dimensions of a base material occurs as a result of temperature increases or the release of internal stresses. This substrate expansion is most common during the baking and lamination phases of board fabrication. It is measured by the coefficient of thermal expansion, which describes how many parts per million the material grows for every degree of temperature change.
The movement is typically different in the thickness direction than it is along the plane of the panel.
Mechanical Impact
Excessive substrate expansion in the Z-axis puts significant strain on the copper plating inside drilled holes. This stress can lead to barrel cracking or pad lifting during the assembly process when the board is heated for soldering. Manufacturers select materials with high glass transition temperatures to limit this movement in harsh environments.
In the X and Y directions, the expansion must be managed to maintain registration between layers and ensure that components align with their pads.
Control Mechanism
Careful control of the thermal profile during production minimizes the risk of permanent deformation. The material eventually reaches a stable state once it cools, but the initial movement must be accounted for in the design phase.