Structural Element
Cured laminate structure consisting of fiberglass fabric and resin bonded between copper foils forms an intermediate building block for complex high-layer-count printed circuit boards. Board fabricators utilize a sub-core to build inner layer pairs that undergo etching and partial lamination before integration into the final multilayer stackup. Processing inner sub-cores independently allows high-resolution trace formation and early optical inspection prior to final press cycles.
Blind vias and buried vias are frequently drilled and plated through individual sub-core units before main lamination. Pre-inspecting sub-cores reduces scrap costs by discarding defective inner layer pairs before consuming expensive outer prepreg materials. The building block designation stops applying once the sub-core is permanently laminated into the completed rigid board structure.
Sequential Lamination
High density interconnect designs require multiple press cycles to join individual sub-core structures into unified multilayer boards. Each lamination cycle subjects sub-cores to high thermal pressure, requiring heat-resistant resin systems to prevent internal registration skew. Dimensional stability testing tracks material movement across sequential press cycles to maintain via-to-pad registration.
Dielectric Tolerance
Sub-core thickness consistency dictates impedance accuracy for high-speed differential signals routed on internal layer pairs. Dielectric layer height variations alter capacitance and signal propagation speed across processing lots. Chemical cleaning prior to sequential lamination removes surface oxides without reducing dielectric resin thickness.
Microsection testing verifies that sub-core dielectric spacing meets impedance model requirements across the finished circuit board panel.
