Fabrication Shift
Cumulative displacement of internal circuit patterns relative to each other or to the outer drilled holes occurs during the lamination of multi-layer boards. The inner layer registration walk refers to the dimensional distortion and material movement that takes place when prepreg and core layers are subjected to high heat and pressure. This shift limits the minimum size of annular rings and via pads.
It terminates the validity of the drill file alignment if the shift is too large.
Material Behavior
Epoxy resin and woven fiberglass sheets have different coefficients of thermal expansion, which causes the material to expand and contract unevenly during the lamination cycle. When the prepreg resin melts and flows, it exerts shear forces on the thin inner layer cores, causing them to stretch, shrink or shift position. This movement is compounded by the release of internal stresses that were locked into the laminate during its initial manufacture.
Fabricators must measure this movement across multiple board lots to establish a statistical model of the shrinkage and expansion. These measurements help to predict how much the layers will walk during processing.
Drill Optimization
Compensation factors are applied to the photolithography artwork to offset the expected material movement. Adjusting the exposure scale of the inner layer patterns ensures that they align with the laser and mechanical drill holes after lamination. This compensation minimizes the occurrence of broken electrical connections or partial annular ring breakouts.
