Rheological Testing Conditions for Thin Prepreg Viscosity
Dynamic parallel plate rheometry under controlled 2 °C/min thermal ramps defines the minimum viscosity window for thin prepreg lamination success
Edge Fracture
Mechanical Separation
Mechanical shearing of board edges during depanelization creates stress concentrations that propagate into laminate layers. Edge fracture refers to this physical degradation of the PCB perimeter where localized pressure causes delamination or micro-cracking of the resin matrix. This defect occurs most frequently when automated router bits or manual V-score break-out tools apply excessive force against the material stack.
Unstable support fixtures beneath the board allow deflection during the cutting operation, which exacerbates the tension on the outer fibers.
Process Requirement
Fabricators maintain clearance distances between copper features and the board edge to mitigate the risks associated with peripheral structural damage. Tight routing tolerances necessitate sharp tooling and specific feed rates to avoid dragging fibers across the epoxy surface. Production environments check for this condition using magnification under standard lighting, focusing on the interface between the substrate and the mechanical cut.
High-density designs with conductive tracks near the perimeter encounter this failure more readily than boards with generous margins.
Acceptance Boundary
Finished goods must exhibit intact edges without peeling or loose fiberglass bundles to ensure dielectric integrity under thermal cycling. Internal IPC standards categorize these mechanical splits as defects when the separation encroaches upon defined conductive widths or reduces the spacing below the minimum isolation limit. Inspection protocols exclude boards displaying fissures that connect separate internal layers or compromise the seal of the laminate.
Minimal cosmetic chipping of the board material beyond the copper foil typically remains acceptable provided the circuit remains protected from environmental contaminants.