Shear Deformation Kinetics of Inner Layer Heavy Copper Features during Vacuum Pressing
Shear forces during vacuum pressing shift heavy copper traces when resin flow velocity exceeds interfacial bond strength, requiring optimized aspect ratios.
Copper Undercut
Etch Defect
Subtractive chemical etching defects occur when the etchant liquid dissolves copper laterally beneath the photoresist mask. The lateral removal distance known as copper undercut reduces the top width of etched conductors relative to their base width. Etchant spray pressure and chemical contact time govern the magnitude of this lateral removal.
High etch factor values indicate minimal lateral erosion relative to vertical depth.
Conductor Geometry
Trace width variations resulting from lateral dissolution reduce the conductive cross-sectional area of signal lines. Trapezoidal conductor cross sections increase high-frequency conductor loss due to skin effect concentration near the trace base. Narrow conductor tops also reduce the surface area available for component soldering on outer circuit layers.
In fine-line designs, excessive undercut leads to trace lifting or complete conductor collapse. Differential impedance values drift higher when lateral etching removes more copper than calculated in initial stackup models.
Process Correction
Photolithography compensation factors adjust original Gerber artwork dimensions to balance over-etching. CAM operators broaden artwork trace widths prior to phototool creation based on copper foil thickness. Thicker copper foils require larger compensation allowances due to longer immersion times in the chemical etching chamber.
Modern alkaline and acid cupric chloride etching systems employ precise spray nozzle geometry to deliver uniform chemical impact across the panel.