Inner Layer Process
(II): Exposure and Development

After pre-treatment, the inner-layer copper surface is laminated with a dry film photoresist, a UV-sensitive material that protects areas not intended for etching.

Once laminated, the board is aligned with a photomask film representing the circuit pattern and exposed to ultraviolet (UV) light. The exposed areas harden to form a protective layer, while the unexposed portions dissolve in the developer, revealing the bare copper. This stage determines the accuracy and clarity of the circuit pattern.

Therefore, light intensity, exposure time, lamination pressure, and development speed must all be tightly controlled. Insufficient or excessive exposure can cause blurred or distorted circuit edges, directly affecting impedance characteristics and line width/spacing tolerances.

As modern PCBs demand higher performance for high-frequency and high-speed signal transmission, the precision of the exposure and development process has become even more critical.

For this reason, manufacturers regularly calibrate exposure and development equipment to maintain accuracy and consistency.