Printed Circuit Board Imaging Process

One of the most important steps in fabricating printed circuit boards is the imaging process. The imaging process takes the circuitry pattern from the design and transfers it to the physical board, creating a mask to allow copper plating to fill in the areas that will become the actual circuitry pattern on the board. This step is critical in the manufacturing process as the imaging of the pattern is often very intricate and small and requires an exact image of the pattern in order to work properly.

Abstract: The imaging process for printed circuit boards is very similar to the imaging process for a silk screen mask or photo plate in the printing industry. The design becomes a negative imaging mask that is applied to the circuit board panel through an ultra violet light sensitive process using a material called a resist. The pattern is exposed to the light sensitive material creating the areas to be plated with copper while masking off the areas that will remain not plated. A developer solution completes the imaging process and the panel is ready to be plated, thus creating the actual circuitry on the printed circuit board panel.

In imaging process the circuitry pattern is transferred to the blank circuit board panel  covered with a resist except where the copper will ultimately remain on the base material. The fundamental principle of resist is to protect selected areas from electrolytic copper plating.

Dry-Film Resist, Dry film resist is an ultraviolet light sensitive photopolymer (photoresist). It is supplied on a roll and applied by processing the panel through heated rollers (hot roll laminator). After the hot roll lamination, the panel is placed in a UV printer frame, and a phototool is positioned to the panel using tooling pins as locators. The emulsion on the phototool forms the circuitry pattern plus any auxiliary features added during the panelization. This emulsion blocks the UV light so that the UV light only passes through the clear portions of the phototool to activate the light sensitive resist. This step in the imaging process is called exposure.

The areas of the dry film exposed to the UV light undergo a chemical reaction called photopolymerization. These areas then become impervious to chemical solutions in the next step of the imaging process, developing.

The panel is placed in a conveyorized developing machine which sprays a developer solution made of water and sodium carbonate monohydroxide on the panel to remove the unexposed resist. An image of the desired circuitry open to the base copper is formed completing the imaging process.