When creating circuit layers, there are two ways to go about the process. Below we break down the additive process and the subtractive process:
Using the additive method begins with very thin (5 microns or less) copper layers on a single side or both sides of polyimide film. Resist is coated on the copper surface, imaged, and developed. This creates channels in the resist where the copper will be plated. The plated copper becomes the pads, traces, and other conductive areas of the circuit. Finally, the resist is removed and the background copper is etched away to isolate the newly created copper features.
The additive process involves more steps than the subtractive process and it costs more. On the plus side, it provides a resolution of .003 inches wide (or smaller) leads at a rectangular lead cross-section. We can create fine-line circuits using the additive process with trace and space patterns as small as .001 inches.
Using the subtractive method starts with copper that is bonded to one or both sides of polyimide film. The thickness of the copper at the start of the process is the thickness it will be upon completion.
Resist is added to the copper surface, then exposed and developed. The result is openings in the resist through which the copper can be attached using an etchant. After the etching process, the resist is taken off, leaving the finished circuitry.