One of the first steps in fabricating a flexible circuit is to create a circuitry pattern. In order to correctly define the circuit pattern, one needs to understand the basic process for building a circuit. Processes tend to fall in one of the following two categories:
1. Subtractive
The substrate starts with copper bonded to the dielectric, the unwanted metal is removed; the remaining metal defines the circuit traces. The basic process steps for a subtractive process are:
- Create resist pattern
- Etch away exposed metal
- Remove resist
- Continue to process
There are many methods possible for the creating the resist pattern, the following are two of the more common ones.
Screen Print
- Screen print resist pattern
- Cure resist
- Etch
- Remove resist
Photo Image
- Apply resist coating
- Photo expose image onto resist
- Develop away unexposed resist
- Etch
- Remove resist
For screen printing a tool is made by forming a negative of the desired pattern on a woven screen mesh. The resist material is pushed through the open mesh and forms the circuit pattern on the substrate. The initial state of the resist material is normally a paste or semi-liquid. After the material is applied to the substrate, the resist needs to be cured. The cured resist protects the covered copper areas during the etching process.
For photo imaging, a photo tool is created that is the negative of the desired pattern. The photo tool is placed over the resist coated substrate and flooded with a UV light. The photo tool allows the light to cure the resist where the pattern is to be created and blocks the light everywhere else. The UV light causes a chemical reaction in the resist and transforms it to have the chemical and physical properties to withstand the etching process. The developing solution strips away all the “un-cured resist” exposing the bare copper to be etched away.
2. Additive
The substrate starts with just the bare dielectric (possibly with a seed coating) the metal is added to the surface to directly form the circuit pattern. The basic process steps are:
- Apply conductive material
- Cure conductive material
- Continue to process
The additive material is usually a paste and is often applied by screen printing. The applied paste is then cured to reach its optimal properties. Materials that have outstanding conductive properties tend to require very high temperatures to cure. The dielectric substrate for flexible materials is a limiting element for the temperature level and tends to restrict the types of additive materials that are used to create a circuit pattern. In general the electrical, physical and chemical properties of this type of circuit are significantly lower than the properties achieved via the subtractive process.