How to achieve this? First of all, we need to understand a concept of"Line negatives". We print the circuit design into film by lithography, then coating the photosensitive dry film on the substrate. The main ingredients of photosensitive dry film are specific spectral sensitivity and create chemical reaction. Photosensitive dry film can be divided into two types: photopolymerizable and photodecomposition type. The photopolymerizable dry film will become hard under light irradiation of particular spectrum, become water-soluble substance from the photolysis of water-insoluble type, while the photodecomposition type is the opposite. Here we cover the photopolymerization type photosensitive dry film on the substrate, then cover a layer of the film on the top to let it exposure, where exposure is black opaque, others is transparent(line portion).
What will happen when light irradiate onto the photosensitive dry film through film? The color of dry film where is transparent become deeper, and the dry film begin to be hard. The cooper foil which tightly wrap the substrate just like printing the wiring pattern on a substrate, then we expose the copper foil which don't require protection of the dry film by development step(washed unhardened dry film by sodium carbonate). This step is called stripping. The next step is to etch the substrate by copper etching liquid (Chemical which can corrode copper), all copper without dry film protection are corroded, leaving circuit diagram(under protection of dry film) on the substrate. This process is called"image transfer", which is very important during the manufacturing process of PCB.
1. Difference in quality and usage
(1) Electrolytic copper is created by electrolytic method to make copper ions absorb on the substrate. It's features are: strong in electrical conductivity, but week resistance during bending
(2) Rolled copper is created by extrusion method. The features are: good resistance during bending, but electrical conductivity is weaker than electrolytic copper, mainly use in clamshell cell phone camera.
If distinguish by extrinsic feature, electrolytic copper is red and roller copper yellowish.