Inner Layer---Multi-layer PCB Manufacturing Process

For multi-layer printed circuit boards, the working layer of the inner layer is sandwiched in the middle of the entire board, so the multi-layer circuit board should first be processed on the inner layer.


Specifically broken down, the inner layer processing of printed circuit boards can be divided into 5 steps, namely Pre-processing, Inner Layer Imaging, Inner Layer Etching, Resist Stripping and Inner Layer AOI inspection.


1. Pre-processing

In the process of manufacturing printed circuit boards, the copper foil substrate is first cut into a size suitable for processing and production, and then pre-processed. The copper foil substrate after cutting has oxides, grease and other impurities attached to the surface, which firstly needs to be pretreated. Generally speaking, Pretreatment has two functions: first, clean the substrate after cutting; second, increase surface roughness of the copper foil to ensure good the combination of the substrate and the dry film. Usually, abrasive blasting,  chemical process and mechanical grinding are three different types used in pretreatment.
 
2. Inner Layer Imaging

Lamination is the process of coating the surface of a pretreated the copper foil substrate with a special photographic material. Dry and wet film are two difference photosensitive materials. Both of them can produce a polymerization reaction and form a stable substance attached to the surface of the copper foil when exposed to UV light. This stable substance will not wash off during the etching phase. As for using dry film or wet film, it depends on manufacturer's own operating environment and needs. 

Dry or wet film is applied to the copper foil substrate under heated and pressurized conditions. Because the dry or film is UV sensitive and to avoid any shorts and opens on the inner layer, the process must be happens in a dust-free and yellow light room.

3. Inner Layer Etching

At the same time, the printed film of the inner circuit has been produced using light plotter. Technicians place the printed film over the copper foil substrate and align them using the registration holes in order to avoid offsetting the position of the printed film. Uv light exposes the printed film, to harden the chemicals of the light-transmitting areas into the printed pattern. These hardened material protects it and the copper under that will remain on the board during etching, while light-tight part does not react which is to be washed out by the developer. The etching section is the core of this production line. Its function is to corrode the exposed copper that is not covered by the hardened material.

4. Resist Stripping

Technicians remove any remaining resist covering the cooper trace of the PCB inner layer to ensure that no substances affect the conductivity of the copper. After removing the resist,  the inner layer circuit has been shaped.

5. Inner Layer AOI inspection

This steps examine the inner layer to look for incomplete patterns or resist that may still be on the surface. The machine compares the digital image  with the original design in the Gerber file to ensure the cooper trace after etching is without any defects.