Prior to lamination, the surface of inner image layer needs to go through an oxide process (brown oxide or black oxide treatment) to roughen and passivate the copper surface, increase the interaction surface area of resin, enhance the bond strength between the copper layer and the prepreg to avoid problems such as delamination and diffusion coating.
Once inner layers have been confirmed as being defect-free, they are ready for lamination. PCB lamination process mainly consists of following steps:
Lay-up
Multi-layer boards with six layers or more require preliminary lay-up, which stacks and fixes core boards with etched circuit pattern and prepreg together in a certain order. In this step, inner layers are fixed by riveting or fusing to ensure that the traces of each layer will not cause open or short circuits because of misalignment. Lay-up step is to stack copper foil, prepreg, inner layers after preliminary lay-up and other materials in the proper order for lamination.
Lamination
The inner layers, which are clamped by iron plates, are placed on stands and then fed into a vacuum heat press machine for lamination. Under high temperature and pressure, the resin in the prepreg melts and flows to fill the core board and laminates the layers together. Once finished, operators remove the top iron plate of the stacked layers and then take away the bearing aluminum plate. Only three or more layers of PCBs need to be laminated while single and double-sided panels do not require.
Post-treatment
After lamination, the boards are processed by bevelling, targets drilling, edge milling and other processes for subsequent production. Among the processes, layer deviation checking by X-RAY and hole milling are the most important parts.
