Multi-layer PCBs

[kellyA]

Have you ever wondered how a multilayer PCB was put together?

It is simple to explain, yet a complex process.

It all starts with the digital files and the requirements for the PCB as to how the layers are arranged and processed, but for a standard 8 layer design this is how we would build it:

1. The internal layers are grouped into pairs. For the standard 8 layers build, we group layers 2 and 3, layer 4 and 5 and layer 6 and 7 together.

2. These pairs are imaged together onto both sides of a copper-clad laminate core, creating 3 distinct double-sided inner layer panels. This image is a representation of the copper image that will represent each layer.

3. Once imaged they are sent to be etched. This step removes the unwanted copper cladding creating spaces, defining traces and pads.

4. Once etched, the layers are inspected to ensure that etched images are defect-free and are sent to the lamination department.

5. The first process in lamination is to chemically roughen up the copper to help it create a strong bond to the epoxy bonding plys (prepregs). There are several different names for this chemical process, oxide, black oxide, alternative oxide are just a few of them.

6. After oxide, the inner layers are ready to be layed up and pressed into a single panel. A multi-layer layup starts with preparing the lamination book, this book contains a top and bottom plate that is made from steel, steel pins for alignment, thermal lagging material, stainless steel shims, Teflon release sheets and of course the manufacturing panels.

7. The manufacturing panel starts by placing copper foil (that will become layer 8 in our standard 8 layer design) onto the alignment pins, followed by prepreg (this is a partially cured glass-reinforced epoxy resin in sheet form) layers. This prepreg has two functions, first, it bonds the external foil to the adjacent copper layer (layer 7 in this case) and creates the dielectric insulator between these layers. Prepreg comes in several different thicknesses to help match the electrical requirements of the design.

8. Next the etched and oxidized core is placed on top of the prepreg, more prepreg, the next core, etc until you get to the top where again copper foil is placed.

9. Depending on the overall panel thickness, multiple panels can be contained within a single book, separator sheets are placed between panels to keep them from sticking together. Shims can be used within a book to help distribute pressure more evenly and help to maintain the flatness of the panels. Thermal lagging material is generally placed between the steel plates on either end of the book and the first panel, the goal is to control the heat flow into the manufacturing panels during amination.

10. Once the book is built it is then “pressed.” Using heat and mechanical pressure under vacuum (to remove volatiles from the prepreg) the prepreg is heated up, the epoxy is liquefied, distributed evenly and then cooled to a hardened epoxy, bonding all of the layers together into a single panel that is ready to have the through holes drilled into it.

...and that is how a multilayer PCB is put together.

Good luck out there!