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Friday, August 14th

Multi Layer PCB Image Process


In our last post we talked about the basic material used to start the PCB manufacturing process.

If we were doing a double sided board it would actually be going into drill at this part of the process. We will get to that eventually. What we are going to do is walk through the multilayer process which is a different process initially. We will pick up at drill and the double sided process a little later on.

In the last article we mentioned the various core thicknesses that are available for PCBs. In this example we will be talking about a how a 4 layer board will be put together.

Essentially what you have to do is make a sandwich. The inner core is going to be the meat of the sandwich. Now to get a final thickness of .062 you might start with a core of .042 thick. The core is copper clad fiberglass.

Before the “meat” of the sandwich can go into the sandwich there are a few things that need to be done.
The first of which is etching the traces and spaces into the copper that is on the material.

This is done through a several step process. The first being the application of a photo or light sensitive material referred to as Dry Film Resist.

Core material with Dry Film Resist


The film is applied with some heat and pressure and covers the entire core material.
The panel is then placed into an exposure unit where the image needed is exposed. The image comes from the design files from the Electrical Engineer. They are referred to as gerber files. (see my post about the common errors with Gerber Files).

This film is used to place an image on the resist. For inner layers the film is a negative image and for outer layers it is a positive image.

Inner layer film is “negative”. That means that the copper patterns left behind after processing the core are the “clear” areas on the film.

Outer layer film is “positive”. The traces and pads that are dark on the film are copper on the outside of the board and the clear areas will be clear of copper.

Everything that is clear in this image will be the copper that is left behind. This is a positive image.

Image Develop

The exposed core is processed through a chemical solution or developer that removes the resist from areas that were not hardened by the light.

Panel after image has been exposed and developed

The next step is an etch process. All the areas not covered by the film will be etched off of the board with will leave a matching pattern of the image.

What you are left with then is the resist still on the remaining copper. The resist must then be removed as well with a chemical process that removes it but leaves the copper behind.

The inner “meat” of the sandwich is now ready to be added to the rest of the sandwich for the next part of the process which we will review in the next post.

In the next post we will talk about the lamination and drill process.

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