Friday, December 09th
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We learned how the inner core of a 4 layer PCB is processed prior to adding the outside layers in our last post. In this post we will take about the next part of our “sandwich” construction – but first, let’s start with a brief introduction of foil lamination.

Foil Lamination

There are a variety of ways that foil lamination is used as well as a variety of materials that are used for it. The most common way is a process of applying some type of adhesive along with pressure and heat to “laminate” the metal material to whatever other material it is being laminated to. Common metals that are used for foil lamination include aluminum, gold and copper. In practical applications you may see foil that is laminated on to a certificate that will have a gold laminated star of achievement. The process for copper foil laminate to be applied to the fiberglass circuit board material is similar in many ways. The foil is laid on top of the prepreg material of fiberglass in a multi-layer circuit board.  Then to make sure that the foil lamination process is complete, the stack up is placed inside of a lamination press. The press then applies pressure and heat for a period of time. Once this is completed the next steps of the process can continue.

The next step, continuing with our example, is to send the now etched panels with traces through an AOI, or Automated Optical Inspection. Inner layers are then inspected against design rules using data from the gerber files. If allowed and practical, some repairs can be made at this point. Information on defects is shared with the appropriate departments to correct any process problems.

The core that has the design pattern now etched in the copper is combined together with prepreg fiberglass material. (See our first post for info on that). In our sandwich analogy it’s kind of like cheese might be.

The bread in our sandwich is going to be the copper foil that is added. Typically it would be .5 oz to 1 oz copper foil that is very thin added as part of the total stackup of our sandwich.
Would you like your sandwich toasted?
If you want to have the cheese melt and have the sandwich “bond” together you do!
The panels are placed into a lamination press. They will undergo heat and pressure that will bond the core, prepreg and the copper foil together.

Desmear –
Multi-layer Boards Only Desmear generally applies only to multilayer boards. It is a chemical process that removes the thin coating of resin from the inner layer connections that is produced by the heat and motion of the drill bits as they create the holes.
Removing the resin smear improves the electrical connectivity.

In order to get consisting copper plating in the holes of the board there are several steps that need to be completed.

Deburr
Deburr is an abrasive mechanical process that removes the raised edges of the metal or burrs surrounding the holes that occur during the drilling process. Any debris that may be left in the holes is also removed at this time.

Desmear –
Multi-layer Boards Only Desmear generally applies only to multilayer boards. It is a chemical process that removes the thin coating of resin from the inner layer connections that is produced by the heat and motion of the drill bits as they create the holes.

Removing the resin smear improves the electrical connectivity.

Electroless Copper Deposition
Once the smear is removed, a thin coating of copper is chemically deposited on all of the exposed surfaces of the panel, including the hole walls. This creates a metallic base for electroplating copper into the holes and onto the surface. The thickness of the electroless deposit is between 45 & 60 millionths of an inch.

The next step then is to apply and etch the image into the copper of the outer layers. This is a similar process as mentioned with the inner layer image process. There are a couple of things that are different but all in all it is about getting the traces and spaces etched into the copper.
In that process the panel will also go through a copper platting bath in order to add copper plating to the holes as well as the copper on the surface of the board. The time for plating varies on the final copper thickness required for the board.

Please ask your questions in the comments below.
In the next post we will talk about the next steps in plating, soldermask, etc.

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3 Responses

  • actpcb (actpcb.com) November 30th, 2011

    PCB 4 Layers allow for signals to run on the inside of your board, you have the ability to pack more components within closer tolerances allowing for a more compact design. Multilayer boards will always be offered with an even number of layers on the PCB. The most common and practical are boards with layers of 4, 6, or 8. A common approach to Multilayer is to dedicate one layer to the ground plane while another is focused on power.

  • Peter Brissette (bayareacircuits.com) November 17th, 2011

    Hi Daniel, thanks for stopping by and checking them out. I have a few more posts coming soon that with further explain the process and talk about some of the finish options you have with PCBs.

  • Daniel Honda November 17th, 2011

    Thank you so much for these posts!!! I was looking for something that explained PCBs in a concise fashion and these articles are really helpful! Thanks again and please keep writing!



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