In our last post we learned how the inner core of a 4 layer PCB is processed prior to adding the outside layers. In this post we will take about the next part of our “sandwich” construction.
The now etched panels with traces will need to go 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.