PCB Layout for the Multilayer PCB Fabrication Process
“Who doesn’t love a good magic trick,” or at least that’s how the expression goes. Well I am definitely one of those people who really loves a good magic trick. More than that, I want to know exactly how the trick is done and I will spend all kinds of time trying to figure it out. There’s got to be a logical way that they made the coin disappear, and I want to know what it is!
In the same way that people want to know how a magic trick works, they also want to understand the magic of how a printed circuit board is created. Fortunately, I'm in a career where understanding PCB manufacturing is sort of a job requirement, and so I’m going to pass on a little bit of that here to you. I can guarantee you that although fabricating a PCB may seem like magic, there isn’t a rabbit being pulled out of a hat here. So if you will permit me, sit back and be amazed as I pull up the curtain on the multilayer PCB fabrication process.
Setting Up for the Multilayer PCB Fabrication Process
When you finish your printed circuit board design and complete all of the design and manufacturing rules checks, your output files will go to a PCB fabricator. The first thing that your fabricator will do is run the same sort of checks that you just did, but they will be checking to make sure your design will work with their specific processes. Once they are satisfied that this board can be built as designed, they will use your design output data to create the images needed for PCB fabrication.
To create these images some fabricators will use a photoplotter to produce tooling film, while others will use more advanced direct imaging techniques. Whether ultraviolet light is used to flash the images on the photoresist through a process called photolithography, or a focused laser beam is used to directly expose the photoresist, the result is the same. The traces, vias, pads, and other metal features of the PCB will be imaged into the photoresist in order to create the physical circuit board. We’ll talk more about the photoresist in the next section as we begin to build up the inner layers of the PCB.
The bare printed circuit board after fabrication before parts are assembled to it
Building Up the Inner Layers of the Circuit Board
The inner layers of a printed circuit board are made up of a core material of epoxy resin and glass fiber commonly referred to as FR-4. There are other other materials as well, but this is the more common materials used to create the board substrate. Copper is also pre-bonded on both sides of this core material to create what is called the inner layer laminate.
The photoresist material is coated onto the copper, and one layer image from the PCB CAD data is exposed to the photoresist. The areas that are exposed will harden and protect the copper that it is covering, while the areas not exposed will stay pliable allowing it to be chemically cleaned off. Once this photoresist is removed, those areas of unprotected copper can now be etched away while the copper that was masked by the hardened photoresist will remain. When the etching is completed, the hardened photoresist is also removed leaving the inner layer metal circuitry of the PCB.
This step is repeated for each pair of inner layers. Once completed, the layers will be inspected by automated systems before being passed onto the lamination stage.
Laminating the Multilayer Board Together
To laminate the layers together, a sheet of fiberglass impregnated with epoxy resin known as “prepreg” is used. Next, the inner layers together with their adjacent layers of prepreg are carefully stacked together to maintain the layer to layer alignment of the copper circuitry. A thin layer of copper foil is then used to cover the external prepreg layers on the top and the bottom of the board, and the compiled PCB sandwich is ready for lamination. To complete the lamination process, the PCB sandwich is heated and pressed together to fuse the layers into one complete circuit board.
At this point the laminated PCB is ready to be drilled, and then followed by a thin layer of copper that is applied to coat the interior of those drilled holes. The next step is to add the top and bottom circuitry similar to how the inner layers were created, only this time a reverse image is used to expose the photoresist. This way the pliable photoresist is cleaned off where copper circuitry needs to be added. Those areas of circuitry are plated, and then covered with tin for their protection. The remaining photoresist is then removed, and any unnecessary copper is etched away leaving only the circuitry which was protected by the tin. Lastly, the tin is removed as well.
Advanced PCB Design CAD tools will allow you to create the best fabricated boards
Finalizing the Board with the Finishing Touches
Once the PCB has been etched and laminated, the final touches are added. Solder mask is applied using a UV process, and surface finishes, silkscreen and other markings are applied as well. The last step in the fabrication process is to electrically test the board for continuity, and then pass it through final inspection for validation before it is shipped to the PCB assembler.
As you can see, fabricating a printed circuit board is not magic, but it is a complex operation that demands precision. To make sure that your next PCB is built correctly you need to make sure that it is designed correctly first. To do that requires PCB design tools that allow you to exactly control component placement and trace routing with rules and constraints that keep your spacings at the correct widths for error free fabrication.
The PCB design tools from Cadence give you these capabilities plus much much more. The design rules and constraints in Allegro PCB Designer will give you the control you need to keep your metal spacing exactly where your fabricator needs it to be. Additionally, Allegro gives you multiple features for component placement and trace routing that will make you the magician to those who rely on you for expertly designed boards.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.