From sports to entertainment, we judge a performance based on what we see on the screen. However, there is much more going on behind the scenes that most of us will never know about. Practice squads, costume designers, trainers, and technicians are just a few of those who contribute to what we see on our TVs, yet most of us are completely unaware of their contribution.
Printed circuit board manufacturing has a similar “behind the scenes” supplement. Most everyone is familiar with the work of a PCB contract manufacturer who assembles electronic components to a circuit board. Many people don’t realize, though, that before it is assembled, the circuit board is produced using a very intricate fabrication process. This job is handled by fabricators who use lamination and PCB etching processes to create the raw circuit board. We will explore this part of circuit board manufacturing with some details about the PCB fabrication process.
Setting Up for PCB Fabrication
In printed circuit board manufacturing, a PCB CM will typically choose a fabricator based on the board’s needs and send the files to them. Once the fabricator has completed their own design review to verify the manufacturability of the board, they transfer the circuitry images onto the board materials. For internal layers, this involves coating photoresist onto sheets of copper pre-bonded to core materials made from epoxy resin and glass fiber, commonly known as FR-4. The circuitry images are then exposed onto the photoresist either with an ultraviolet light through photo tooling or directly imaged with a laser.
Whether the fabricator uses ultraviolet light or a laser for exposing the photoresist, the results will be the same. Where the circuitry images are exposed, the photoresist will harden or polymerize, forming a protective coating over the copper. All other areas of photoresist that did not get exposed will remain pliable and can be chemically cleaned off for etching. However, before we continue with the fabrication process, let’s first examine what etching actually is.
What is Etching?
PCB etching is the process of removing copper from a circuit board layer that is not protected by the hardened photoresist by immersing it in a chemical solution. The etching used by industrial circuit board fabricators is broken into two different processes: acidic etching and alkaline.
Acidic etching
Acidic etching is used for removing copper from internal layers in standard rigid FR-4 circuit boards due to its precision and less aggressive action. The acidic process won’t react with the photoresist and will produce less of an undercut in the metal it etches. The trade-off, however, is that acidic etching will take longer than other methods.
Alkaline etching
Alkaline etching is used for the outer layers of the circuit board, where a quicker process is more to maintain the uniformity of the remaining copper. Alkaline is more active than acidic and requires careful monitoring to ensure its accuracy.
The acidic and alkaline etching processes are ideal because they have a high etching rate at a relatively low operational cost and effectively etch many different metal materials. PCB fabricators must precisely maintain their etching processes to guard against exposure to hazardous liquids and fumes and prevent any chance of water contamination. Now, let’s see how this etching is used in the fabrication of printed circuit boards.
The PCB Etching Processes to Fabricate the Board
With the hardened photoresist on the copper of the inner board layers protecting the traces, vias, and other areas of metal circuitry, the remaining photoresist is cleaned off, revealing bare copper. The layers are then etched to remove unprotected bare copper leaving only the desired circuitry. After the hardened photoresist is removed, the inner layers are left with only their copper circuitry. This step is repeated for each pair of inner layers until all of the board layers are completed.
The next step is to laminate all of the inner layers together with sheets of fiberglass impregnated with epoxy resin, known as “prepreg.” Once the top and bottom of the board are covered with a thin layer of copper foil, the board is ready to be laminated together with heat and pressure. After lamination is completed, the thru-holes in the board are drilled, and it’s ready for the next etching process.
First, the top and bottom surfaces of the board will be covered with photoresist material just as the inner layers were for imaging. Following that, ultraviolet light or a laser will expose the photoresist, except a reverse image will be exposed instead. For these surface layers, all the photoresist will be hardened except for the metal circuitry patterns. Once those areas of circuitry have their pliable coating of photoresist washed off, the thin layer of copper foil underneath is exposed.
Now the exposed copper circuitry on the surface layers of the board will be electroplated with a thick layer of copper, building up the thin copper foil. After copper plating, the circuitry is plated again with a layer of tin to protect the copper during the final etching process. The hardened photoresist on the rest of the board surface is cleaned off, and the unprotected copper foil is etched away, leaving only the tin-protected copper circuitry. Once the tin is removed, the board is ready for its final processing. It’s important to note that etching is only one part of the overall process of fabricating the board.
Ready for PCB Assembly
The raw circuit board is built at this point, but some final steps need to be completed. Protective surface finishes will be plated to the exposed copper, and a solder mask applied across the board’s surface. Silkscreen reference designators and other markings will be added as well. The board will also go through a series of tests and inspections to ensure that there aren’t any electrical shorts between nets before being sent back to the PCB CM for component assembly.
As you can see, PCB fabrication consists of many precise processes to build each board per its requirements. The PCB etching process is applied differently depending on which portion of the board is being built and is carefully monitored to ensure the trace widths and spacings are within their specified tolerances. PCB CMs understand these needs and will choose the best fabrication partner for the job. At VSE, we have our own list of fabricators that we work with based on their capabilities and experience to ensure that your board is built at the highest level of quality.
