The PCB Manufacturing Process
PCBs are the basis of all electronics designs. They allow complex designs to be manufactured in a reliable and repeatable way. While the manufacture of PCBs is typically handled by a PCB fabricator, it is important for PCB designers to understand the manufacturing process. With an understanding of the process a designer has an insight into some of the opportunities for errors to be introduced into their PCB designs and how to avoid them. Furthermore, PCB designers can also benefit from understanding all of the capabilities of modern PCB fabricators and take advantage of these capabilities when tackling their next PCB based application.
Broadly, this article will discuss how a PCB goes from being output by PCB design software to being manufactured into a physical PCB by a PCB fabricator. As an initial point of reference, let us first look at a diagram of a four layered PCB board:
Starting from the center of our four layered board PCB diagram and proceeding out: The PCB is based on a fiberglass substrate, with copper layers stacked on top of the substrate and further copper layers separated by additional layers of substrate. Finally, the board is terminated on both sides in a solder mask layer. The substrate provides the PCB with mechanical stability, the copper layer functions as the conductive portion of the board and the solder mask protects the copper from shorting to the outside environment. Additionally, vias are used to connect copper traces from different layers of the design. The solder mask is also typically marked with informative marking on a layer that is called the silkscreen. Markings on the silkscreen include the names and references of components as well as information about the board revision and manufacturer.
With the general description of a multilayer PCB board in hand, we can continue to discussing how a PCB is taken from its basic components and made into a final product:
1.Design Files are Sent to a Fabricator
Once PCB design files have been submitted to a PCB fabricator, the first step in the PCB manufacturing process is running a design rule check on these design files. Commonly called Design for Manufacture (DFM) check, when a design is received by a PCB fabricator from a customer, the fabricator will check the design files to see if they conform to the minimum tolerances of his manufacturing process. Some common checks include checking a design for minimum trace width, trace spacing, minimum drill hole sizes and board edge spacing. The file standard for designs is extended Gerber, which is an industry standard file format for the description of PCBs. Almost all PCB design software is capable of producing Gerber Files, so they can be sent to the fabrication houses the designer chooses for manufacture.
2.Turn PCB Design Files into Photo Films
Once the PCB design files have been confirmed they are then sent to a plotter, which is essentially a specialty laser printer, to generate photo-films. Photo-films start as clear plastic sheets. The plotters then print the PCB designs onto the photo-films with black ink. When completed, the photo-films for the inner layers of the PCB are essentially negatives of the PCB design, where the portion of the design that is going to be copper is black and the portion that is not conducting is clear. The Photo-films used on the outer layers of the PCB are the opposite. On the Photo-films for the outer layers the board the layer where copper is to remain is clear and the portion where copper is to be removed is black. Photo films are made for each layer of the PCB design and the solder mask, so a design of two layers would have four layers of photo-films printed, a four layer design would have six printed and so forth. The last step in the production of the photo-films is to punch them so that they can be precisely aligned in later steps of the PCB manufacturing process.
3.Printing Designs on the Copper Clad Substrate (Inner layers)
This step (along with 4, 5 and 6) is only completed when we have more than a two layer board. In the event that we are manufacturing a two layer board we would skip to step 7 (drilling).
With the photo films in hand, we can now turn our attention to the substrate and base copper layers of the PCB board that is going to be produced. Blank copper clad fiberglass boards are the basis of the vast majority of PCB designs. These copper clad panels are initially cleaned and then coated with a photo resist layer. The photo resist layer is a layer of photo reactive chemicals that harden when exposed to ultra violet light. Once the photo resist layer has been installed on the copper clad substrates, the photo films are placed onto the board and the board is exposed to an ultra violet light source. The portions of the photo film that are opaque prevent the photo resist layer from hardening, while the clear portions allow for the resist to harden. After the ultra violet light exposure portion of the process is complete, the photo films are removed from the board and then the board is washed with an alkaline solution to remove the unhardened photo resist. What is left at the end of this process is a copper clad board with resist over the portions of the board that are to remain copper in the final design.
4.Etching Inner Layer Copper
Once we have the resist layer printed on the copper, we can now proceed to remove the unwanted copper portion of the board. This is done by exposing the copper board with resist to powerful copper solvent solutions which remove all of the copper from the fiber glass substrate that was not covered in resist during the previous step. It is of note that different weights of copper require different amounts of exposure to copper solvents, which in turn, can indicate different track spacing requirements. With the unwanted copper removed, the resist that protected the desired copper can now be removed. The result is the substrate with the desired copper layer or layers.
5.Inner Layer Registration and Optical Inspection
Once the substrate with inner layers has been produced, the resulting product is given alignment punches to allow it to be aligned to other layers in the process correctly. The copper layers in the resulting process can also be inspected for correctness at this point. This sort of inspection is typically done using an optical inspection system, which compares the original design files to the actual copper traces produced by the etching process.
6.Lay-up and Bonding of Outer Layer
At this point, if your design has more than two layers, additional layers must be bonded to the substrate. This is done by laying what is called prepreg (short for pre-impregnated) and a copper foil on the top and bottom of the original substrate, with it’s now etched copper traces. Prepreg is essentially fiber glass with epoxy impregnated into its structure. The substrate/prepreg/copper stack up must now be bonded together. This is done by placing layers in a metal clamp and heating them while they are under pressure. This is typically achieved in specialty bonding presses, which can heat and cool the layers in the correct fashion as they press the layers together, to insure that they are well bonded.
7.Drilling the PCB
Once we have the blank outer layers of the PCB in place, we can proceed with drilling out all of the required holes in the PCB design. The drilling is done with a computer controlled drilling machine. The drilling machine takes the drilling file from the submitted design files and places drill holes accordingly. The copper stack up is placed in the drilling machine and aligned to insure that the drill holes are properly placed. Entry and exit material is used to insure that the drill holes do not mare the copper during the drilling process. Finally, excess copper is cut off from the edges of the production panel using a profiling tool. The drill holes from this step in the manufacturing process will become the vias and mechanical mounting holes of the design, once they are plated later in the process.
With the drill holes in place in our panel, we now proceed in plating these holes to connect the different layers of the design together. This plating process is done via a chemical deposition process. The drilled panel is cleaned and then dipped into a series of chemical baths, which results in a very thin layer of copper being plated on all of the holes of the design and coincidentally the outer layer of copper of the panel.
9.Image the Outer Layers
The next step in the process is imaging of the outer layers of the copper stack-up. Once again a layer of photo resist is applied to the outer copper on the panel. The photo films with the outer layers of the design printed on them are then used to preferentially expose portions of the PCB where copper will not remain to ultra violet light. This is the opposite of the inner layers, where the portion of the board that is exposed is the portion that will not remain. The result is the board with resist covering all the areas that will eventually be removed.
Now the board will go through the electro-plating process. As the board now stands, the exposed portions of the board are the portions of the board that were left exposed in our last step and the previously chemically plated copper through holes. Once the initial copper electro-plating is complete then the board is typically plated with tin. This will allow for the removal of all of the unwanted copper that is still remaining on the board, while the tin protects the portion of the board that we want to remain during the final copper removal process.
Our board now has a layer of resist along with the tin plated copper traces that we want to remain. The next step in the process is to remove the resist layer and finally the unwanted copper that is left under the resist. This is done by a chemical process which removes the exposed copper, but does not remove the tin plated portion of the outer layers. At the end of this step we have all of our conducting areas and connections in place.
12.Apply Solder Mask
The next step in the process is to apply a layer of solder mask to each side of the board. Panels must first be cleaned and then coated with an epoxy solder mask ink. The boards are then once again exposed to UV light through a solder mask Photo Film. The portions of the board that are not exposed to UV light are left soft and are therefore able to be chemically removed later in the process. Once the unwanted solder mask has been removed the PCB is further cured in an oven to insure the solder mask stays intact for the life of the PCB.
Now that we have the tin finished exposed copper pads, we further plate the exposed PCB pads for solder-ability. Commonly PCBs are chemically plated with Gold or Silver. PCBs can also be supplied with the pads having undergone a hot air leveling process. The hot air leveling uses hot air to insure that the pads are all manufactured to a highly similar and precise depth.
Now that the board is largely complete it is time to apply a silk screen layer to the board. The silk screen indicates which components go where during assembly and how they are oriented. While this layer is commonly referred to as the silk screen layer, it is no longer commonly implemented using a silkscreen. Rather, it is typically printed directly onto the PCB using an ink-jet type process. Once the silkscreen has been placed on the board, the board goes through a final coating and curing process.
With a complete board in hand, the board can be electrically tested. This is typically done through an automated process in which the integrity of the different nets of the design is tested. This is automated, by having the original design file define the location of the nets to be tested and then having a “flying-probe” test that the different nets of the design are in fact isolated.
16.Profiling and V Scoring
The last step in the manufacturing process is mechanically cutting out the different boards housed in the original panel. This can be done with a router which leaves small tabs along the board edges or with a v-groove which cuts diagonal channels along both sides of the board. Both approaches allow for the individual units of the boards to be snapped out of the panel manually. Quite often the boards will remain in a panel all the way through assembly.
Hopefully this article help illuminate the PCB manufacturing process. While the details of the process can change somewhat from manufacturer to manufacturer, the process used will closely resemble that outlined above.