In this series on the common problems associated with circuit boards, we’ve already looked at issues related to design and components. As we have seen, many different aspects of design and components can affect the final circuit board. The next set of problems we are going to explore is the fabrication of the raw circuit board. How the PCB is designed can affect the fabrication process, but specific issues can also arise due to improper fabrication processes.
The production of a high-quality bare fab circuit board is essential to the successful assembly of the final product. We’re going to start with some of the differences you can expect between different circuit board fabrication shops and how to choose between them. Then we will explore some of the more common circuit board problems with fabrication and what you can do to ensure the highest quality in your next PCB project.
Which PCB Fabricator Should You Choose?
When choosing a printed circuit board fabricator to work with, the first step is to determine your needs. Will you be building a prototype only, or are you planning on transitioning that prototype to regular production? If you are building a production run of circuit boards, what kinds of quantities will you need over what time span?
The next step is to start researching circuit board fab shops according to the technologies and complexities they build. For instance, some shops will only build boards with limited layer counts, while others will handle large multi-layer configurations. The same goes for circuit density, trace widths, and hole counts. Some shops may not handle the materials necessary for high-frequency designs, while others may not be set up for large backplanes. The important thing here is to verify that the circuit board fabricator you eventually choose will serve your needs.
With the board shop selection taken care of, the next step is to ensure that your board is built to the highest quality. For that, let’s take a look at some of the more common PCB fabrication problems.
Six Common Printed Circuit Board Problems with Fabrication
Here are some of the problems you should be aware of that can happen to a circuit board while being fabricated and how designers can avoid some of those problems.
1. Shorts and opens
A short is when the copper of different nets unexpectedly comes into contact with each other, while an open is when the continuous copper of a single net is broken. These problems can be caused by the following:
- Improper fabrication techniques: The etching process needs to be tightly controlled by the fabricator to prevent under or over-etching of the copper. This problem can result in broken traces or copper that shorts to other copper. The boards must also be thoroughly cleaned after etching to remove any chemicals that may deteriorate the copper.
- Design errors: Many fabrication problems can create opens or shots related to design. These problems include creating acid traps with acute or odd angles of metal that trap chemicals that continue to erode the copper, slivers, and islands of free-floating copper, which can detach and short other nets. Incorrect trace widths and spacing can cause traces to under etch and break during fabrication.
2. Electrical performance failures
While a short or an open is an undeniable failure, circuit boards can also suffer from fabrication issues that will degrade their ultimate electrical performance:
- Trace degradation: Like an open or short, under etching a trace can alter its desired shape creating more of a mushroom when viewed as a cross-section. Not only does this lessen its structural strength, but it can introduce signal integrity problems into the board by changing the characteristic impedance of the trace.
- Moisture: Residual moisture not removed after fabrication can create condensation and eventual corrosion of the board while packaged. This type of corrosion can also affect the characteristics of the PCB traces.
- Oxidation: Copper traces and pads that oxidize will affect the characteristics of the traces during board operation and can also impact how well the parts will solder during assembly.
3. Incorrect controlled impedance routing parameters
High-speed designs rely on controlled impedance routing for some of their sensitive signals. Controlled impedance trace routing requires precise line widths and spacing values and specific board layer configurations, materials, and dielectric thicknesses. If these parameters are not adhered to during fabrication or are specified incorrectly by design, the board may suffer electrical performance problems. The controlled impedance signals may reflect or create other signal integrity problems and degrade the board’s performance.
4. Inadequate board materials
The different variations of standard epoxy resin (FR-4) substrate materials used for building circuit boards are satisfactory for most projects. However, some high-frequency applications or extreme environmental operating conditions, including heat, vibration, shock, and pressure, may require the use of more exotic materials. Some of the problems of inadequate board materials include degraded electrical performance and delamination of the board’s copper layers from the dielectric material. Engineers must make board material decisions before the design starts, as the material’s physical characteristics will alter how the design’s layout will affect its optimum performance.
5. Warp and twist
Circuit board stackups that are not symmetrical can create problems for PCB fabrication. Dense copper on one side of the board can overwhelm the board’s bare areas leading to warping during fabrication. This unevenness of copper can also cause problems during PCB assembly, where the high heat of soldering can twist the board as it is processed. For circuit boards with dense areas of copper, designers should balance the design by adding copper fills to the bare areas of the board.
6. Increased cost
Circuit board designers are not always aware of how their decisions can impact the fabrication of the board. While technically not a “fabrication problem,” increased costs and production time can create unexpected problems downstream for the project. For instance, not every board needs the narrowest trace widths and spacing or blind and buried vias. These elements can pile up the fabrication costs for your design, and in some cases, increase the amount of time it will take to build the board.
After reviewing all of these different fabrication problems, the next question would naturally be, “how can I avoid all of this trouble?” One of the best answers is to plug into a resource that understands PCB fabrication and has a lot of experience working with circuit board fab shops.
Use Your PCB CM to Help Avoid Circuit Board Fabrication Problems
Your local PCB contract manufacturer has experience working with PCB fabricators. PCB CMs schedule boards to be built by fab shops every day and have a network of preferred shops that they use depending on the board’s technology and complexity requirements.
When VSE assembles your circuit boards, we will schedule the fabrication of the boards. We have a list of approved vendors that we regularly work with, and will choose the right vendor for your project based on your needs. Our engineers will work with you to determine the circuit density, board materials, signal integrity requirements, and PCB technology type to provide the best fabrication shop fit for your board.