In our ongoing series describing the more common problems associated with printed circuit boards, we have looked at design, component, and fabrication issues. Next, we will turn our attention to some of the problems that can affect the circuit board while going through its assembly process. While some of these issues can be caused by production errors, most trace their origins to how the board was designed. These problems can range from minor annoyances like miss-marked polarity indicators to production show-stoppers like missing components in the bill of materials.
Often, design for manufacturability (DFM) problems originate during PCB layout but aren’t given the attention they deserve. When this happens, it falls on the manufacturer to find the errors, return the design for corrections, or make the alterations themselves. This circumstance could turn into an expensive and time-consuming process that could be avoided if design teams were better prepared in the early stages. To help identify these potential issues and ensure PCB designs are production-ready before they are sent out, here are some common printed circuit board problems in the assembly that you should know.
Eight Common Printed Circuit Board Problems with Assembly
Many circuit board problems during assembly are due to design and layout issues of which designers are unaware. Here are eight of those common problems that can slow down or stop the manufacturing of a PCB.
1. Bill of materials (BOM) problems
Your PCB contract manufacturer will review your BOM before starting on your project, and they will be looking for the following:
- Component availability: Some parts are out of stock or are no longer being manufactured.
- Price: In some cases, the parts you want are costly, and functional alternatives may be available at a much lower price.
- Counterfeits: Some parts do not come as advertised, but the CMs staff is trained to spot these counterfeits and avoid them.
- Supply chain: PCB CMs have a well-developed network of component manufacturers, distributors, and brokers that they can use to work around many of the current supply chain difficulties.
You can avoid delays due to BOM problems by working with your CM early on in the design cycle so they are prepared with the parts when you are ready for manufacturing.
2. Incorrect PCB footprints
If built incorrectly, PCB footprints can create multiple DFM problems, including poor solder joints, mismatched pins, or lack of physical room. It is essential to use industry or manufacturing specifications when building CAD footprint models and ensure they are correctly matched to the part numbers in the schematic.
3. Component placement errors
Components that violate the manufacturer’s DFM rules by being too close together may cause the automated assembly systems difficulties. Pick and place machines as well as wave and reflow soldering require minimum spacing clearances for optimum results. Adequate component spacing is also necessary for manual rework. Getting a soldering iron into an area can be challenging when the component is overshadowed by another larger part. There is also the chance of collateral damage to nearby components trying to fit tools into tight spaces.
4. Soldering defects
DFM rules violations can also create various solder defects in a printed circuit board. Here is a sampling of what may occur:
- Solder bridging: When solder forms a connection, or a short, between two metal conductors that are not intended to be connected, it is known as a solder bridge. This defect can happen if components are incorrectly placed for wave soldering, the solder mask is incorrectly designed and allows too much solder on the pad, or the solder stencil has design or alignment problems.
- Shadowing: Larger parts that proceed smaller parts into the wave solder could shadow the smaller parts and rob them of the solder they need for a good connection.
- Bad solder joints: In thru-hole components, this is often due to insufficient thermal relief pads allowing too much heat to be pulled away from the connection during soldering.
- Tombstoning: In small two-pinned surface mount parts, too much metal on one pin will create a thermal imbalance causing the solder on the other pin to melt faster, and pull the part off the colder pin.
- Sunken joints: Thru-hole pins that are too large for their pins may not retain melted solder as they should, causing an incomplete connection.
5. Thermal issues
Circuit boards with layer stackups that are not symmetrical may experience warping and twisting with the heat applied during soldering operations. These same thermal problems may also adversely affect components that are heat sensitive requiring these parts to be manually assembled later.
6. Lack of testability
PCB manufacturers use circuit board tests to validate the assembly process by ensuring that each component pin has a good connection with their associated net. Without full coverage of test points placed according to design for test (DFT) rules, the tests can not be fully completed, requiring a manual validation process.
7. Inadequate silkscreen markings
Accurate silkscreen markings, like reference designators and polarity indicators, are essential for assembly validation, rework, and test. Test technicians will have a much more time-consuming and challenging job finding the right spots on the board to work on without these markings.
8. Incomplete or unclear assembly documentation
Assembly documentation needs to be complete and up-to-date for the board being built. Drawings need to be readable, and all of the required data and information files need to accompany the design for manufacturing.
Why the Choice of a PCB Contract Manufacturer Matters
As you can see, many potential PCB assembly problems can happen due to how the circuit board is designed. However, with the help of a good circuit board manufacturer, many of these problems can be mitigated. A good PCB CM will work with you throughout the PCB design process to validate your BOM before manufacturing and supply you with current and up-to-date DFM and DFT rules and constraints.
At VSE, we have made working with our customers a top priority to ensure they have the information needed to design a successful board. Our engineering staff is ready to work with you on everything from components selections to PCB layout tactics, and we will help you ensure your design is production-ready. Not only will this save you time and money in the long run, but it will help create the highest quality in the circuit boards we will build for you.
