Printed circuit board manufacturing aims to deliver a completely assembled and functional circuit board in the least amount of time and for the best price. To accomplish this requires coordinating many different pieces of the manufacturing puzzle from part selection to final QA while avoiding several potential design pitfalls along the way. Specifically, the following six areas can cause problems for assembly if not given due consideration:
- Incomplete or incorrect bill of materials information
- Non-standard PCB footprints
- Automated assembly errors
- Defects in the soldering processes
- Incomplete test coverage of the circuit board
- Inadequate access for rework or final assembly
To find out more about circuit board manufacturing problems and how to guard against them in PCB design, here’s a closer look at these six common PCB assembly defects.
PCB Assembly Problems and How They Will Affect You
Indeed, circuit board assembly defects may not be as problematic for prototype designs or smaller volumes boards. It may be frustrating to make a few manual corrections here and there, but those alterations can be more efficient than redesigning the board. For larger production runs, however, this is not the case. Assembly defects can have a severe impact on a project in these three areas:
- Time: Assembly defects can impact your schedule with unexpected delays. It will take time to replace incorrectly ordered parts, repair assembly, and soldering defects, and manually assemble parts that should have been installed automatically.
- Expense: The delays and the extra work will also impact your project expenses. There will be additional charges for board re-work labor and for re-ordering faulty parts.
- Performance: Undetected assembly defects may also cause unexpected performance problems with the circuit board or outright failures that must be debugged and corrected.
It is crucial to design the board for error-free production to eliminate the unexpected time and expense associated with assembly defects. To do that means staying up-to-date and familiar with clean design by watching for these six areas of common assembly defects.
The Top Six Common PCB Assembly Defects
1. BOM problems:
Bill of materials that contain incorrect data can cause the wrong parts to be ordered and installed on the board. Additionally, BOMs can collect outdated information and specify parts that are not available for manufacturing. Before a circuit board is sent out for manufacturing, review its BOM for the following:
- Correct parts
- Part numbers that match their descriptions
- PCB footprints that are correctly associated with their parts
- Parts that may not be available due to being obsolete (OBS), recommended for new designs (NRND), or end of life (EOL)
2. Incorrect PCB footprints
As we just stated, it is imperative to check the BOM to make sure that the right footprint is associated with the part. A review of the PCB footprints shouldn’t stop with just the BOM, though. Check that the footprint is built correctly according to industry standards and the manufacturer’s datasheet. This scrutiny should also include the footprint pads to ensure that the part will sit down and solder correctly to the circuit board.
3. DFM errors
Design for manufacturability is one of the most vital criteria for designing a circuit board. Ensure that the components are placed with correct spacing to each other and the edge of the board or other mechanical board objects. Without this spacing, the parts may not be assembled using automated equipment, which will add a lot of time and cost to the manufacturing of the board.
4. Soldering defects
Not only do you need to ensure that the components are spaced correctly for automated pick and place machines, but they also must be arranged correctly to avoid soldering defects. For wave soldering, the parts should be rotated to enter the wave perpendicular to it. Also, do not place taller components where they will proceed shorter parts into the wave as this could create a shadow effect and reduce the strength of the solder joints on the pins. For parts going through the solder reflow oven, ensure that both pins of smaller two-pin discreet parts are thermally balanced. This practice will ensure that one side does not heat faster than the other and pull the part onto that pin in what’s known as tombstoning.
5.Lack of testability
PCBs are run through automated testing to verify the strength and connectivity of all their pins after soldering. To do this means that testing must be built into the board during the design phase. A test point has to be included for each net on the board and located where the probes of a test fixture can access each one without hitting other components. When the design is ready for manufacturing, include a separate file that details the location of each testpoint with the rest of the manufacturing data.
6. Inadequate human access
Without sufficient space to work, re-work technicians won’t be able to change parts, repair other assembly defects, or test the board. This lack of space adds time and expense to PCB assembly as the technicians have to work around the obstacles and risk damaging other components. Additionally, installing the circuit board into its system may be difficult if the connectors and other human interfaces aren’t placed for easy access.
These six areas of assembly defects cause the most pain for PCB manufacturing. Next, let’s take a look at some ways to reduce or eliminate the potential for these defects to happen.
The Best Methods of Eliminating PCB Assembly Defects
During PCB design, take time to review all aspects of the design before releasing it for manufacturing. This review includes providing a readable schematic synchronized with the PCB layout and ensuring that all the necessary manufacturing documentation is included. Also, be sure to examine your bill of materials for miss-matches or out-of-date parts as described above.
Another critical aspect of PCB layout is to follow the PCB contract manufacturer’s recommended design for manufacturability rules. Following DFM rules reduces or eliminates the potential of many PCB assembly defects. Additionally, by creating a manufacturable PCB layout without defects, the design is streamlined for full production. Finally, by engaging with a PCB CM early on in the design phase, many later problems that result in assembly defects are avoided.
At VSE, we have been working with PCB design innovators for many years, and understand the different challenges. Our engineering teams are ready to assist in making the right choices for your design, so it’s error-free during manufacturing, saving both time and money.
