Printed circuit board manufacturing aims to deliver a completely assembled and functional circuit board quickly and for the best price. Accomplishing this task requires coordinating many pieces of the manufacturing puzzle, from part selection to final QA, while avoiding several potential design pitfalls. Foremost among these difficulties is a board that lacks manufacturability – not only is this likely a cost adder by a failure to adhere to the tried-and-true production methods, but it can also reduce the yield and reliability of PCBAs. Designers must meet the action items outlined in design documents without sacrificing the manufacturability (or if the design is uncompromising, understanding the yield will fall and cost will rise proportionally). Ultimately, designers and manufacturers must collaborate to reduce the incidence and prominence of PCB defects with an approach that prevents their occurrence in the first place.
Action Plan for Common PCB Defects Occurring During Assembly
| Type | Issue | Rectification |
|---|---|---|
| Open solder joint | No/poor continuity; the connection may only form during operation when the board heats and the solder joint partially remelts (if solder is present). | Manual resoldering if the issue is present in a few locations; lots of open solder joints should be grounds for rework and potential redesign. |
| Solder bridging | Unintentional solder formation between pads can cause damaging shorts or poor/unexpected performance. | Ensure the temperature profile during soldering does not have an excessive temperature ramp that could cause sputtering. |
| Component shifting | Component movement during automated placement or soldering can impact the integrity of solder joint formation or completely disrupt the connection. | Create a thermal balance for two-pin components to prevent tombstoning; for more general cases, check automated placement for excessive motion. |
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The Bottom Line of PCB Defects
Indeed, circuit board assembly defects may be less problematic for prototype designs or smaller volumes of boards. Making a few manual corrections here and there may be frustrating, but those alterations can be more efficient than redesigning the board. However, larger production runs do not have this luxury. Assembly defects can severely impact a project in these three areas:
- Time – Assembly defects can impact production schedules with unexpected delays. It will take time to replace incorrectly ordered parts, repair assembly and soldering defects, and manually assemble parts (as opposed to automation).
- Expense – The delays and the extra work will also increase 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.
Designing the board for error-free production is crucial 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 some areas of common assembly defects.
Common PCB Defects During Assembly
Component Issues
A bill of materials (BOM) that contains incorrect data can lead to incorrect part orders and installation. Additionally, BOMs can collect outdated information and specify parts unavailable for manufacturing. Before a circuit board departs for manufacturing, review its BOM for the following:
- Part numbers that match their descriptions
- PCB footprints that are correctly associated with their parts
- Parts that may be unavailable due to being obsolete (OBS), not recommended for new designs (NRND), or end-of-life (EOL)
Incorrect PCB Footprints
A review of the PCB footprints shouldn’t stop with just the BOM. Check that the footprint is built correctly according to industry standards and the manufacturer’s datasheet. This scrutiny should include the footprint pads to ensure the part will sit down and solder correctly to the circuit board.
Design for Manufacturability Concerns
DFM is the most vital criterion for producing a circuit board. Ensure that the component placement has the necessary spacing between each other and the edge of the board or other mechanical board objects. A lack of space may complicate automated assembly, adding unnecessary time and cost to PCBA manufacturing.
Soldering
Components require a minimum spacing for automated pick-and-place machines and proper orientation to avoid soldering defects. The parts should be rotated for wave soldering to enter the wave perpendicular to it. Also, do not place taller components where they will precede 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.
Lack of Testability
After soldering, PCBs are run through automated testing to verify the strength and connectivity of all their pins. Because testing is necessary for every board due to quality control measures, the ability to test quickly and accurately is a high priority. A test point is necessary for each net on the board; locational accessibility is key so that 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 test point with the rest of the manufacturing data.
Poor Manual Reworkability
Without sufficient space to work, rework 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 easily accessible.
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 while ensuring all necessary manufacturing documentation is present. Examining the bill of materials for mismatches or out-of-date parts can significantly reduce assembly headaches by vetting part availability and suitability before procurement.
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 defects while streamlining designs for full production. Finally, engaging with a PCB CM early in the design phase avoids many later problems resulting in assembly defects. At VSE, we have worked 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 time and money.
