As an eminent SMT assembler, PCB Trace Technologies Inc. also aims to minimize all types of defects in PCB assembly. To realize and sustain improvements in product quality, we strive to gain an understanding of defects through their root causes. According to our long experience in PCB assembly and related services, the top three assembly defects amounting to almost 75% of all the defects we encounter in PCBs are related to:
- Open Solder Joints (35%)
- Solder Shorts (20%)
- Component Misalignment (20%)
When customers choose to get their PCBs assembled by machines rather than by human hands, it is because they expect their assembled PCB to be free from defects. However, PCB assembly almost always tends to perfection, without ever reaching it. Even with the state-of-the-art equipment, a minor percentage of the assembly might occasionally face quality issues. This is because machines can develop unexpected faults and/or encounter unexpected errors that lead to quality issues.
We always try to be aware of these issues to be able to help our customers minimize or prevent these defects by optimizing our throughput. This is how we analyze assembly defects:
Open Solder Joints
An open solder joint on a pad on the PCB is almost impossible to detect, unless we are using a magnifier. Usually, all that a visual check does is confirm whether the components are all in their proper places on the PCB. However, it cannot confirm a vertical gap exists between the pad and the component lead, which prevents the component from functioning properly. There can be several root causes that bring about such a situation.
| Basic Defect | Visual Appearance | Probable Root Cause | Remedy |
| Open Solder Joint | Vertical gap between component lead and pad | Component tilt or tombstoning due to asymmetric pads | PCB design change |
| Insufficient Solder | Improper aspect ratio | Stencil design change | |
| Insufficient wetting of component lead | Corrosion or contamination of component lead | Change source or Improve material handling | |
| Open via on pad wicking solder | PCB design change | ||
| Insufficient wetting of PCB pad | Corrosion or contamination of PCB pad | Improve PCB handling and environmental exposure | |
| Dry solder | Wrong reflow profile | Improve reflow profile | |
| Solder paste contamination | Improve solder paste handling and environmental exposure |
Solder Shorts
Solder shorts or bridges are PCB assembly defects occurring when molten solder crosses and connects one lead to another. Solder shorts can cause serious damage when powering up the PCB assembly for electrical testing. Bridges occasionally form between the leads of fine-pitch components, and being tiny, easily escape visual inspection. A variety of factors can cause solder shorts to occur, for instance, wide pads with insufficient gaps. Incorrect stencils depositing large amounts of solder paste is another common cause.
| Basic Defect | Visual Appearance | Probable Root Cause | Remedy |
| Solder Short | Short between two components | Excess paste deposition | Change stencil design |
| Component pads too close | PCB design change | ||
| Short between adjacent pins of fine pitch IC | Excess paste deposition | Change Stencil design | |
| Improper stencil registration | Improve stencil registration | ||
| Pads too large for IC | PCB design change | ||
| Solder smear from stencil | Improve stencil printing process | ||
| Component Shift | Change Pick and place machine accuracy | ||
| Short between adjacent vias | Improper stencil registration | Improve stencil registration | |
| Vias too close | PCB design change | ||
| Short between via/pad and neighboring track | Solder mask opening is too wide | Improve mask printing process | |
| Track to via/pad separation is too small | PCB design change |
Component Misalignment
Visual inspection is adequate for detecting component misalignment during PCB assembly, except for ICs with leads under the package. Physical factors like high vibrations of the conveyor are the major causes of component misalignment, but one cannot rule out bent leads of the component itself and wrong placements.
| Basic Defect | Visual Appearance | Probable Root Cause | Remedy |
| Component misalignment | Some pins of IC are misaligned | Bent pins | Change source or improve material handling |
| Specific IC misaligned | Placement error | Improve Pick and place machine accuracy | |
| All components misaligned by equal amounts and in same direction | Improper stencil registration | Improve Stencil registration | |
| Random component misalignment | Excessive vibration or rapid change in handling system | Reduce conveyor speed and improve handling | |
| Excessing flux outgassing during preheating | Improve reflow profile | ||
| Misalignment due to excess solder | Excess paste deposition | Change stencil design | |
| Misalignment of small components next to a large one | Small components too close to large one | PCB design change |
Preventing PCB Assembly Defects
PCB Trace Technologies Inc. offers various evaluation services to customers when they present their designs for fabrication. These are Design for Assembly or DFA and Design for Manufacturing or DFM. Our experienced engineers and technicians are always ready to help customers with their design.
DFA and DFM can uncover several design related issues that may be the cause for potential problems during assembly. Along with these evaluation services, PCB Trace Technologies Inc. also offers superior visual inspection capabilities. We have automated optical inspection or AOI equipment for comparison of boards during inspection.
For components with leads covered by the body, such as ICs with J-wing and BGAs, AOI is inadequate as the leads and balls are not visible. For such components and for checking defects within multi-layered boards, we use X-ray equipment to scan the balls of solder located between the component and the PCB.
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Reducing Automotive PCB Defect Rates
As shown above, proper design of stencil is imperative to reducing PCB assembly defects. We advise our customers to get stencils designed to match the pad size for ensuring the proper amount of solder paste deposition.
Conclusion
PCB Trace Technologies Inc. advices PCB designers to keep PCB assembly defects in focus while designing. This can help prevent, or even reduce potential issues. Using a proper PCB design software, and using DFA and DFM checklists help in a big way.
