How to avoid PCB Assembly Defects

Minimization of all solder joint defects should be the goal for any SMT manufacturer. Through understanding the defect, its root causes, as well as how to prevent it, you can greatly improve the quality of all assemblies you manufacture. According to industry statistics, the top 3 PCB assembly defects which account for 74% of all manufacturing defects are Opens, Solder bridging, and Component shift.

Component Shift  15%

Component shift may be described as the misalignment of an item to its target. Component shift may occur during reflow due to the components ability to float on the molten solder. Components with many pads such as BGA components may realign themselves due to the surface tension of the molten solder, however many times the components will remain where they are placed, which is a good reason to make sure that components are placed exactly where they are supposed to be in the middle of the pad or land areas.

Root causes

1. A mismatch in part to pad geometry will cause a part to pull towards the closest thermal mass.

2. Misshapen & bent leads.

3. Asymmetrical component heat sinks.

4. Oxidation of component leads

5. Excessive vibration or rapid speed changes in the conveyor system.

6. Poor solder placement (Wrong location [off the pads] or incorrect volume).

7. Convection rate too high.

8. Heating ramp rate too high causing flux to outgas.

9. Small components placed next to large ones where the heated gas is directed from the side of big component toward the smaller ones.

Prevention

1. Abide by recommended temperature and humidity requirements.

2. Improve the accuracy of component placement in pick and place machines.

3. Minimize the amount of movement the unreflowed assembly sees.

4. Improve the solderability of the components or PCBs by using a more aggressive flux.

Solder bridges (shorts) 15%

Shorts, sometimes referred to as Solder Bridging, occur when solder is improperly crossing and connecting one lead to another. This short can be microscopic in size and extremely hard to detect. If a short goes undetected, it can cause serious damage to the circuit assembly, like a burn-up or blow-up of a component and/or burn-out PCB trace.

Root Causes

1. Reflow profile not suitable i.e. Initial ramp rates too steep.

2. Soldering pads too big relative to gap between pads

3. Too much solder on the pads due to incorrect stencil specification.

4. Solder paste not active enough

5. Bad seal between the stencil and the board during printing.

6. Mismatch between the stencil and PCB

7. Poor component placement or poor component lead to PCB pad size relationship.

Prevention

1. Use the appropriate reflow profile.

2. Use the appropriate solder paste metal to flux weight ratio for the appropriate application.

3. Ensure paste deposition is in good resolution and quality without slump or smear prior to reflow.

4. Reduce stencil aperture dimensions by 10% or reduce the thickness of the stencil to reduce the amount of solder paste being deposited.

5. Pay close attention to alignment of stencil apertures to pads.

6. Ensure proper pressure and accuracy for component placement

Opens 34%

Open Solder Joints, occurs when there is no bonding between the lead and pad, or other point of connection on a PCB, causing an open connection. It can also occur when solder appears only on the PCB pad while no solder at component lead.

Root Causes

1. Lace of solder paste or inconsistency of solder paste deposit.

2. Gap between PCB and the component lead.

3. Contamination or corrosion at component’s lead

4. Solder paste not active enough

5. Poor reflow profile does not allow all surfaces to come up to reflow temperature.

Prevention

1. Ensure a correct aspect ratio,Which is defined as the ratio of aperture width to stencil thickness.

2. Investigate lead co-planarity issues and monitor operator material handling procedures.

3. Avoid solder paste contamination by avoiding extreme environmental effects in the manufacturing process.

4. Investigate fabrication issues with PCB supplier.

Conclusion

Failure in a PCB assembly is dependent on whether or not the assembly is considered a class 2 or class 3 products. IPC standards state that Class 2 products are expected to maintain continued performance and extended life is required, however uninterrupted service is desired but not critical. Class 3 products however, demand continued high performance or performance-on-demand is critical, and equipment downtime is unacceptable. Additionally, class 3 products may need to function in extreme environments, and the equipment must function when required, such as in life support or other critical systems. In either case, ensuring every assembly is of the highest quality and is in proper working condition before it leaves your shop will help to prevent latent failure in the field.

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