**Common Defects and Solutions in the PCB Dispensing Process**
1. **Wire Drawing/Tailing**
1.1 **Definition and Causes**
Wire drawing or tailing is a common defect in the PCB dispensing process. It occurs when excess adhesive forms strings or tails after the dispensing nozzle is lifted. The primary causes of this defect include:
– **Small nozzle diameter**: A nozzle with an insufficient inner diameter may cause excessive backpressure, leading to irregular dispensing.
– **High dispensing pressure**: Excessive pressure can cause the glue to flow uncontrollably, resulting in strings.
– **Excessive nozzle-to-PCB distance**: If the dispensing nozzle is too far from the PCB surface, it can lead to uneven adhesive deposition.
– **Aging or poor-quality adhesive**: Adhesive that is either out of date or of low quality may have inconsistent flow properties.
– **High adhesive viscosity**: If the adhesive is too thick, it will be difficult to control, causing irregular flow.
– **Cold adhesive**: When adhesive is taken out of cold storage but not allowed to reach room temperature before dispensing, it can behave unpredictably.
– **Excessive adhesive volume**: Dispensing too much adhesive at once can lead to messy deposition.
1.2 **Solutions**
– **Larger nozzle**: Switching to a nozzle with a larger inner diameter can help reduce backpressure and improve flow consistency.
– **Reduce dispensing pressure**: Lowering the dispensing pressure can prevent the formation of strings and tails.
– **Adjust nozzle height**: Fine-tune the “stop” height of the dispensing nozzle to maintain a consistent distance from the PCB.
– **Use fresh, high-quality adhesive**: Ensure the adhesive is within its shelf life and has been properly stored. Select adhesives with suitable viscosities for the process.
– **Allow adhesive to reach room temperature**: Before dispensing, allow refrigerated adhesives to warm up to room temperature (typically around 4 hours) to ensure proper flow.
– **Control adhesive volume**: Dispense the appropriate amount of adhesive to avoid overapplication, which can lead to defects.
2. **Glue Nozzle Blockage**
Blockages in the glue nozzle can significantly disrupt the dispensing process. This issue is often caused by:
– **Cured adhesive buildup**: If the nozzle is not cleaned regularly, hardened adhesive can accumulate and block the flow.
– **Viscosity mismatch**: High-viscosity adhesives may become more prone to clogging, especially if not handled correctly.
– **Improper storage**: Adhesive that is not stored properly can develop clumps or solidify, leading to nozzle blockages.
**Solutions**
– **Regular cleaning**: Clean nozzles frequently to remove any accumulated adhesive and prevent blockages.
– **Use low-viscosity adhesives**: Where possible, select adhesives with lower viscosities to minimize the risk of clogging.
– **Proper storage**: Ensure adhesives are stored according to the manufacturer’s recommendations to prevent premature curing or thickening.
By identifying the causes of common dispensing defects and implementing these solutions, PCB assembly processes can become more efficient, leading to higher quality products and fewer production delays.
**2.1 Insufficient or Missing Glue Output**
*Phenomenon*: The glue amount dispensed from the nozzle is either too small or completely absent.
*Cause*: Common causes include incomplete cleaning of the pinhole, contamination of the patch glue with impurities, nozzle clogging, or mixing of incompatible glues.
**Solution**:
– Replace the needle with a clean one.
– Use high-quality patch glue, ensuring compatibility with the machine and process.
– Avoid mixing different brands of patch glue to ensure consistency and reliability.
**3. Glue Dispensing Without Output (Empty Play)**
*Phenomenon*: The dispensing process initiates, but no glue is dispensed.
*Cause*: This can occur when air bubbles are present in the patch glue or when the glue nozzle is clogged.
**Solution**:
– Remove air bubbles from the glue in the injection cylinder, particularly if you manually loaded the glue.
– Replace the glue nozzle to ensure consistent and uninterrupted dispensing.
**4. PCB Component Shift After Curing**
*Phenomenon*: After curing, components shift, and in extreme cases, the component pins no longer align with the pads.
*Cause*:
– Uneven glue application, such as one glue point being significantly more or less than the other.
– The components may have shifted during the patching process, or the initial adhesion of the glue was too low.
– The PCB was left for too long after dispensing, leading to semi-curing of the glue.
**Solution**:
– Inspect the glue nozzle for blockages and eliminate uneven dispensing.
– Adjust the placement machine’s working parameters to ensure proper glue application.
– Use a better quality or appropriate glue.
– Ensure the PCB is not left idle for extended periods after dispensing (preferably less than 4 hours).
**5. Component Detachment After Wave Soldering**
*Phenomenon*: After curing, components exhibit insufficient bonding strength, sometimes even leading to chipping when touched.
*Cause*:
– Incorrect curing parameters, especially an insufficient curing temperature.
– Large PCB components absorbing excessive heat.
– Aging of the UV curing lamp.
– Insufficient glue applied.
– Contamination on components or the PCB surface.
**Solution**:
– Adjust the curing process parameters, particularly increasing the curing temperature. For thermal curing adhesives, ensure the peak temperature reaches approximately 150°C to avoid adhesive failure.
– For light-curing adhesives, inspect the UV lamp for aging or blackened tubes, as these reduce curing efficiency.
– Ensure adequate glue quantity and confirm there is no contamination on the PCB or components.
**6. Floating or Shifting of Component Pins After Curing**
*Phenomenon*: Component pins float or shift after curing, potentially causing soldering issues like short circuits or open circuits due to solder entering beneath the pad.
*Cause*:
– Uneven, excessive, or misaligned glue application during dispensing.
– Component deviation during the patching process.
**Solution**:
– Adjust the dispensing process parameters to ensure consistent glue output.
– Control the dispensing volume to avoid excess glue.
– Refine the patching process to ensure precise alignment and adhesion.
1. **Wire Drawing/Tailing**
1.1 **Definition and Causes**
Wire drawing or tailing is a common defect in the PCB dispensing process. It occurs when excess adhesive forms strings or tails after the dispensing nozzle is lifted. The primary causes of this defect include:
– **Small nozzle diameter**: A nozzle with an insufficient inner diameter may cause excessive backpressure, leading to irregular dispensing.
– **High dispensing pressure**: Excessive pressure can cause the glue to flow uncontrollably, resulting in strings.
– **Excessive nozzle-to-PCB distance**: If the dispensing nozzle is too far from the PCB surface, it can lead to uneven adhesive deposition.
– **Aging or poor-quality adhesive**: Adhesive that is either out of date or of low quality may have inconsistent flow properties.
– **High adhesive viscosity**: If the adhesive is too thick, it will be difficult to control, causing irregular flow.
– **Cold adhesive**: When adhesive is taken out of cold storage but not allowed to reach room temperature before dispensing, it can behave unpredictably.
– **Excessive adhesive volume**: Dispensing too much adhesive at once can lead to messy deposition.
1.2 **Solutions**
– **Larger nozzle**: Switching to a nozzle with a larger inner diameter can help reduce backpressure and improve flow consistency.
– **Reduce dispensing pressure**: Lowering the dispensing pressure can prevent the formation of strings and tails.
– **Adjust nozzle height**: Fine-tune the “stop” height of the dispensing nozzle to maintain a consistent distance from the PCB.
– **Use fresh, high-quality adhesive**: Ensure the adhesive is within its shelf life and has been properly stored. Select adhesives with suitable viscosities for the process.
– **Allow adhesive to reach room temperature**: Before dispensing, allow refrigerated adhesives to warm up to room temperature (typically around 4 hours) to ensure proper flow.
– **Control adhesive volume**: Dispense the appropriate amount of adhesive to avoid overapplication, which can lead to defects.
2. **Glue Nozzle Blockage**
Blockages in the glue nozzle can significantly disrupt the dispensing process. This issue is often caused by:
– **Cured adhesive buildup**: If the nozzle is not cleaned regularly, hardened adhesive can accumulate and block the flow.
– **Viscosity mismatch**: High-viscosity adhesives may become more prone to clogging, especially if not handled correctly.
– **Improper storage**: Adhesive that is not stored properly can develop clumps or solidify, leading to nozzle blockages.
**Solutions**
– **Regular cleaning**: Clean nozzles frequently to remove any accumulated adhesive and prevent blockages.
– **Use low-viscosity adhesives**: Where possible, select adhesives with lower viscosities to minimize the risk of clogging.
– **Proper storage**: Ensure adhesives are stored according to the manufacturer’s recommendations to prevent premature curing or thickening.
By identifying the causes of common dispensing defects and implementing these solutions, PCB assembly processes can become more efficient, leading to higher quality products and fewer production delays.
**2.1 Insufficient or Missing Glue Output**
*Phenomenon*: The glue amount dispensed from the nozzle is either too small or completely absent.
*Cause*: Common causes include incomplete cleaning of the pinhole, contamination of the patch glue with impurities, nozzle clogging, or mixing of incompatible glues.
**Solution**:
– Replace the needle with a clean one.
– Use high-quality patch glue, ensuring compatibility with the machine and process.
– Avoid mixing different brands of patch glue to ensure consistency and reliability.
**3. Glue Dispensing Without Output (Empty Play)**
*Phenomenon*: The dispensing process initiates, but no glue is dispensed.
*Cause*: This can occur when air bubbles are present in the patch glue or when the glue nozzle is clogged.
**Solution**:
– Remove air bubbles from the glue in the injection cylinder, particularly if you manually loaded the glue.
– Replace the glue nozzle to ensure consistent and uninterrupted dispensing.
**4. PCB Component Shift After Curing**
*Phenomenon*: After curing, components shift, and in extreme cases, the component pins no longer align with the pads.
*Cause*:
– Uneven glue application, such as one glue point being significantly more or less than the other.
– The components may have shifted during the patching process, or the initial adhesion of the glue was too low.
– The PCB was left for too long after dispensing, leading to semi-curing of the glue.
**Solution**:
– Inspect the glue nozzle for blockages and eliminate uneven dispensing.
– Adjust the placement machine’s working parameters to ensure proper glue application.
– Use a better quality or appropriate glue.
– Ensure the PCB is not left idle for extended periods after dispensing (preferably less than 4 hours).
**5. Component Detachment After Wave Soldering**
*Phenomenon*: After curing, components exhibit insufficient bonding strength, sometimes even leading to chipping when touched.
*Cause*:
– Incorrect curing parameters, especially an insufficient curing temperature.
– Large PCB components absorbing excessive heat.
– Aging of the UV curing lamp.
– Insufficient glue applied.
– Contamination on components or the PCB surface.
**Solution**:
– Adjust the curing process parameters, particularly increasing the curing temperature. For thermal curing adhesives, ensure the peak temperature reaches approximately 150°C to avoid adhesive failure.
– For light-curing adhesives, inspect the UV lamp for aging or blackened tubes, as these reduce curing efficiency.
– Ensure adequate glue quantity and confirm there is no contamination on the PCB or components.
**6. Floating or Shifting of Component Pins After Curing**
*Phenomenon*: Component pins float or shift after curing, potentially causing soldering issues like short circuits or open circuits due to solder entering beneath the pad.
*Cause*:
– Uneven, excessive, or misaligned glue application during dispensing.
– Component deviation during the patching process.
**Solution**:
– Adjust the dispensing process parameters to ensure consistent glue output.
– Control the dispensing volume to avoid excess glue.
– Refine the patching process to ensure precise alignment and adhesion.
