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The "invisible killer" of solder paste printing: 9 reasons causing short circuits, tombstones, and empty soldering.
The "invisible killer" of solder paste printing: 9 reasons causing short circuits, tombstones, and empty soldering.
In SMT (Surface Mount Technology) soldering processing, more than 60% of the soldering defects can be traced back to the solder paste printing stage. What's more challenging is that many problems do not surface initially - they are often discovered in large quantities as short circuits, voids, or incomplete soldering occur at the end of the reflow soldering process.
These flaws do not suddenly emerge; instead, they are gradually laid down by one "invisible killer" after another.
This article summarizes 9 core reasons that lead to short circuits, monument erection, and empty welding, and provides corresponding troubleshooting directions.
1. The core reason for short circuits
Short circuits are the most obvious and destructive defect in solder paste printing, usually indicating the complete scrapping of the entire board.
Excessive solder paste → Bridge short circuit
Symptom: The solder paste between adjacent pads is connected after reflow soldering.
Root cause:
The opening area of the stencil is too large
The stencil is too thick
The excessive printing pressure causes the solder paste to be squeezed out and overflow
Troubleshooting direction:
Check whether the design of the steel mesh holes complies with the IPC-7525 standard.
For fine pitch devices (0.4mm pitch IC), it is recommended to use a steel mesh with a thickness of 0.10–0.12mm.
Reduce the pressure of the scraper appropriately.
✅ When printing 0.4mm pitch QFP or connectors, the control of solder paste volume is more important than the strength.
2. Solder paste collapse → Bridging occurred before reflow
Description: After the solder paste is released from the steel mesh, it gradually collapses and spreads during the holding or mounting process.
Root cause:
The solder paste viscosity is too low or its thixotropic property is insufficient.
The ambient temperature is too high (>28℃).
The solder paste has not been adequately re-warmed or has expired.
Troubleshooting direction:
Select a solder paste with high visco-elasticity (suitable for fine pitch)
Control the temperature and humidity in the workshop at 23±3℃ / 45–65%RH
Allow the solder paste to warm up for at least 2–4 hours. Do not use it immediately after taking it out.
3. Residual steel mesh → Tin paste contamination
Symptom: After printing several boards, tin paste gradually adheres to the bottom of the steel mesh, causing contamination outside the pads of the next board.
Root cause:
Low frequency of steel mesh cleaning
Inappropriate cleaning mode (inappropriate combination of dry/wet/vacuum)
Insufficient steel mesh tension, not adhering properly to the PCB
Troubleshooting direction:
Suggested wiping frequency: 2-3 boards per time
Use the method of wet wiping + dry wiping + vacuuming
Regularly check the tension of the steel mesh (recommend > 30N/cm²)
II. The main reason for erecting the monument
The erection of the monument is commonly seen in small-sized resistive and capacitive components such as 0201 and 01005. In essence, the forces on the two solder pads are extremely unbalanced.
4. Excessive difference in the amount of solder paste at both ends
Symptom: One end of the component is fully immersed in the solder, while the other end stands up like a "tombstone".
Root cause:
Asymmetrical holes in the steel mesh
Printed circuit deviation, resulting in less solder paste at one end
Troubleshooting direction:
The holes in the small component steel mesh must be centered and symmetrical.
Check the matching degree between the PCB pads and the holes in the steel mesh.
Control the printing offset within 15% of the size of the steel mesh hole.
✅ The essence of the sign erection problem is not a welding issue, but a mechanical imbalance.
5. Solder paste printing position deviation
Symptom: There is solder at both ends, but the deviated end is far away from the end of the component, and it "can't be pulled through" during melting.
Root cause:
Insufficient alignment accuracy between the PCB and the steel mesh
The clamping plate is not tight, and the PCB moves slightly during the printing process
Troubleshooting direction:
Check the stability of the Mark point recognition of the equipment
Confirm whether the PCB clamping mechanism is secure
Enable 2D solder paste inspection (SPI) to monitor the offset
Insufficient solder paste activity + Oxidation of the solder pads
Performance: The solder paste on one end of the solder pad cannot effectively wet, while the other end is normal.
Root cause:
The PCB pads have been contaminated or oxidized.
The activity of the solder paste flux is insufficient to remove the oxide layer.
Troubleshooting direction:
Check if the PCB is damp or has exceeded its storage period.
Replace the solder paste with a higher activity level (e.g. ROL1 → ROL2).
III. The core reasons for void soldering
Void soldering is one of the most concealed defects. It has electrical connectivity but is unstable, and the risk of failure in the later stage is extremely high.
7. Low tin content / Insufficient printing volume
Symptom: The amount of solder at the solder joints is significantly reduced, and the device pins are either floating or have poor contact.
Root cause:
The holes in the steel mesh are too small.
The solder paste does not release properly, remaining inside the holes of the steel mesh.
The scraper moves too fast, resulting in insufficient filling.
Inspection direction:
For devices with small pitch, the recommended ratio of hole width to thickness is >1.5, and the ratio of area to hole size is >0.66.
Use nano-coated steel meshes or electro-polished steel meshes to improve demolding.
The scraping speed is recommended to be controlled within 30–60mm/s.
8. Solder paste "dog ears" / protrusions → Poor contact
Symptom: After demolding, the shape of the solder paste is in the form of a peak. During the mounting process, it is distorted and cannot form a complete solder joint after reflow.
Root cause:
The surface of the steel mesh holes is rough
The demolding speed is too fast or the distance is too large
Inspection direction:
Check if there are any burrs on the mesh hole walls.
The recommended demolding speed is 0.5–2mm/s (slow demolding).
Choose round holes or inverted trapezoidal openings to reduce resistance.
- PCB / steel mesh surface dirtiness。
Description: Repeated empty soldering at specific locations, while other areas are normal.
Root cause:
There are remaining solder mask oil or finger oil residues on the PCB pads.
There are dry solder paste residues in the stencil holes.
Inspection direction:
Use a lint-free cloth and alcohol to wipe the PCB pad area before printing.
Manually perform deep cleaning of the screen mesh every 4-6 hours.
IV. Summary Table
Defect TypeCore CauseMost Critical Troubleshooting Actions
Short CircuitExcessive solder paste / Collapse / Steel net contaminationSteel net thickness + Cleaning frequency + Temperature and humidity
Bump FormationInsufficient solder quantity / Deviation / Insufficient activityEqualization of opening positions + SPI deviation monitoring
LeakageInsufficient solder / Tip pulling / DirtinessRelease parameters + Hole wall quality + Cleaning
V. Conclusion
Paste printing is a step on the SMT production line that is "invisible but decisive".
Even if your reflow soldering furnace has the most perfect temperature curve and your surface mount machine has the highest precision, if you fail to pass the printing stage, all your efforts will be in vain.
The three most effective suggestions:
Perform SPI (2D or 3D solder paste inspection) - Stop relying on visual inspection for printed components。
Establish a ledger for steel mesh management - Record the through-hole rate and usage frequency of each steel mesh。
Treat temperature and humidity as process parameters for control - The sensitivity of solder paste to the environment is far greater than imagined。