Key takeaways
- Locate the defect stage first — printing, placement or reflow — then check material, stencil, parameters, components and profile in that order.
- Incomplete release, slumping and thickness variation usually trace to stencil apertures, squeegee pressure, paste viscosity and mixing.
- Solder balls, bridging, tombstoning, insufficient solder and voids often involve paste condition, pad design, placement accuracy and reflow profile together.
- First-article inspection, SPC records and saved reflow profiles prevent more defects than rework.
Troubleshooting solder paste defects by chasing the symptom often fails. The same bridging defect can come from stencil apertures, printing parameters, placement offset, paste viscosity or the reflow profile. A better approach is to locate the stage first, then eliminate variables in a fixed order.
This guide covers printing, placement and reflow defects, followed by a practical shop-floor troubleshooting sequence.
Locate the stage first
Before changing parameters:
- Where is the defect? Whole board, fixed locations or random? Fixed locations usually point to stencil/pad design; random distribution suggests material or environment.
- When does it occur? At first article or after running for a while? The former points to design and setup; the latter to paste condition, environment or equipment drift.
- Run a controlled comparison: fresh paste, new stencil, different PCB lot — change one variable at a time.
1. Printing defects
Printing is a high-risk stage. Paste viscosity, stencil apertures, squeegee pressure, print speed and release speed all affect the deposit.
| Defect | Possible causes | Actions |
|---|---|---|
| Incomplete release | Clogged apertures; paste viscosity too high or low; poor powder uniformity; worn squeegee; dirty aperture walls | Clean stencil and apertures; select suitable paste; replace squeegee; check warm-up and mixing |
| Slumping | Excessive squeegee pressure; paste viscosity too low; powder too fine; PCB not fixed | Adjust pressure; secure the PCB; use a medium-viscosity paste |
| Burrs on edges/surface | Rough aperture walls (etched stencil); PCB misalignment; low viscosity | Select suitable paste; adjust stencil-to-PCB spacing; optimise aperture and stencil thickness |
| Uneven thickness | Stencil not parallel to PCB; uneven mixing; poor powder uniformity | Adjust parallelism; mix thoroughly; use paste with uniform particle distribution |
| Poor paste formation | Stencil too thin; excessive squeegee pressure; poor paste flow | Use suitable stencil thickness; adjust pressure; select matching flow |
Shop-floor tip: inspect aperture walls and paste edges at 10–20×. If paste remains in the apertures, clean the stencil and check whether the paste was warmed up sufficiently or is too viscous.
2. Placement defects
Placement defects show up as shifted or dropped components and paste smearing. Besides machine accuracy, paste tack and operator technique matter.
| Defect | Possible causes | Actions |
|---|---|---|
| Components wobble | Unstable manual placement; low paste tack; excessive board movement | Use suitable paste; place in one accurate motion; adjust conveyor support |
| Flying/dropped components | Insufficient tack; nozzle timing inconsistent with placement; contaminated component | Change paste; calibrate nozzle and placement height; clean components |
| Paste smearing/bridging | Paste tack too low; placement offset; excessive paste | Adjust paste and printing; calibrate placement coordinates; check stencil apertures |
3. Reflow defects
Reflow defects often involve printing, placement and material condition together.
Solder balls
| Possible cause | Action |
|---|---|
| Stencil aperture and pad mismatch, apertures too large | Redesign stencil apertures |
| Stencil too thick | Select suitable thickness |
| PCB moisture or substrate absorption | Bake the PCB per process requirements |
| Incorrect paste viscosity, fine powder or high oxygen content | Change paste; check storage and warm-up |
| Improper paste use (short warm-up, over-mixing) | Standardise 4–6 h warm-up and 1–3 min mixing |
Bridging
| Possible cause | Action |
|---|---|
| Unreasonable stencil apertures, openings too large | Redesign stencil |
| Paste already connected during printing | Adjust pressure, speed and release |
| Paste squeezed together during placement | Calibrate placement height and coordinates |
| Poor paste viscosity/flow | Select suitable paste |
| Insufficient flux activity | Evaluate activity or change paste |
Tombstoning
| Possible cause | Action |
|---|---|
| Severely oxidised pads | Change board or remove oxidation |
| Excessive paste or asymmetric pad design | Optimise stencil and pad design |
| Insufficient flux activity | Evaluate paste activity |
| Uneven reflow heating | Adjust zones and conveyor direction |
| Oxidised component terminations | Remove oxidation or change components |
| Placement offset | Calibrate placement coordinates |
Insufficient solder
| Possible cause | Action |
|---|---|
| Stencil too thin | Increase stencil thickness |
| Excessive squeegee pressure | Reduce pressure |
| Apertures too small | Optimise area ratio |
| Poor paste release | Change paste or adjust viscosity |
Voids and pinholes
| Possible cause | Action |
|---|---|
| Insufficient preheat or reflow temperature | Adjust reflow profile |
| PCB substrate outgassing or moisture | Bake the board or change material |
| Solvent not fully evaporated | Extend preheat; optimise ramp rate |
Dull joints
| Possible cause | Action |
|---|---|
| Temperature too low or high-temperature dwell too long (yellowing) | Adjust reflow profile |
| Matte-type paste by design | Confirm the specification; not a defect |
| Uneven powder oxidation | Check paste storage and batch |
| Large pads, uneven paste thickness | Optimise stencil and printing |
| Insufficient solder, poor reflection | Adjust aperture-to-pad ratio |
Recommended troubleshooting sequence
- Verify paste condition: stored at 5–10°C, warmed 4–6 hours, mixed 1–3 minutes, within shelf life, not repeatedly frozen.
- Inspect the stencil: clogged apertures, width/thickness and area ratios, tension, parallelism.
- Review printing parameters: pressure, speed, release speed, snap-off.
- Check placement: coordinates, height, nozzle, component solderability.
- Measure the reflow profile: peak, time above liquidus, ramp/cooling rates against the paste datasheet.
- Check materials and environment: PCB moisture, component oxidation, 18–24°C / 40–50% RH.
Change one variable at a time and record the result. First-article inspection, parameter records and saved profiles are more effective than rework.
Prevention checklist
- First-in, first-out paste; store at 5–10°C; warm 4–6 hours; mix 1–3 minutes.
- Clean stencils regularly; design apertures with W/T ≥ 1.5 and area ratio > 0.66.
- Maintain 18–24°C and 40–50% RH; avoid direct airflow.
- Bake PCBs as required; store components dry.
- Measure the reflow profile for each board thickness and assembly density.
- Build a defect image library and corrective-action log for fast comparison.
If a defect is not covered here, prepare photos, PCB stack-up, stencil parameters and the reflow profile, and contact STAR.I’s technical team for analysis.
FAQ
What causes solder balls most often?
Common causes include oversized or too-thick stencil apertures, moisture in the PCB, incorrect paste viscosity or fine powder, poor warm-up/mixing, and an unsuitable reflow profile. Check the stencil and PCB baking first, then paste and profile.
Why does tombstoning happen on small components?
Small components have low thermal mass. If one end melts before the other, surface tension lifts the part. Asymmetric pads, uneven paste, placement offset and uneven reflow heat all contribute.
Does no-clean paste residue need cleaning?
No-clean paste leaves low residue, but cleaning depends on the reliability class, whether residue affects subsequent coating or testing, and customer acceptance. High-reliability products should assess insulation resistance and ionic contamination.
How can I tell whether the paste or the stencil is the problem?
Print with the same stencil and a fresh paste batch, then with the same paste and a new stencil. If fresh paste improves the result, focus on paste condition; if a new stencil improves it, focus on apertures, thickness and tension.
References
- STAR.I TECHNOLOGY solder paste troubleshooting technical data
- IPC-A-610 Acceptability of Electronic Assemblies
- IPC J-STD-005 Requirements for Solder Pastes
- STAR.I solder paste product data (storage, printing and reflow parameters)
