Common Issues And Solutions in Reflow Soldering

Reflow soldering is a critical process in SMT (Surface Mount Technology), and its quality directly impacts the reliability of electronic components. The following are common issues in reflow soldering and their solutions, categorised by typical defects:

1. Solder Short Circuits (Bridging)

Symptoms: Solder bridges form between adjacent solder joints, causing short circuits.

Causes:

Too large stencil openings or solder paste printing offset

Insufficient placement accuracy (component offset)

Unreasonable reflow temperature curve (insufficient preheating or excessively high peak temperature)

Solutions:

Process optimisation: Reduce stencil opening size (e.g., reduce opening size by 5%–10%), adjust printing pressure and squeegee angle.

Equipment calibration: Check placement machine accuracy (ensure within ±0.05mm), use SPI (solder paste inspection system) to monitor printing quality.

Temperature adjustment: Extend preheating time (90–120 seconds), reduce peak temperature (typical peak 235–245°C), avoid excessive solder paste flow.

2. Solder balls

Symptom: Small solder balls scattered around the solder joints.

Causes:

Too rapid temperature rise in the preheating zone, resulting in insufficient evaporation of the solder paste solvent.

Solder paste has absorbed moisture or is expired.

Excessive flux residue during reflow.

Solutions:

Curve optimisation: Control the temperature rise rate in the preheating zone to 1-3°C/second to ensure sufficient solvent evaporation.

Solder paste management: Strictly follow storage conditions (2-10°C refrigeration, warm up for 4 hours before use) to avoid using expired solder paste.

Parameter adjustment: Select low-residue no-clean solder paste, or increase the dwell time (60-90 seconds) in the constant temperature zone (150-180°C).

3. Cold solder joints

Symptoms: Rough solder joint surface with poor wetting.

Causes:

Insufficient peak temperature or too short reflow time (e.g., below the solder liquidus line)

Oxidation of pads or component leads

Insufficient solder paste activity

Solutions:

Temperature verification: Use a KIC temperature meter to confirm the actual peak temperature (Sn-Ag-Cu solder requires ≥217°C).

Material handling: Check the MSL rating of components and PCBs; bake moisture-affected materials (125°C/24 hours).

Solder paste selection: Switch to high-activity solder paste (e.g., with a stronger flux formulation).

4. Tombstone effect

Symptoms: One end of the surface-mount component lifts off the pad.

Causes:

Significant thermal capacity difference between the two ends of the pads (e.g., ground pads cool faster)

Uneven solder paste printing causing asynchronous melting times at both ends

Solutions:

Pad design: Symmetrical pad size design, add thermal isolation grooves to ground pads.

Printing optimisation: Symmetrical stencil openings to ensure consistent solder paste volume at both ends.

Temperature adjustment: Reduce the ramp rate in the heating zone (e.g., 2°C/second) and extend the time above the liquidus line (60–90 seconds).

5. Solder joint voids

Symptom: Bubbles or voids are present inside the solder joint.

Causes:

Obstructed escape of solder paste volatiles (e.g., BGA solder joints)

Excessively fast temperature rise rate during reflow

Solutions:

Curve optimisation: Extend preheating time and reduce temperature rise rate to below 1.5°C/second.

Material improvement: Use low-void solder paste or preformed solder sheets, and adopt vacuum reflow soldering for high-density solder joints.

Process control: Optimise stencil aperture (increase exhaust channels) to prevent solder paste collapse.

6. Component displacement

Symptom: Components shift position during the reflow process.

Causes:

Excessively high reflow oven airflow speed (>1.5 m/s)

Insufficient solder paste viscosity or collapse

Solutions:

Equipment adjustment: Reduce the airflow speed inside the furnace and use laminar flow mode.

Solder paste control: Select high-viscosity solder paste (e.g., Type 4) to ensure no collapse after printing.

Placement optimisation: Increase placement pressure (for large components) or use bottom dispensing for fixation.

7. PCB deformation

Symptom: PCB bending causes poor soldering.

Causes:

Low PCB material Tg value (e.g., standard FR4 Tg=130℃)

Steep temperature curve rise and fall

Solutions:

Material upgrade: Use high Tg boards (Tg≥170℃) or flexible PCBs.

Fixture design: Add support fixtures (e.g., clamps or carriers) to prevent high-temperature deformation.

Curve optimisation: Reduce heating/cooling rates (e.g., 3°C/s → 1.5°C/s).

Key process control points

Real-time monitoring: Use SPI (solder paste inspection), AOI (automated optical inspection), and X-ray (BGA inspection) for full-process monitoring.

Equipment Maintenance: Regularly clean the reflow oven tracks, calibrate thermocouples, and ensure stable temperature curves.

DFM Review: Engage during the PCB design phase to optimise pad, stencil, and component layout.

Data Analysis: Evaluate process stability using CPK (Process Capability Index) and continuously improve yield rates.

By systematically analysing the root causes of issues and implementing targeted measures, reflow soldering quality can be significantly improved, and rework rates reduced.