What causes a lot of solder paste residue?

The problem of solder paste residue is usually caused by a combination of process parameters, material properties, equipment condition and environmental control.The following combines industry practice and the latest research, common causes and solutions for systematic sorting and supplement:

I. Printing process problems

1. Improper squeegee parameters

Cause:

Insufficient squeegee pressure: resulting in the solder paste not being able to fill the stencil holes sufficiently, insufficient amount of solder paste after printing, which may increase the risk of residue.

Squeegee speed is too fast: the squeegee moving speed exceeds a certain range (e.g. 150mm/s), the solder paste is not filled sufficiently, resulting in uneven printing or residue.

Removal speed is too high: Removal speed is too fast may pull up the solder paste, increasing the risk of residue.

Solution:

Adjust squeegee pressure: the recommended pressure range is 30-50N (per 100mm squeegee length), which needs to be dynamically adjusted according to the paste viscosity and stencil thickness.

Control the printing speed: the printing speed is controlled at 50-150mm/s to ensure that the solder paste is fully filled.

Optimise demoulding speed: set the demoulding speed at 0.1-1mm/s to reduce the probability of the solder paste being pulled up.

2. Stencil design defects

Cause:

Unreasonable size/shape of stencil opening: e.g. width/thickness ratio <1.5, resulting in poor solder paste release.

Rough hole wall: slag from laser cutting or hourglass shape from chemical etching increases the risk of residue.

Insufficient cleaning: solder paste residue at the bottom of the stencil, affecting the quality of the next print run.

Solution:

Optimise opening design: Improve solder paste release by using a tapered design (e.g. 10% smaller diameter at the bottom than at the top) or nano-coating.

Improve aperture wall quality: Use laser cutting or electroforming process to reduce aperture wall roughness.

Regular cleaning of the stencil: clean the residue at the bottom of the stencil every hour to avoid clogging.

II. the material factors

1. Solder paste performance mismatch

Reason:

Viscosity is too high: > 400 Pa-s solder paste release difficult, easy to residue.

Insufficient metal content: ＜88% when the proportion of flux is too high, resulting in increased residue.

Improper storage conditions: the solder paste is not refrigerated or insufficient time to return to the temperature, affecting the fluidity.

Solution:

Choose the right solder paste: low viscosity (80-300 Pa-s), high metal content (88-92%) of the paste.

Standardise storage conditions: Refrigerate the solder paste (2-10°C), rewarm to room temperature and stir well before use.

Check the expiry date: Avoid using expired solder paste.

2. Flux characteristics

Reason:

Highly reactive rosin-type flux: Residues are evident after reflow and may cause electrical performance problems.

Low temperature solder paste compatibility: Low temperature solder paste needs to match the special flux to avoid residue.

Solution:

Switch flux type: Use no-clean or low residue water-soluble flux.

Optimise reflow profile: Ensure flux evaporates fully to reduce residue.

III. Environment and equipment

1. Temperature and humidity out of control

Cause:

Temperature is too high: > 28 ℃ to accelerate the solder paste volatilisation, resulting in unstable printing volume.

Humidity is too low: <30% to make the paste dry, increasing the risk of residue.

Solution:

Control the workshop environment: keep the temperature at 22-26 ℃, humidity 40-60%.

Use constant temperature and humidity equipment: Ensure the solder paste printing machine works in a stable environment.

2. Abnormal status of equipment

Cause:

Squeegee wear: edge unevenness > 0.05mm when it affects the printing quality.

Insufficient stencil tension: <35N/cm² leads to stencil deformation, increasing residue.

Solution:

Replace squeegee regularly: replace squeegee after every 500,000 prints.

Check the stencil tension: Check and adjust the stencil tension regularly to ensure it meets the standard.

IV. Process Control

1. Printing retention time is too long

Reason: PCB placed > 4 hours, solder paste solvent evaporation, reflow residue carbonisation.

Solution: 2 hours after printing to complete the patch reflow to avoid paste drying.

2. Reflow curve is not appropriate

Cause:

Preheating too fast: temperature rise > 2 ℃ / s resulting in flux is not fully evaporated.

Peak temperature is not enough: not reached above the melting point of the alloy 25-35 ℃, affecting the quality of welding.

Solution:

Optimise the reflow profile: preheat 60-120s, peak temperature 25-35°C above the melting point of the alloy.

Real-time monitoring of the reflow process: Use temperature sensors to monitor the reflow profile to ensure consistency.

V. Systematic troubleshooting steps

1. Quick Verification

Method: Use new product stencil/paste for trial production to confirm whether it is a consumable problem.

2. Parameter Comparison

Method: L9(3^4) orthogonal test using Taguchi's method to screen key influencing factors (e.g. squeegee pressure, speed, paste viscosity, etc.).

3. Quantitative testing

Tools:

3D SPI: Measure solder paste thickness to ensure printing quality.

Ionic contamination test: Residue should be <1.56μg/cm² NaCl equivalent to avoid electrical performance problems.

VI. Supplementary technical programmes

1. Vacuum demoulding device

Application: For precision packages (e.g. 01005 components), vacuum release can reduce the release residue by more than 90%.

2. Selective soldering process

Note: The nozzle aperture needs to be matched with the rheological properties of the solder paste to avoid residue.Dynamic pressure control technology is recommended to optimise the paste spraying effect.