Introduction

Stencils are used in surface mount technology (SMT) printed circuit board assembly to apply solder paste pattern on the PCB pads accurately and consistently. Cleaning the stencils regularly is crucial to maintain print quality and avoid defects. While automatic stencil cleaners are commonly used in production, manual cleaning is preferred in case of lower volume or prototype PCB assembly. This article provides a detailed procedure on how to effectively clean stencils manually.

Need for Stencil Cleaning

Some key reasons why regular stencil cleaning is required:

• Residual solder paste on stencil apertures can cause insufficient or inaccurate solder deposit. This leads to poor prints and missing joints.
• Paste residues also result in solder balls and mid-chip solder beads affecting assembly yield.
• Dust, flux and other contaminants on stencil lower print quality and solder paste release.
• Clogged apertures due to dried paste can alter print definition and stencil life.
• Insufficient cleaning increases number of misprinted boards.

Thus, stencil cleaning is vital to avoid print defects and maintain process stability in SMT assembly.

Cleaning Frequency

The frequency of stencil cleaning depends on factors like:

• Volume of PCBs produced per day
• Size and complexity of solder paste deposits
• Type of solder paste – some are more sticky and prone to smearing
• Environmental conditions like humidity and temperature

Typical cleaning frequencies for manual stencil cleaning process:

More frequent cleaning is needed at start to check process and adjust cleaning procedure. The frequency can then be optimized based on print defects.

Materials Required

Following materials are required for manually cleaning the stencils:

• Cleaning solvents (isopropyl alcohol, acetone etc.)
• Non-abrasive wipes and swabs
• Metal squeegee/scrapers
• Plastic spreader/squeegee
• Lint-free gloves
• Set of metal brushes and foam swabs
• Adhesive tape rolls
• ESD mat for stencil
• Protective eyewear

Solvents and wipes should be cleanroom grade. Foam swabs and brushes must be made of non-metallic materials to avoid damage.

Step-by-Step Cleaning Procedure

The detailed manual stencil cleaning process is as follows:

Step 1: Visual Inspection

• Visually inspect underside of the stencil for dried solder paste, debris and other contamination.
• Check for paste residues in apertures and open areas of stencil surface.
• Take note of heavily soiled areas for concentrated cleaning.

Step 2: Preparation

• Clean hands with solvent or wear lint-free gloves to avoid fingerprints.
• Place stencil securely on an ESD mat kept on a flat surface.
• Prepare required cleaning solvents, swabs, wipes etc. Keep different wiping materials for top and bottom side.

Step 3: Loosening Dry Paste

• Use adhesive tape to remove large chunks of dried paste. Discard after 1-2 uses.
• Gently roll non-abrasive swabs across underside to loosen paste residues. Apply solvent using swab to assist.

Step 4: Cleaning Apertures

• Dip soft brass brush in solvent and gently brush inside apertures and around edges to remove clogging.
• Swipe foam swabs soaked in solvent through apertures to extract remaining paste.

Step 5: Bottom Side Cleaning

• Saturate cleaning wipe with solvent and wipe the stencil underside in single strokes.
• Wrap wipe around squeegee and scrape to remove paste buildup on bottom side.
• Replace wipes as soon as they appear soiled. Change solvent when saturated with paste.

Step 6: Top Side Cleaning

• Moisten foam swab in solvent and softly rub in direction of stencil tension to clean top side.
• Use adhesive tape strips to remove paste residues from top surface.

Step 7: Final Cleaning

• Wipe entire stencil bottom side edge to edge with solvent and wipe in single pass.
• Similarly, clean stencil top side thoroughly with swab and solvent.
• Use lens cleaning tissue for final wipe down. Ensure no material is left on squeegee/wiper side.

Step 8: Drying

• Allow stencil to air dry completely before further use. Drying time depends on solvent used.
• Alternatively, blow dry with clean compressed air to speed up drying.

Step 9: Inspection

• Visually examine stencil under bright lights for any remaining paste or contamination.
• Recheck problem areas and apertures closely to ensure thoroughly clean.
• Swab test – rub swab across stencil and check if any paste residue shows up on swab.

This completes the stencil cleaning process. Carry out further print trials to validate cleaning effectiveness.

Best Practices for Manual Cleaning

Some recommended best practices for manual stencil cleaning:

• Always use lint-free gloves to prevent fingerprints on stencil.
• Apply just enough solvent using swabs/wipes to maximize cleaning without waste.
• Frequently replace swabs, brushes and wipes to avoid spreading contamination.
• Use unidirectional wipes in the direction of stencil tensioning.
• Avoid excessive brushing or scrubbing to prevent damage to stencil surface.
• Clean apertures before cleaning stencil surface to avoid redepositing contamination.
• Allow sufficient drying time before reusing stencil after cleaning.
• Maintain a separate cleaning area to prevent solvent contamination in production area.

Effect of Cleaning on Print Quality

Proper manual cleaning of stencils improves solder paste printing by:

• Preventing insufficient paste deposits due to clogged apertures.
• Eliminating solder balls caused by dried paste particles on stencil.
• Removing other contaminants that affect wetting and release of paste.
• Improving print definition by clearing apertures edges.
• Allowing consistent volume and shape of paste deposits.
• Reducing frequency of printing defects.
• Increasing process control and stability.
• Maximizing stencil life by preventing damage.

Conclusion

Manual cleaning is an economical yet effective method for cleaning stencils during prototype runs or low volume SMT production. Using the right technique along with suitable solvents, swabs and wipes allows removing paste residues without damaging the stencil. This improves paste transfer consistency and avoids print defects related to contaminated stencils. With appropriate cleaning frequency and effective inspection, stencil life can be increased considerably. Thus, following the proper protocol for manual cleaning is critical for any facility involved in SMT PCB assembly.

FAQs

Q1. How is manual stencil cleaning different from ultrasonic cleaning?

Manual cleaning relies on mechanical force applied through wipes, squeegees etc. along with solvents to remove paste. Ultrasonic cleaning uses high frequency sound waves and solutions to dissolve contaminants.

Q2. What solvents are used for manual stencil cleaning?

Common solvents like isopropyl alcohol, acetone and ethanol are used. Semi-aqueous solvents are also available. Solvent must be compatible with solder paste flux chemistry.

Q3. What causes solder balls during SMT assembly?

Residual solder paste particles left on stencil due to insufficient cleaning get dislodged and transferred on PCB during printing. These particles later form solder balls.

Q4. How does stencil cleaning affect process yield?

Insufficient stencil cleaning directly contributes to solder paste printing defects which lower process yield. Cleaning improves paste transfer efficiency.

Q5. When should stencil apertures be brushed during cleaning?

Apertures should be brushed first before cleaning the stencil surface. This allows dislodged residues to be removed from stencil surface rather than re-enter apertures.