Stencil Cleaning Methods: Manual vs Automatic

For precise and reliable solder paste pattern application on the PCB pads in Surface Mount Technology (SMT) printed circuit board assembly, stencils are utilized. To keep the print quality high and avoid mistakes, it's important to clean the cutouts often. Although automatic stencil cleaners are frequently used in manufacturing, hand cleaning is recommended when assembling prototype PCBs or in smaller quantities. Because it determines the process's yield and return on investment, stencil cleaning is a crucial stage. This article compares cleaning techniques and offers a thorough explanation of how to clean stencils both manually and automatically. For those who need customized stencils to match specific board designs, JLCPCB provides flexible stencil options.Visit JLCPCB and order your custom stencil now.

Need for Stencil Cleaning

The same issue is frequently encountered by numerous PCBA assembly factories: dirty stencils! How should these stencils be cleaned? Although this is a straightforward issue, it now leads to a lot of headaches and subsequent problems including labor concerns, cleaning uniformity, and quality.

Some key reasons why regular stencil cleaning is required:

• Regular stencil cleaning is necessary for the following main reasons:
• Inaccurate solder deposit may result from residual solder paste on stencil apertures.
• Solder balls and mid-chip solder beads are also caused by paste remnants.
• The quality of the print is reduced by dust, flux, and other impurities on the stencil.
• Dried paste clogged apertures can change stencil life and print definition.
• More misprinted boards happen when there isn't enough cleaning.

How Do You Clean the SMT Stencil?

Cleaning the SMT stencil is essential to enhancing the printing process' quality, as it is commonly stated that 70% of product flaws in the mounting process are brought on by the solder paste printing process. There is a chance of errors like too much solder paste or not enough solder being printed if dirt or other debris blocks the SMT stencil's apertures.

Additionally, there is a chance that manual cleaning with a wiping cloth soaked in IPA won't be able to fully clean and remove the solder paste or solder particles if the pitch narrows as a result of the increasing density of mounting.

Three Ways to Clean SMT Stencils:

1. Cleaning by Soak by Hand:

Tools and materials: Cleaning agent, Duster cloth, Special gloves, Container

Steps of Cleaning:

1) If the solder has condensed into fragments, immerse the stencil in a container for a while or spray it with a cleaning solution.

2) Put on gloves and swab the solder paste-covered areas with a dusting cloth.

3) Using pressurized air to dry

2.Pneumatic Spray Cleaning

Tools and materials: Pneumatic stencil cleaning machine, Cleaning agent, Special gloves

Steps of Cleaning :

1) Turn on the air compressor.

2) Establish the drying and cleaning times.

3) Put the stencil in and let the operator work on other projects.

4) After returning, remove the stencil and insert a new one (this is a completely automated operation that only requires loading and unloading).

3.Ultrasonic Stencil Cleaning

Tools and materials: Ultrasonic stencil cleaner machine, Cleaning agent

Steps of Cleaning:

1) Turn on the power.

2) Add the stencil and specify the cleaning and drying times.

3) Remove the stencil and insert a new one.

Comparison between Cleaning Methods:

Knowledge about Wiping Process:

Once solder paste has flowed into the holes, it hardens and adheres to the stencil surfaces and the pads. The forces holding the deposit to the pad must be greater than the ones holding it to the stencil walls at separation.The paste next to the wall cracks and strains. A portion of the paste will release to the PCB and some will remain in the aperture, depending on the pad wall area ratio. A small amount of paste could cling to the underside of the stencil as a result of stringing or pump out.

There are three types of Wipes:

• Dry Wipe: The most popular kind, dry wipe, is used to remove solder balls from the bottom of the stencil that have bled out of the aperture.

• Vacuum Wipe: Vacuum wipes the aperture to clear out paste and clear clogged areas.

• Solvent wipe: This tool is used to remove flux leftovers from the bottom of the stencil.

Wipe Frequency Description:

Frequency of Wiping The following describes the wipe frequency for each print:

At the beginning, more frequent cleaning is required to assess the process and modify the cleaning protocol. Print flaws can then be used to optimize the frequency.

Step-by-Step Cleaning Procedure:

The following is the comprehensive manual stencil cleaning procedure:

Step 1: Visual Inspection

Inspect the stencil surface for paste residues in the apertures and exposed areas. Visually check the stencil's underside for contaminants, debris, and dried solder paste.

Step 2: Preparation

On an ESD mat that is kept on a level surface, firmly place the stencil. Get the necessary swabs, wipes, and cleaning chemicals ready. Keep the top and bottom sides of the wipes separate.

Step 3: Loosening Dry Paste

Large pieces of dried paste can be removed with sticky tape. After one or two uses, discard. To help, apply solvent with a swab.

Step 4: Cleaning Apertures

To eliminate blockage, carefully brush within apertures and around edges using a soft brass brush dipped in solvent. In order to remove any leftover paste, run solvent-soaked foam swabs into the holes.

Step 5: Bottom Side Cleaning

Use a cleaning wipe that is saturated with solvent to wipe the bottom of the stencil in single strokes. To get rid of the paste buildup on the bottom side, wrap, wipe the squeegee around, and scrape. As soon as the wipes seem dirty, replace them. When the paste is saturated, switch the solvent.

Step 6: Top Side Cleaning

To clean the top side, wet a foam swab with solvent and gently rub it in the direction of the stencil strain. To get rid of paste remnants from the upper surface, use strips of sticky tape.

Step 7: Final Cleaning

Clean the bottom side of the stencil from edge to edge with solvent and do this all at once.

Likewise, use a brush and solvent to thoroughly wipe the top side of the stencil.

Step 8: Drying

Before using the stencil again, let it air dry completely. The solvent used determines the drying time. Alternatively, to expedite drying, use clean compressed air to blow dry.

Step 9: Inspection

Check the stencil visually for any contamination or leftover paste under bright lights.

To make sure everything is clean, carefully inspect the apertures and trouble spots again.

Conclusion:

In conclusion, manual cleaning is a cost-effective yet efficient technique for stencil cleaning in low-volume SMT manufacturing or prototype runs. However, automated stencil cleaning methods are more costly and can be applied to assembly lines that produce large quantities.

Paste residues can be removed without causing damage to the stencil by using the proper method in conjunction with the appropriate solvents, swabs, and wipes. This prevents print flaws brought on by contaminated stencils and enhances paste transfer consistency. JLCPCB’s combination of nano-coated and electropolished stencils provides a non-stick surface that is easier to clean, further improving print reliability and reducing maintenance effort.Stencil life can be significantly extended with proper cleaning frequency and thorough inspection