How Does Peelable Soldermask Work?

Peelable soldermask is typically made of a latex-based or synthetic rubber material that is applied in liquid form and then cured to create a solid, yet flexible coating over the PCB surface. The key characteristics of PSM are:

  1. Thermal stability: Withstands high temperatures during soldering without degrading
  2. Solderability: Prevents molten solder from wetting and adhering to coated areas
  3. Peelability: Easily removed by hand without leaving residue or damaging the PCB

The PSM is applied by screen printing, spray coating, or dip coating methods depending on the specific requirements and PCB design. It covers and protects areas that need to remain solder-free, such as:

  • Test points for probing and debugging
  • Contact pads for connectors or switches
  • Bare copper areas for grounding
  • Existing surface mount components

During the soldering process, the high surface tension of molten solder prevents it from displacing the PSM coating. This allows solder to only wet and bond to the intended exposed metal surfaces on the PCB. After soldering, the PSM is simply peeled away by hand to reveal the clean, solder-free areas underneath.

Advantages of Using Peelable Soldermask

There are several key benefits to using PSM in PCB assembly:

1. Enables Selective Soldering

PSM allows assemblers to solder only specific areas of the board while protecting others. This is especially useful for:

  • Adding components to an already populated board
  • Soldering boards with mixed through-hole and surface mount parts
  • Protecting sensitive or pre-attached components

2. Eliminates Masking and Cleaning Steps

Without PSM, areas that must remain solder-free are typically covered with high-temperature masking tapes or dots. This is a time-consuming manual process. After soldering, the masking must then be removed and the board cleaned of adhesive residue.

In contrast, PSM can be quickly applied over large areas, withstands soldering, and is easily peeled off without leaving any residue behind. This streamlines the assembly process and reduces material and labor costs.

3. Improves Solder Joint Quality

By preventing solder bridges, balls, and spillover, PSM helps ensure solder only bonds to intended surfaces. This improves solder joint reliability and reduces defects and rework. PSM’s clearly defined edge also results in a neat and consistent soldermask outline.

4. Protects Components and Surfaces

PSM shields sensitive components like LEDs, connectors, and switches from heat and solder during wave soldering or reflow. It also guards critical exposed contacts and conductors against solder bridging or damage.

Comparing Peelable Soldermask to Other Masking Methods

There are several methods used to mask off areas on PCBs prior to soldering. Here’s how PSM stacks up:

Method Advantages Disadvantages
Peelable Soldermask – Easily applied/removed
– No residue
– Withstands soldering
– Enables selective soldering
– Higher material cost
– Requires curing
– Limited shelf life
Kapton Tape – Heat resistant
– Leaves no residue
– Tedious manual application
– Difficult to completely seal
– Can leave adhesive residue
Dots/Discs – Inexpensive
– Quick to apply
– Time-consuming to remove
– May leave residue
– Tend to absorb solder
– Only protect small areas
Liquid Latex – Easy to apply
– Inexpensive
– Not suitable for high temps
– Tends to peel/flake during soldering
– Hard to fully remove

For high volume, selective soldering of complex boards, PSM is often the most efficient and effective option. It offers the best balance of ease-of-use, durability, clean removal, and compatibility with standard SMT processes.

Selecting and Applying Peelable Soldermask

When choosing a peelable soldermask, consider the following factors:

  1. Base material compatibility (FR4, polyimide, flex, etc.)
  2. Curing requirements (time, temperature)
  3. Temperature resistance during soldering
  4. Peelability and cleanliness of removal
  5. Shelf life and storage conditions
  6. Viscosity and coatability for your application method
  7. Compliance with any regulatory requirements (RoHS, etc.)

Work with your supplier to select a PSM formulation that meets your specific needs. They can also advise on the proper methods for applying, curing, and removing the PSM for optimal results.

In general, the steps for using PSM are:

  1. Clean the PCB of any contaminants
  2. Apply PSM by screen printing, spray coating, or dipping
  3. Cure PSM per manufacturer’s recommendations
  4. Perform soldering operations (wave, selective, hand, etc.)
  5. Allow PCB to cool
  6. Peel away PSM by hand
  7. Inspect PCB to verify complete removal

Some tips for success:

  • Ensure even, consistent coverage of PSM, especially on irregular surfaces
  • Avoid touching or contaminating cured PSM surfaces prior to soldering
  • Allow adequate curing time and temperature for optimal PSM durability
  • Peel PSM at a steady, low angle to avoid lifting pads or damaging the PCB
  • If a thin residue remains after peeling, remove with IPA and a soft brush

With proper selection and processing, peelable solder mask provides a highly effective and efficient method of masking PCBs for selective soldering.

Peelable Soldermask and PCB Design Considerations

While PSM is compatible with most standard PCB designs, there are a few points to keep in mind during layout and manufacturing:

1. Pad and Trace Spacing

To ensure clean peeling and prevent solder bridging, provide adequate space between PSM-masked pads/traces and adjacent exposed features. A minimum gap of 0.015″ (0.4mm) is typically recommended.

2. Soldermask Clearance

PSM is peeled away after soldering, so it should not overlap or encroach on any final permanent soldermask openings. Allow for at least a 0.010″ (0.25mm) soldermask pullback/clearance around PSM coated features.

3. Component Placement

Avoid placing tall components near PSM coated areas, as they may interfere with the peeling process or be damaged during removal. Provide at least 0.040″ (1mm) of clearance if possible.

4. Fabrication Drawing Callouts

Clearly identify and document any areas requiring PSM coating on your fabrication drawings and files. Use distinct colors, hatch patterns, or labels to differentiate PSM from final soldermask.

5. Assembly Process Sequencing

Consider the order of operations when planning your assembly process. PSM should be applied after any bare board electrical testing steps, as probing can pierce and damage the coating. Peeling should occur after soldering but before any conformal coating, encapsulation, or potting steps.

By factoring these design and process considerations into your PCB development, you can ensure successful and trouble-free use of peelable soldermask.

Peelable Soldermask FAQs

1. How does PSM compare in cost to other masking methods?

In terms of material cost, PSM is typically more expensive than masking tapes, dots, or liquid latex. However, it often proves more economical in the long run by eliminating the labor time and costs associated with manual masking and residue cleaning. The improved quality and reliability PSM provides can also reduce defects and rework.

2. Is PSM suitable for use with lead-free solders?

Yes, modern PSM formulations are designed to withstand the higher temperatures associated with lead-free soldering processes. They maintain their integrity and peelability even after exposure to reflow or wave soldering with SAC or other lead-free alloys. Consult with your PSM supplier to confirm compatibility with your specific soldering requirements.

3. Can PSM be used on flexible circuits or non-FR4 PCB materials?

PSM is available in formulations compatible with a variety of base materials, including polyimide, PEEK, ceramic, and metal core substrates. The key is selecting a PSM that matches the thermal, mechanical, and chemical properties of your specific PCB material. Some specialized materials may require custom formulations or application methods.

4. How does the shelf life of PSM compare to other maskants?

The shelf life of peelable soldermask is generally shorter than that of masking tapes or dots, which can be stable for years under proper storage conditions. Most PSM formulations have a shelf life of 6-12 months when stored between 40-90°F (5-32°C). Freezing or elevated temperatures can degrade performance. Always check the manufacturer’s recommendations and use the oldest stock first.

5. Are there any environmental or health concerns with using PSM?

Peelable soldermasks are typically non-hazardous and comply with RoHS and REACH regulations. However, some formulations may contain natural latex or other sensitizing agents, so review the safety data sheets (SDS) and take appropriate precautions if allergies are a concern. Proper ventilation should also be provided during the application and curing process to minimize exposure to any fumes or vapors.

In summary, peelable soldermask offers numerous advantages over traditional masking methods for selective soldering of printed circuit boards. By understanding the properties, benefits, and proper use of PSM, assemblers can achieve optimal results in terms of solder quality, process efficiency, and overall PCB reliability.

Categories: PCBA


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