Peelable Solder Mask

Introduction

Solder mask, also known as solder resist, is a thin lacquer-like layer of polymer that is usually applied to the surface of printed circuit boards (PCBs) to protect the copper traces from oxidation and prevent solder bridges from forming between closely spaced pads during soldering. Peelable solder mask is a special type of solder mask that can be selectively removed or “peeled” away after soldering to allow access to solder joints, components and conductors beneath it. This makes debugging and rework of assemblies much easier compared to having to work through or around a permanent solder mask.

In this article, we will take a closer look at what peelable solder masks are, their composition and properties, how they are applied, where they are useful, and the pros and cons of using a peelable mask versus a traditional permanent solder mask on PCBs.

What is Peelable Solder Mask Made Of?

Most solder masks consist of two main components:

• Photoimageable polymer resins - This forms the main structure and bulk of the solder mask material. Some common resins used are epoxy acrylate, polyimide, silicone, etc. The resin needs to form a tough, flexible and dielectric coating on the PCB.
• Photoinitiators - These are special chemicals that initiate polymerization of the resin when exposed to certain wavelengths of light, usually UV. This allows the mask to be selectively exposed and cured to form openings over pads, vias, etc.

Peelable solder masks contain some additional compounds and modifiers to the basic photoimageable resin that allow them to be peeled off with some force while still being resistant to common solvents:

• Release agents - Silicon oils and waxes are added to reduce adhesion to the copper and board substrate.
• Weak boundary layers - The mask-substrate interface is modified to have weaker bonds that can be broken.
• Flexible resins - The bulk resin used is optimized for flexibility, elongation and low tensile strength.

Typical Composition

A typical composition of a peelable solder mask is:

• 50-70% base resin (epoxy, polyimide, acrylic etc.)
• 15-20% silicon release agent
• 5-10% acrylate monomer
• 1-5% photoinitiator
• 1-5% other additives like flexibilizers, adhesion promoters etc.

The exact formulations and ratios used by different solder mask vendors are proprietary information. But most follow a composition designed to balance peelability vs solvent resistance.

How is Peelable Solder Mask Applied?

Peelable solder mask is applied in almost the same way as regular permanent solder mask. The key steps are:

1. Surface prep - The PCB is first thoroughly cleaned to remove any contaminants, oils or oxides from the copper that could affect mask adhesion.
2. Lamination - A layer of the liquid mask material is deposited over the entire board surface through methods like screen printing, curtain coating, spray coating or roll lamination.
3. Curing - The solder mask is then dried and cured using heat to solidify the material. Partial curing may be done at this stage.
4. Imaging - Just like normal solder mask, the peelable mask is then selectively exposed to UV light through a phototool mask. The exposed areas will later form the openings in the mask.
5. Developing - The board is developed in a chemical solution to remove the unexposed, uncured mask material, leaving behind the cured mask layer with openings.
6. Final curing - A final UV and/or thermal cure is done to fully harden the mask material.

The key parameters that are controlled to get the right peelable properties are the lamination thickness, the degree of curing, and the interfacial boundary with the board.

Where is Peelable Solder Mask Used?

Peelable solder mask provides a number of advantages over traditional permanent mask in certain applications:

Prototyping and Rework

During prototyping and development, changes often need to be made to a PCB after initial assembly and bring-up. Components may need to be changed, wire fixes added, or conductor access required for debugging or probing.

Peelable mask allows easy access to the PCB surface beneath by simply peeling off sections of the mask by hand. This avoids having to risk damage to a board by abrading or solvent-wiping permanent mask.

Low Volume Production

For short production runs where time and cost are critical, peelable mask can help reduce rework costs if any assemblies fail during testing. Defects can be quickly fixed by peeling the mask rather than a complex mask rework process.

Customized Access

In dense complex boards, peelable mask can be used to provide customizable access over certain components like test points, programming interfaces, potentiometers etc. After soldering, only the required area of mask can be peeled off rather than having permanent openings.

RF/Microwave Circuits

In high frequency RF and microwave PCBs, any solder mask can affect signals by adding capacitance. So peelable mask may be used under components to avoid this, and peeled off after soldering.

Conformal Coating Prep

To improve adhesion of conformal coating on a board, peelable mask can be used to provide a clean, exposed copper surface for the coating to adhere to. After soldering, the mask is peeled away where coating is needed.

Reworkable Adhesives

Solder mask material itself can be used as a reworkable adhesive by applying a thin peelable layer between components and PCBs. This allows easy removal of components for replacement.

Pros and Cons of Peelable vs Permanent Solder Mask

Here is a comparison between peelable and traditional permanent solder mask:

Peelable MaskPermanent MaskAllows access to covered conductorsNo access possible after applicationHelps debugging and prototypingFor well-defined production boards onlyLower material usage as only applied where neededEntire board is coatedReworkable and replaceableDifficult to rework or replaceRequires careful peel off methodMore abrasion and solvent resistanceCan leave residual adhesivePermanently adhered to boardLess mechanical strengthMore rigid and durableHigher cost per areaVery low cost

Typical Properties of Cured Peelable Solder Masks

Peelable solder masks have slightly different properties when cured compared to traditional solder masks due to the modifications that provide their peel-off capability.

Typical values are:

Property Value

Peel Strength:2-4 N/mm

Tensile Strength: 10-20 MPa

Elongation at break: 20-60%

Hardness (Shore D): 50-80

Glass transition temp: 100-130°C

Coefficient of thermal expansion: ~50 ppm/°C

Dielectric breakdown voltage: 15-25 kV/mm

Dielectric constant (1MHz): 3.2-4.5

Insulation resistance: 1x10^13 Ohm-cm

Moisture absorption: 0.5-1%

Temperature range: -55°C to +150°C

These properties can vary across different material formulations from different manufacturers. But in general peelable masks have moderate adhesion strength, lower mechanical toughness but high flexibility and elongation compared to standard masks.

Peelable Solder Mask Removal Process

While peelable solder mask can be removed easily by hand, the process requires some care to avoid damaging underlying components or pads:

• First, identify the areas of the board requiring access underneath the mask. Also decide sections where mask needs to stay.
• Using a sharp hobby knife, gently cut through the mask around the perimeter of the area to be peeled off. Cutting through the mask first allows it to peel cleanly.
• Next, use tweezers to grasp the edge of the cut mask section and slowly peel it off at a shallow angle. Acute peel angles work better than perpendicular pull off.
• Take care while peeling near soldered components. Slowly work around the component to free the mask from its adhesive forces. Apply even tension.
• After removal, clean the exposed area with isopropyl alcohol to remove any mask adhesive residue.
• Avoid tearing off pieces of mask residue as it can be time consuming to remove if bonded to circuit features underneath.
• The peeled off mask can be retained and potentially reapplied using adhesive if further rework is needed in the same area.

Frequently Asked Questions

Q1. Can you solder through peelable solder mask?

Yes, peelable solder mask can withstand the typical heat of soldering processes and does not debond or peel during soldering. Openings in the mask are created at the desired places for component leads to be soldered to the PCB pads.

Q2. Is peelable solder mask reworkable?

One of the key features of peelable mask is its reworkability. Mask areas that have been removed can be peeled again and replaced if required, as long as the adhesive residue is cleaned off. Some masks also allow the peeled off sections to be reapplied.

Q3. Does peelable mask provide solder bridging protection?

Yes, like standard solder mask, peelable mask acts as a dielectric barrier and prevents bridging between closely spaced PCB features. It maintains this protection even after being peeled and reapplied.

Q4. Is peelable mask available in different colors?

Peelable mask is available in a range of colors just like permanent solder mask, including green, red, yellow, black, blue and clear. Different colored masks are usually available from solder mask manufacturers.

Q5. Can conformal coating be applied over peelable mask?

In most cases, conformal coatings will adhere poorly over peelable mask because of its low surface energy formulation. So peelable mask may need to be avoided where conformal coating is required.

Conclusion

Peelable solder mask is a unique solder masking solution for applications requiring post-assembly access to circuits. It offers the protective benefits of regular solder mask but with the reworkability to peel off sections without damaging the PCB. With its customized peel-off access and easing of rework, prototyping and debugging, peelable mask usage is growing for advanced PCB designs, especially in lower volume and high mix environments.