Introduction
A printed circuit board (PCB) is the foundation of electronics. It provides the mechanical structure to mount and interconnect electronic components using copper traces etched from copper sheets laminated onto a non-conductive substrate. To protect the copper traces from corrosion and short circuiting, a protective coating called solder mask is applied over the entire PCB except for areas requiring soldering.
Peelable solder mask, also known as temporary solder mask, is a type of solder mask coating that can be partially peeled off to expose selected areas of the PCB copper for soldering components, thereby eliminating the need to undergo a complex masking and etching process to create openings in the solder mask.
This article provides an in-depth understanding of peelable solder masks, their properties, types, applications, and methods of application and removal.
What is Peelable Solder Mask?
Peelable solder mask, as the name suggests, is a protective solder mask coating that can be selectively peeled off to expose underlying PCB copper traces for soldering electronic components. It is a temporary mask which is meant to be removed after soldering is completed, unlike permanent solder mask which remains on the PCB throughout its lifespan.
Peelable solder mask is typically a thin film coating, around 0.03 to 0.10 mm thick. It uses acrylic, silicone, polyurethane or epoxy based polymers which exhibit selective adhesion to copper and FR-4 substrate. This allows the mask to be peeled off copper pads while remaining adhered to the PCB substrate.
Key Properties
The properties of an ideal peelable solder mask include:
- Easy peelability from copper while adhering to FR-4 substrate
- Clean peel without any residue
- Good solderability through the peelable mask
- Chemical, thermal and mechanical resistance
- Thin application
- Accuracy and consistency in peel strength
Why Use Peelable Solder Masks?
Peelable solder masks provide the following benefits:
1. Eliminates Solder Mask Etching
- In standard PCB fabrication, openings are created in the solder mask through an additional masking and chemical etching step to expose underlying copper pads. This requires an extra process step.
- Peelable masks avoid this solder mask etching process by allowing selective peeling to expose copper pads.
2. Simplifies Prototyping and Design Changes
- For prototypes and low volume production, creating a new solder mask for each board revision is time-consuming and expensive.
- Peelable masks allow quick turnaround of prototypes and design changes by avoiding repeated solder mask etching steps. Solder mask openings can be modified by simply peeling the required areas.
3. Allows Late Stage Solder Mask Customization
- With standard solder masks, the mask openings must be predefined prior to manufacturing, limiting flexibility.
- Peelable masks permit late stage customization of solder mask openings according to needs, thereby providing flexibility and reducing inventory.
4. Reduces Risk of Solder Mask Slivers
- Etching openings in solder mask carries the risk of mask slivers remaining along the edges of copper pads which can interfere with soldering.
- Peelable masks avoid this problem since the mask is cleanly peeled off to expose copper pads.
Types of Peelable Solder Masks
Peelable solder masks are classified based on their peel strength as:
1. Ultra-Low Peel Strength Masks
These masks have a peel strength of less than 0.2 oz/in (5 g/cm). They can be peeled off manually without using any tools due to their very low peel strength.
However, their adhesion is so low that they can prematurely release even with minor mechanical or thermal stresses. This makes them unsuitable for processing steps like wave soldering.
2. Low Peel Strength Masks
These masks have a peel strength between 0.2 to 4 oz/in (5 to 100 g/cm). A simple hand tool can be used to peel them off from copper pads. They offer a good balance between peelability and adequate adhesion to withstand PCB fabrication processes.
3. Medium Peel Strength Masks
Their peel strength ranges from 4 to 10 oz/in (100 to 250 g/cm). They require a peel tool with higher grip or leverage to remove them. Offer better adhesion than low peel masks.
4. High Peel Strength Masks
These masks have a high peel strength of 10 to 16 oz/in (250 to 400 g/cm) and must be removed using a pneumatic peel tool. Provide very high adhesion but can be difficult to peel manually.
Peelable Solder Mask Materials
Some common materials used to manufacture peelable solder mask products are:
- Acrylic based: Offer good solderability but have low chemical and temperature resistance. Popular for simple, low temperature applications.
- Polyurethane based: Provide high peel strength and chemical/temperature resistance. Widely used for wave soldering applications.
- Silicone based: Have very high temperature resistance but poor chemical resistance. Used where high heat is involved.
- Epoxy based: Offer a balance of chemical resistance, peel strength and heat range. Used for many moderate temperature applications.
- UV Curable: Applied as liquid which is then cured using UV light. Allows in-house application and customization.
Application of Peelable Solder Masks
Peelable solder masks are applied using the following methods:
Liquid Photoimageable Method
This is similar to standard liquid photoimageable solder mask application:
- Liquid peelable solder mask is applied over full PCB surface by screen printing, roller coating, or curtain coating.
- It is then dried and exposed to UV light through a film mask.
- Unexposed mask is developed away, leaving solder mask only in desired areas.
- The remaining mask is then cured by heat or UV exposure.
Lamination Method
This involves lamenting a dry peelable solder mask film:
- The solder mask film is first laminated onto a carrier film to create a sandwich structure.
- This 3-layer sandwich is then laminated onto the PCB under heat and pressure.
- The carrier film is subsequently peeled away, leaving the peelable mask securely attached to the PCB.
Electrodeposition Method
This method allows in-house application of liquid peelable solder mask:
- The PCB acts as the cathode while an anode is used to electrolyze the liquid solder mask solution.
- This deposits a thin even coating of the mask uniformly across the PCB surface.
- The mask is then cured by heating to finalize the coating.
Pre-Coated Copper Foil Method
This avoids post-etch application of solder mask:
- Peelable solder mask is coated directly onto bare copper foil prior to lamination onto FR-4 substrate.
- This pre-coated copper foil is then laminated onto substrate along with other layers.
- The mask remains on top of copper traces, avoiding any post-etch application steps.
Removal of Peelable Solder Masks
Peelable masks are removed from copper pads using the following techniques:
Manual Peeling
Low peel strength masks can be peeled off manually using fingers, tweezers or a simple hand tool. This provides limited control and is not suitable for stronger masks.
Pneumatic Peel Tools
Pneumatic suction cups are used to grip and lift off the peelable mask from copper pads. The vacuum adheres to the mask while edge rollers peel it cleanly. It provides good control and works on stronger masks.
Pre-Patterning with Cuts
The peelable mask is laser cut or scored along the boundaries of intended peel areas prior to peeling. This weakens the mask adhesion in required areas so that it can be peeled off easily and precisely along existing cuts.
Thermal Shock Method
A high temperature air blower is used to heat the mask until it softens and loses adhesion. The heat-weakened mask can then be peeled cleanly from copper pads using a tool.
Applications of Peelable Solder Mask
Some common applications of peelable solder mask in PCB fabrication and assembly are:
- Prototyping – Allows quick turnaround by avoiding mask etch steps.
- Low volume production – Ideal for small batch manufacturing.
- Design modifications – Enables easy design changes by peeling mask in required areas.
- Mixed PCB assembly – Supports assembling components with different footprints on same board.
- Rework and maintenance – Helps modify solder mask for reworking and repairs.
- Wave soldering – Compatible with high temperature wave soldering process.
- 3D packaging – Used in advanced 3D packaging like Package-on-Package.
- COB assembly – Useful for chip-on-board assemblies.
- RF/high speed – Suitable for high frequency boards which need exposed pads.
Pros and Cons of Peelable Solder Masks
Pros
- Eliminates solder mask etching step
- Allows late stage customization of mask openings
- Simplifies PCB modifications and rework
- Improves solder mask adhesion around component pads
- Compatible with high temperature processes like wave soldering
- Peelable masks cost less than solder mask etching
Cons
- Can increase overall PCB fabrication cost
- Patchy residue may be left after peeling
- Mask adhesion varies across different materials
- Requires precise peel tools and skilled operators
- Peel strength reduces over time if not peeled immediately
- More difficult to control compared to standard mask etching
Frequently Asked Questions
Q1. Can peelable solder mask withstand soldering heat?
Yes, most types of peelable solder masks are formulated to withstand the high temperatures involved in soldering processes like reflow soldering and wave soldering. Polyurethane and epoxy based masks have good thermal resistance.
Q2. Does peelable mask provide soldering quality comparable to etched solder mask?
Peelable masks allow equivalent soldering quality and reliability as obtained through standard solder mask etching, provided the masks are cleanly removed from copper pads. No residue should be left.
Q3. Can peelable solder mask be reapplied after peeling?
Once peeled off, the same peelable solder mask cannot be reapplied on copper pads. A fresh coat of mask will have to be applied using the standard application methods.
Q4. How long can a PCB with peelable mask be stored before peeling?
Peelable mask should ideally be removed within 7-10 days of PCB fabrication to ensure optimal peel strength. Many masks lose peelability over time if left for several weeks before peeling.
Q5. Does peelable mask increase PCB fabrication cost?
Peelable solder masks typically cost more compared to traditional solder mask that requires etching. But they eliminate the cost associated with solder mask etching. Overall, they may result in cost reduction due to process simplification.
