Solder mask (solder resist) is a protective layer of polymer that is applied to the surface of printed circuit boards (PCBs) to control solder spreading and prevent solder bridges during assembly. It also helps protect traces from environmental corrosion and electrical leakage. This article covers solder mask types, properties, functions, design considerations, application methods and defects to help PCB designers utilize this essential material correctly.
What is Solder Mask / Solder Resist?
Solder mask, also known variously as solder resist, solder stop, or solderable mask, is a thin lacquer-like insulating layer that is permanently applied to the surface of completed PCBs. The key functions of solder mask include:
- Protecting exposed copper from oxidation and environmental corrosion
- Preventing solder bridges from forming between closely spaced pads during assembly
- Electrically insulating live nets to prevent shorting
- Allowing exposed areas to be selectively coated or plated
Green colored solder mask is ubiquitous on most finished circuit boards, covering the majority of the surface while leaving metalized pads and traces exposed for soldering. Along with silkscreen, it gives production boards a distinctive appearance.
But solder mask serves important roles far beyond just looks. When designed and applied correctly, it greatly enhances PCB reliability and manufacturability.
Why Use Solder Mask on PCBs?
While solder mask adds cost and processing steps versus bare boards, it provides several functional advantages that usually justify its use:
Reduces Solder Bridging
- Prevents solder spreading between adjacent pads during assembly
Insulates Conductors
- Electrically isolates traces preventing shorts
Protects Surfaces
- Reduces corrosion and dendrite growth on traces
Enables Selective Plating
- Exposed pads can be selectively metalized while masked areas remain protected
Improves Aesthetics
- Provides a “finished” look with identifying board colors
Strengthens Pads
- Reinforces pads, especially important on flex PCBs
Enhances Assembly
- Allows solder pastes and adhesives to adhere during placement
For most multilayer and double-sided circuit boards, the benefits of solder mask outweigh its incremental costs.
Solder Mask Types
Several types of solder mask formulations are used, with each having particular benefits suited to different applications:
Epoxy Liquid Photoimageable Solder Masks (LPC/LPI)
- Most common variety, cost-effective
- Cured by UV light exposure
- Available in many colors like green, red, blue etc.
Epoxy Powder Coating
- Applied as dry powder then cured
- Lower cost but less capable
- Limited mainly to consumer products
Silicone-Based and Urethane Acrylate Solder Masks
- Withstand high temperatures and flexibility
- Used on flex boards or high temp boards
- Cost more than epoxy
Flexible Solder Masks
- Maintain flexibility after curing
- Used on flexible circuits
- More complex processing
Flame Retardant Solder Masks
- Meet flammability standards for safety compliance
- Often used on consumer goods
Each formulation has beneficial properties for particular environments and applications.
Solder Mask Design Considerations
Proper solder mask design enhances both assembly ease and board reliability:
- Specify final finished copper and mask fiducials for best alignment
- Keep clearances between mask and pads consistent
- Follow IPC guidelines like 0.2mm pullback from pads
- Verify pullbacks don’t fully expose traces
- Mask-defined land pads improve registration
- Sizes defined in both copper and mask layers
- Mask-over-pads options for easier assembly
- Select suitable mask dam sizes to control paste volumes
- Specify mask tenting over drilled vias where needed
Paying attention to solder mask design details results in boards that assemble, solder and perform reliably.
Solder Mask Properties
Important properties to evaluate when selecting a solder mask include:
- Dielectric Strength – Insulation resistance and breakdown voltage ratings.
- Adhesion – Bonding to the PCB substrate and copper.
- Resolution – Line widths and spacing it can reliably reproduce.
- Flexible and Thermo-Mechanical Properties – Strain tolerance and CTE.
- Solder and Chemical Resistance – Withstanding reflow, fluxes and cleaners.
- Flame Retardance – Burning characteristics.
- Temperature Range – Functional max and min temperatures.
- Moisture and Corrosion Resistance – Hermetic sealing effectiveness.
Verifying the mask vendor’s reported property test results ensures the material is suitable for each application and operational environment.
Applying Solder Mask to PCBs
Solder mask is applied onto PCB surfaces through a range of additive methods:
LPI – Liquid Photoimageable Ink
- Most common method
- Rolled, sprayed or screen printed
- Precise exposure cures and defines openings
Powder Coating
- Electrostatically coated with powder
- Cured either thermally or using UV light
Preformed Solder Masks
- Cured sheets laminated onto boards
- Limited applications due to alignment challenges
Dispensed Epoxies
- Dispensed over areas needing limited coating
- Manual process for rework/repair
LPI automated processing provides the best combination of quality and cost for solder mask application in volume PCB production.
Solder Mask Processing Overview
The core steps in a typical LPI solder mask application process include:
Surface Prep – Thorough cleaning and roughening for adhesion
LPI Application – Rolled, sprayed or screen printed
Pre-cure – Partial UV cure to solidify mask
Exposure – UV light transferred through artwork to define openings
Develop – Unexposed mask washed away
Cure – Full UV/thermal cure and post-treatment
Inspection – Validation of proper openings, coverage and cure
This sequence results in a precisely patterned solder mask intimately bonded to the PCB surface.
Solder Mask Defects
Common solder mask defects include:
- Bubbles – Trapped air or solvent pockets
- Insufficient Coating – Areas with inadequate thickness
- Smearing or Running – Blurred uneven edges
- Lifting – Detached areas or edges
- Pinholes – Microscopic holes penetrating the mask
- Tenting – Sagging that improperly coats through-holes
- Misregistration – Pattern inaccurately aligned
Careful process controls during application and curing minimizes issues. Thorough inspection also helps catch any defects.
Solder Mask Design Guidelines
To leverage solder mask effectively, follow these design guidelines:
- Specify sufficient clearance between mask openings and adjacent pads/traces based on registration capabilities to prevent shorts. Common values are 0.2mm clearance or greater.
- Maintain clearance around pads consistently – varying pullback allowance risks nearby shorts.
- Avoid acute angles between mask and pads which can lead to separation during thermal shock events.
- Size apertures to provide adequate solder paste volumes for assembly. A 0.1mm annular mask dam is common.
- Specify areas needing tenting over through-holes to prevent solder wicking down barrels.
- Mark all fiducials on both copper and mask layers to aid alignment verification.
- Review minimum track/space rules based on solder mask resolution limits.
Collaborating with your PCB manufacturer ensures you design effectively for their solder mask process capabilities.
Examples of Good Solder Mask Design
Here are examples of effective solder mask design and application:
Sufficient Pullback Clearance
Avoid Acute Angles
Consistent Pullback
Following solder mask design best practices prevents defects and reliability issues.
Summary of Solder Mask Uses in PCBs
In summary, key uses of solder mask in PCB fabrication and assembly include:
- Insulating conductors and preventing solder bridges between pads
- Protecting exposed copper traces from corrosion and dendrite growth
- Allowing selective metal plating of exposed pads
- Providing mechanical reinforcement of pads
- Enabling adhesive bonding of components
- Improving aesthetics with color coding
Along with silkscreen, solder mask finishing gives PCBs a quality appearance while also enhancing manufacturability and reliability.
Conclusion
The green lacquer-like surface finish of solder mask may initially seem like an aesthetic afterthought, but in fact serves many vital functions for robust, reliable printed circuit boards. When designed, selected and applied appropriately, high-performance solder mask tailored for each application improves PCB assembly yields, strengthens solder connections and extends operational lifetimes. Collaborating closely with your board fabrication partners ensures solder mask it optimized based on their capabilities and your product needs.
Frequently Asked Questions
What are some key considerations when selecting a solder mask type?
Key solder mask selection considerations:
- Capable of resolving required trace spacing and masks dams
- Suitable dielectric strength for isolation needs
- Withstands temperature cycling, vibration, chemical exposure expected
- Adheres well to substrate and copper through product lifetime
- CTE closely matches PCB substrate to avoid stresses
- Optimal color for aesthetics, branding, visible light or IR performance
Discuss these requirements with your PCB fabricator when specifying a solder mask.
What causes solder mask scratches, nicks or tears?
Common root causes of solder mask surface damage:
- Insufficient initial surface roughening for strong adhesion
- Inadequate mask thickness and coverage
- Excessive flexing stressing the mask
- Thermal shocks cycling over the material’s glass transition temperature
- Mechanical abrasion during handling like sliding PCBs together
- Chemical degradation when exposed to incompatible solvents or fluxes
Mask properties, application quality and handling care minimize these risks.
When is solder mask touch-up or reapplication needed?
Scenarios where solder mask rework may be warranted:
- Insufficient or defective mask identified after fabrication
- Damage during handling, assembly or use exposing conductors
- Thermal stresses or chemical exposure lifting mask over time
- Alterations to a design requiring modified mask areas
- Aesthetic considerations like fading or yellowing after deployment
Assess the risks of exposed conductors and address as needed based on the application.
Solder mask is a thin coating of polymer required to protect the Printed Circuit Board (PCB) from any uncertain short circuits and maintaining the integrity of the copper traces. Resin is by far the most commonly used element in its manufacturing. It is a trade-off between quality and finances. However, it does more good than harm.
Solder masks are available in many different colors including white, black, red and yellow. The most commonly available color is, however green, as most PCBs are compatible with it. Different colors can be chosen based on the customer’s demand e.g. some R&Ds are most likely to opt red PCB solder mask for their PCB prototypes in the stage of NPI so to make them distinct from the stock of other boards. Black solder mask is suitable to encourage compatibility with the color of end product if the boards need to be exposed in the products such as Tesla Coil etc. For instance, the soldering mask of Raspberry pi 3 have been shown below.
Figure 1: Solder Mask of Raspberry pi
Arduino Uno is also available in solder masks of different colors. The accompanied figure clarifies the fact.
Figure 2 : Arduino Uno with different Solder Masks
Use of Solder Mask:
PCB board Solder mask becomes increasingly significant due to the abundant increase in the demand of circuit boards in the concerned market. Technically speaking, solder mask is important due to the following reasons:
1) It prevents tarnish to take place on the covered area.
2) It helps in proper mounting of SMT (Surface Mount Technology) boards through solder paste as it helps in keeping things together.
3) In the absence of solder mask copper will be connected with the solder paste which causes short circuit and the reliability of the PCB is highly compromised.
4) Prevents Copper traces from oxidation and dust particles.
Manufacturing Process:
Solder mask manufacturing involves the cutting-edge technology Solder mask manufacturing is never an easy job. First of all, staunch regulations have to be taken into the account. Secondly, manufacturing process consists of a couple of stages involving high precision and up-to-date equipment. Therefore, it is not possible to manufacture these at home for complicated and large projects. The flow chart below shows the hierarchy of the processes involved.
- Hierarchical Process for Solder Mask Manufacturing:
1. Board Cleaning:
This step aims to clean the board from any sort of possible dust. This is a preliminary process to take a step ahead into literally anything significant.
2. Solder mask ink coating:
Once the cleaning is done the board is then forward for the solder mask coating. The solder coating is made thick considering the need of the project including reliability and sustainability. However, the choice is not that simpler as the board is not smooth all over. Therefore, the thickness differs for different sections of the board. However, standard thickness of 0.8 mils is more than enough for most of the projects.
3. Pre-Hardening:
This steps is conducted to harden the coating and the rest of the pcb substrate left behind is eradicated from the PCB. This helps in quality enhancement of the board.
4. Imaging & Hardening:
In this stage the printed image of the circuit traces are hardened onto the board by means of Ultra Violet (UV) exposure. This stage allows copper to be isolated from the copper traces of the circuit to prevent any short circuits. Correct alignment is to be ensured for proper functioning of the board.
5. Developing:
This stage involves cleaning of the unwanted solder mask so that the designated copper foil can be exposed properly.
6. Final Hardening & Cleaning:
This is the final stage before the assembly and surface finish. This involves complete mounting on mask onto the surface of the Printed Circuit Board.
Precautionary Measures:
Following precautionary measure are taken to protect PCBs from any malfunctioning:
1) No mismatching between copper pads in circuit plane.
2) Careful design file modification.
3) Insufficient solder mask used.
It is highly recommended to contact with RayPCB to be aware of the capability of the solder mask.
Reach RayPCB for suggested solder mask:
RayPCB promises to bring the best for the customers in terms of any sort of solder mask design or color specifications. The design and prototypes shall be eloquently prepared with maximum assurance.
