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A solder mask is one of the essential and often overlooked components of any printed circuit board (PCB). As an integral protective coating applied during manufacturing, PCB solder mask serves several important functions that ensure your circuit board functions as intended. However, for many engineers working with PCBs, a solder mask's exact purpose and role are often unknown or taken for granted.
In this comprehensive guide, we will explain in detail what a solder mask on PCB is, the different types available, and how it critically impacts the reliability and lifespan of your PCBs. Read on to learn their essential functions and why proper solder mask coverage and selection is vital for application-specific requirements.
● Insulation: The solder mask layer insulates the board and prevents shorts between closely spaced traces or pads, thus improving electrical reliability.
● Component Placement: The solder mask defines pads and areas for component placement, aiding automatic assembly.
● Solderability: The solder mask prevents oxidation on copper traces and pads, improving their solderability. It ensures solid and reliable connections.
● Contamination Protection: The coating protects against environmental contaminants and corrosion, extending the usable lifetime of the board.
● It is highly reliable ● Protection against various hazards ● Able to coat complex geometries uniformly Cons:
● Relatively high material and processing costs
● Offers good adhesive strength to the PCB ● It has excellent chemical and thermal resistance
Cons:
● Strict environmental controls needed for the photolithography process
● Solventless process ● Uniform thin film thickness ● Easy handling and registration ● Good insulation and adhesion
● It's hard to coat complex board shapes/geometries uniformly
● Tented Vias: It is how the solder mask is applied over plated through holes (PTH) or vias on a PCB. Instead of the solder mask being fully set into the hole, it tents or domes slightly over the holes, which helps prevent solder bridges from forming between closely spaced holes or vias during assembly.
● Solder Mask Clearance: This refers to the small gap left between the edges of the solder pad areas and the solder mask material. This clearance is essential to ensure the solder has a clean area to properly wick and flow during reflow without being inhibited by the mask. It is usually a minimum of 100 micrometers around each pad.
● Solder Mask Opening: It is where the solder should be deposited and components attached. They are where the solder mask layer is absent, exposing the bare copper pads and circuitry below. Precise openings aligned with the pad areas are critical for reliable solder joints.
● Solder Mask Expansion: This refers to extending the mask slightly beyond the defined pad areas or edges of the circuit features. This small amount of overhang helps prevent solder bridging or short circuits between closely placed pads or traces during the assembly process. It provides a gap or boundary for the molten solder.
Always leave a small gap between the solder mask and the components or copper pads it covers. This gap, typically 0.005 inches, prevents the mask from bridging or coming into contact with the pads, which could cause defects. It also creates a pathway for the solder to flow and properly adhere.
Be sure not to place the solder mask over any anneal lines on the PCB. Anneal lines help relieve stress on the board and should remain free of any covering. Solder masks over annealing lines can fracture during the manufacturing process.
Verify any slots or openings in the solder mask have rounded corners and gradual tapered edges. Sharp inside angles or corners are prone to cracking, possibly exposing the underlying copper. All edges should have a minimum 0.002-inch radius.
Be sure to maintain a consistent solder mask thickness. Variations can lead to Bridging, while a thinner coat may fail to isolate pads fully. A thickness between 0.0015-0.002 inches appropriately balances protection and manufacturability.
● IPC-SM-840 outlines requirements and test methods for liquid photo-imageable solder mask materials. It covers items like recommended film thickness, curing temperature, adhesion properties, and resistance to solvents.
● IPC-SM-781 covers standard electrical and physical tests for solder masks. This includes inspection of pinhole and void defects, pad cratering, filleting, and impact resistance. Correct solder mask coverage helps prevent short circuits.
● IPC-SF-818 establishes guidelines for solder mask definitions and abbreviations. This standardizes terminology for features like solder mask color, legend alignment, and other attributes. Consistent language prevents specification errors.
● Solder bridging: Occurs when the solder flows between two closely spaced contacts or pads, causing an electrical short. It is often caused by too much solder paste being applied, flux traces left on the board, or inadequate cleaning after soldering.
● Pinholes: Pinholes are small holes or gaps in the solder mask layer that expose the underlying copper. They can be caused by contamination during solder mask application or inadequate curing. Pinholes allow the solder to wick underneath during reflow, potentially causing shorts.
● Peeling/Delamination: Peeling or delamination of the solder mask occurs when the mask layer separates from the board material. It is usually due to inadequate surface preparation of the board before solder mask application or incompatible board and solder mask materials. Peeling exposes copper that can oxidize and lead to poor solder joint formation or electrical issues over time.
● Cost/time to apply the mask.
● Risk of defects from the application.
For your next project, consider sending your Gerber files to PCBasic. As a leading PCB manufacturer and assembly house, we can produce boards for you with precision-applied solder masks. Our experienced technicians and quality processes ensure every project is done right. Contact us today to discuss your circuit board needs and get a fast turnaround on a prototype or production run.
Looking for a PCB or PCBA quote? Contact us now.
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