Home > Blog > Industry News > How is solder mask applied on PCB? If you鈥檙e evaluating a PCB Fabrication Service, solder mask is one of the fastest 鈥渢ell me how you build鈥 checkpoints in the PCB manufacturing process. On the surface, it looks like a colored coating. In reality, solder mask on PCB is a precision photolithography step that controls where solder can鈥攁nd cannot鈥攇o, protects copper from oxidation, and helps keep fine-pitch assemblies stable through reflow and years of service.
This guide explains how is solder mask applied in real production: materials, coating and imaging steps, tolerances, DFM rules, via treatments, common failure modes, and how to specify mask correctly so your boards assemble cleanly the first time.
Solder mask is both an electrical insulation layer and a process control layer. A good solder mask process:
Improves assembly yield by preventing solder bridges, especially between fine-pitch pads.
Protects reliability by reducing corrosion, moisture attack, and ionic contamination exposure on copper.
Stabilizes fine-pitch performance by keeping solder volume and wetting behavior consistent during reflow.
Supports long-term durability by resisting cracking, peeling, and chemical attack through handling and cleaning.
A common misconception is 鈥渋t鈥檚 just green paint.鈥 But modern solder mask is usually a photoimageable polymer applied to a controlled thickness, patterned with UV exposure, chemically developed, and fully cured鈥攙ery similar in concept to how circuit patterns are imaged and etched.
What you鈥檒l learn:
What solder mask does (and does not do)
Material types (LPI vs dry film vs epoxy)
The PCB solder mask process step-by-step
Thickness, clearance, registration, and via rules
Defects, root causes, and practical prevention
How to call out solder mask correctly in design files
Solder mask on PCB is a protective dielectric coating applied over copper circuitry, leaving openings only where soldering or electrical contact is needed (pads, test points, certain via types).
Core functions:
Electrical insulation: reduces accidental shorts and leakage risk, especially on dense routing.
Solder bridge prevention: stops solder from wetting unintended copper areas during assembly.
Oxidation protection: shields copper from air exposure, improving long-term stability.
Chemical & moisture resistance: helps resist flux residues, cleaning agents, and humidity.
These are often confused, but they solve different problems.
Solder mask is applied during PCB fabrication, patterned by imaging, and primarily controls solderability and protects copper.
Conformal coating is usually applied after assembly (PCBA), covering components and solder joints to protect against moisture, dust, and harsh environments.
Key differences:
Coverage: solder mask is selective (pads are opened); conformal coating usually covers almost everything unless masked off.
Thickness & purpose: solder mask is a fabrication dielectric; conformal coating is an environmental barrier.
LPI solder mask (Liquid Photoimageable) dominates modern fabrication for a reason: it balances resolution, throughput, and durability.
Why most modern boards use LPI:
Good resolution for dense SMT
Compatible with automated coating lines
Strong adhesion and chemical resistance when processed correctly
Typical traits:
Liquid polymer system applied by spray or curtain coating
UV-imaged through a phototool (or via direct imaging)
Developed to open pads and vias
Final thermal cure for chemical/thermal stability
Dry film solder mask is laminated as a film, then imaged and developed.
Why it is widely applied:
鈼 Excellent at thickness uniformity
鈼 Good for certain high-density needs where the thickness of consistent dam matters
鈼 When very rigorous requirements for mask
Trade-offs:
鈼 Lamination might struggle over aggressive topography
鈼 Process windows can be less forgiving on rough surfaces or uneven copper
Epoxy solder mask (screen printed, non-photoimageable) is older and simpler.
Where it still appears:
Low-density boards
Some quick prototypes
Cost-driven applications where fine features are not required
Limitations:
Lower resolution and less precise pad definition
More variability in thickness and edge quality
Not a great match for fine-pitch or tight mask dams
Flexible PCB solder mask: may use polyimide-based coverlays or flexible photoimageable materials designed for bending.
High-temperature solder mask: formulated for elevated thermal stress, automotive, or harsh processing.
UV-curable / inkjet-applied mask (emerging): can reduce phototool dependency and improve agility for some workflows.
Below is the practical PCB solder mask process most fabrication lines follow. Small variations exist by material, equipment, and board type, but the logic is consistent.
Before coating, the copper surface must be prepared to bond reliably.
Typical steps:
Cleaning: remove fingerprints, oils, and processing residues
Micro-etch: lightly roughen copper for mechanical adhesion
Activation: chemical or plasma treatment depending on process
Why this stage is make-or-break:
Poor prep is one of the top causes of mask peeling, undercutting, and delamination after reflow.
Contamination can cause pinholes or weak ad
Once clean, mask is applied using one of several methods:
Spray coating: good for uniform coverage on complex topography
Curtain coating: high throughput and consistent film build on flatter panels
Screen printing: legacy method, still used for some lower-density needs
Thickness control (conceptually):
Coating starts as a wet film.
After tack cure and full cure, it becomes a dry film with a stable final thickness.
The goal: enough thickness for insulation and durability, but not so much that it floods fine features or causes slivers.
This step partially dries the coating:
Drives off solvents
Stabilizes the film so it can be handled and imaged
Helps prevent flow or sag during alignment/exposure
If tack cure is too short, the film may deform during imaging; too long, and it can become harder to develop cleanly.
This is where 鈥減aint鈥 becomes 鈥減attern.鈥
鈼 A phototool (or direct imaging system) defines which areas will remain as solder mask and which will open as pads/vias.
鈼 Alignment uses fiducials and registration targets to align mask openings precisely to copper pads.
Outcomes depend on the material system, but the practical outcome is the same: openings on pads must be cleanly with accurate edges.
Panels go through a developer (often alkaline) that removes the intended areas and creates:
pad openings
via windows (if vias are not tented)
clearance around features based on defined expansion rules
This step must balance:
clean pad definition (no residue)
minimal undercut
stable dams between pads
Final cure locks in:
chemical resistance
heat resistance for lead-free reflow
adhesion strength
long-term stability against cracking
A well-cured solder mask should tolerate assembly heat cycles without softening, blistering, or becoming brittle.
Inspection looks for:
misregistration
pinholes
mask slivers
pads partially covered
unexpected openings or blocked vias
Tools and methods:
Visual inspection + magnification
AOI (Automated Optical Inspection) for consistent detection
Touch-up may be possible within acceptance limits, but heavy rework is usually a red flag for process stability.
Process Flow Diagram
Surface Clean 鈫 Micro-etch/Activation 鈫 Mask Coating
鈫
Soft Bake / Tack Cure
鈫
UV Exposure (Alignment)
鈫
Development (Open Pads/Vias)
鈫
Final Thermal Cure
鈫
Inspection (AOI/Visual) 鈫 Touch-up (if allowed) 鈫 Final Release
Method | Typical Use | Resolution | Thickness Uniformity | Cost/Throughput Fit | Best For |
LPI (liquid photoimageable) | Most modern boards | High | Good | Strong for volume + quality | Fine-pitch SMT, general purpose |
Dry film photoimageable | Specific high-control needs | High | Very good | Depends on panel topography | Tight dams, controlled thickness |
Screen-printed epoxy | Legacy / low density | Low鈥揗edium | Variable | Simple, less equipment | Low-density, basic prototypes |
Exact thickness will be different from the variants of material & spec, but the principle is consistent.
The most important thing is not 鈥渢hick or thin,鈥 but consistent鈥攊nconsistent thickness causes unpredictable assembly behavior.
Clearance is typically handled by mask expansion rules:
鈼 Mask-defined openings must fully expose pads without encroaching.
鈼 Minimum solder mask dam between adjacent pads must be insulated by dam, to prevent bridging and shorts.
Fine-pitch reality:
鈼 As pad spacing shrinks, mask dams become fragile 鈥渟livers.鈥
鈼 Slivers could lift during reflow, raise risk for solder bridging and shorts.
鈼 Many designers intentionally open mask between ultra-fine pads (or use NSMD pads) based on assembly approaches.
Mask positioning is the alignment accuracy between the mask openings and the underlying copper features.
If positioning drifts:
鈼 pads might be partially covered (not fully and poor solder wetting)
鈼 dams can become too thin (sliver lift)
鈼 clearance can shrink (bridging or inspection failures)
Good positioning leads to stable imaging, robust fiducials, and process monitoring.
A tented via is covered by solder mask.
Benefits:
鈼 reduces solder wicking
鈼 improves cosmetic and cleanliness outcomes
鈼 reduces risk of solder balls near vias
Risks:
鈼 poorly formed tents can crack or trap residues
鈼 tenting may fail if via diameter is too large or the mask thickness is insufficient
鈼 Plugged vias use resin or mask plug to close the hole opening.
鈼 Filled vias (often for via-in-pad) provide a flat surface and stronger solder control.
Non-conductive vs conductive fill:
鈼 Non-conductive is common for preventing wicking and achieving planarity.
鈼 Conductive (copper) filling is higher-performance but more process-demanding.
Some vias should stay open:
鈼 test points
鈼 ground stitching vias for EMI strategy (when specified)
鈼 vias designed for heat dissipation or inspection
Solder mask and surface finish don鈥檛 鈥渃ompete鈥濃攖hey have to work together at the pad edges. Most solder mask issues show up where three things meet: mask edge + exposed copper/pad + finishing chemistry/heat. If that interface isn鈥檛 stable, you鈥檒l see problems like edge lifting, poor adhesion, or messy pad openings.
ENIG is generally very mask-friendly for fine-pitch boards because pad planarity is good. The risk is usually not the finish/treatment itself, but mask edge quality:
鈼 Typical issues: thin 鈥渕ask lips鈥 at pad edges, micro-lifting after reflow, rough pad opening edges if imaging/development is off.
鈼 What helps: tight registration control, clean development to avoid ragged edges, and a solid final cure so the mask doesn鈥檛 soften during assembly.
HASL introduces more topography (uneven solder thickness), and lead-free HASL typically runs at higher temperatures鈥攂oth can stress the mask.
鈼 Typical issues: mask thinning on sharp height changes, small cracks near pads, occasional edge pull-back after thermal cycling.
鈼 Optional: choose coating methods that handle uneven surfaces (spray is often more forgiving), maintain proper tack cure to prevent flow, and ensure the cure profile which matches lead-free thermal loads.
OSP relies heavily on cleanliness and handling discipline, which also affects mask adhesion near pads.
鈼 Typical issues: contamination at the copper surface leading to weak mask bonding, poor-looking pad edges if prep is inconsistent, sensitivity to rework/extra heat cycles.
鈼 Proper surface prep (clean + controlled micro-etch), strict process cleanliness (no fingerprints/ionic residues), and well-controlled bake/cure so the mask locks in adhesion.
These finishes can perform well, but they鈥檙e more sensitive to storage conditions and process residues, which can indirectly trigger mask-edge problems.
鈼 Typical issues: staining/tarnish concerns near openings, mask-edge discoloration, occasional adhesion complaints if pre-cleaning and post-process rinsing aren鈥檛 tight.
鈼 What helps: controlled storage/packaging, disciplined rinsing/drying, and confirming compatibility between the mask system and the finish line chemistry.
Most 鈥渕ask vs finish/treatment鈥 problems are actually caused during process. The most common root causes and solutions:
鈼 Poor surface preparation 鈫 peeling / edge lifting
Solution: strengthen cleaning + micro-etch + activation, reduce time between prep and coating.
鈼 Under-cure or over-cure 鈫 soft mask or brittle mask
Solution: validate cure profile (time/temperature) for your mask system and lead-free requirements.
鈼 Coating too thick/thin for feature density 鈫 slivers or pad encroachment
Solution: tune coating method and thickness target; relax dam requirements via DFM for ultra-fine pitch.
鈼 Positioning drift 鈫 partial pad coverage or weak dams
Solution: improve fiducials and panel stability; align mask expansion rules with fabrication capability.
Symptoms: pad partially covered, uneven dams, shifted openings
Causes: poor alignment, unstable imaging, insufficient fiducials, panel dimensional changes
Prevention: tighter positioning control, better tooling, DFM review for tight-pitch areas
Symptoms: tiny openings that expose copper
Causes: contamination, trapped air, coating defects, poor filtration
Prevention: better cleaning, controlled coating environment, material handling discipline
Symptoms: mask lifts near pads or along traces
Causes: weak surface preparation, under-cure, chemical incompatibility, moisture contamination
Prevention: robust micro-etch/activation, correct cure profile, moisture control
Symptoms: cracks over areas or near board edges
Causes: brittle mask selection, over-cure, mechanical stress, wrong material for flexing
Prevention: flex-appropriate materials, design rules for bend zones, controlled cure strategy
Symptoms: thin dams peel and float, creating bridge risk
Causes: the width of dams is lower than constraint width, aggressive expansion rules, poor positioning, heavy topography
Prevention: DFM-driven dam rules, consider opening mask between pads in ultra-fine pitch, improve alignment control
Responsibility split:
鈼 Design-driven: impossible dams, too-tight clearances, missing notes
鈼 Process-driven: preparation, coating, imaging, development, cure control
You typically provide:
Top solder mask
Bottom solder mask
These layers define where mask is removed (openings) relative to copper pads.
Key point: your CAD mask expansion rules must align with fabrication capability and assembly needs.
At minimum, specify:
solder mask type (e.g., LPI)
color (if required)
any special requirements (high-temp, flex zones, via tenting rules)
acceptance priorities (pad exposure, dam requirements)
Before release, check:
minimum dam width in tight pitch zones
via-in-pad rules (filled/capped requirements)
high-voltage spacing rules (mask isn鈥檛 a substitute for clearance)
test point openings defined clearly
consistent mask expansion strategy
Myths vs reality:
Color does not magically change 鈥渆lectrical performance鈥 in normal designs.
What matters is the material system and cure, not pigment.
Where color does matter:
inspection visibility (contrast with silkscreen and copper)
optical applications (LED boards, sensors) where reflectivity and stray light matter
heat absorption differences can matter in niche cases, but it鈥檚 rarely the main driver
Choose color based on inspection, branding, and optical needs鈥攏ot assumptions.
IPC standards matter because they turn 鈥渓ooks OK鈥 into measurable acceptance criteria鈥攅specially when you鈥檙e building boards that must survive heat, vibration, humidity, and long service life.
IPC-SM-840 is mainly about the solder mask material itself鈥攈ow the mask system is qualified and what performance it should meet (adhesion, insulation performance, chemical resistance, durability, etc.).
In practice, it helps buyers and engineers confirm the mask ink is not just 鈥渁ny epoxy,鈥 but a controlled material system suitable for the intended reliability level.
IPC-6012 is a broader PCB qualification/acceptance standard. For solder mask, it connects the mask layer to board-level requirements, such as:
鈼 coverage and consistency on the PCB
鈼 acceptable cosmetic vs functional defects
鈼 reliability expectations based on product class
Think of it as: IPC-SM-840 = material standard, while IPC-6012 = finished PCB acceptance standard.
IPC product classes reflect the reliability target, and they influence how strictly solder mask issues are judged:
鈼 Class 1 (General electronics): basic functional requirements; cosmetic issues are often tolerated if they don鈥檛 affect soldering or insulation.
鈼 Class 2 (Dedicated service / industrial): tighter control; mask alignment, coverage, and defect limits become more important because boards must be more stable over time.
鈼 Class 3 (High reliability): the strictest level; solder mask must be highly consistent because any weakness can become corrosion paths, leakage risk, or assembly defects.
In medical, aerospace, automotive, and other high-reliability environments, solder mask is treated as a functional protection layer, not decoration. Standards and class targets help control risks like:
鈼 moisture ingress and corrosion
鈼 leakage or creepage failures at higher voltages
鈼 solder bridging and fine-pitch assembly fallout
鈼 long-term insulation breakdown after thermal cycling
Case guidance:
High-density SMT / fine pitch: LPI or dry film with proven resolution and registration stability
Power electronics: focus on adhesion, thermal stability, and chemical resistance
RF boards: prioritize consistent thickness and stable dielectric behavior; coordinate with impedance strategy
Automotive / harsh environments: higher reliability classes and strong corrosion resistance
Flex and rigid-flex: choose materials designed for bending; define bend zones clearly in fab notes
Not always, but it鈥檚 standard for most production boards because it protects copper and improves assembly yield.
Yes, in high-speed designs solder mask acts as part of the dielectric environment. Consistent thickness and controlled stackup assumptions matter.
Common causes include poor surface prep, insufficient cure, contamination, or too-thin dams that lift under thermal stress.
NSMD pads are defined by copper geometry; SMD pads are defined by mask opening. Depends on package, pitch, and assembly strategy.
Solder mask is one of those fabrication steps that quietly decides whether a project could be assembled smoothly or turns into rework & yield loss. When the process is controlled鈥攕urface prep, coating uniformity, UV imaging alignment, development precision, and full cure鈥攜ou get stable insulation, predictable soldering outcomes, and better long-term corrosion resistance.
If your project includes tight fine-pitch, via-in-pad, HDI features, or might be working in harsh-environment, don鈥檛 take solder mask as a default checkbox. Align mask rules with the PCB manufacturing process, confirm manufacturable dams and clearances, and communicate with your manufacturer at early stage for a DFM. That鈥檚 how you turn 鈥渟older mask on PCB鈥 from 鈥済reen paint鈥 into a reliability.
PCB Manufacturing Process Step-by-Step
PCB Inner Layer Imaging & Etching Process
What is the PCB Lamination Process?
How do drilling (mechanical/laser) processes work in PCB manufacturing?
What is the plating process in PCB Manufacturing?
Ultimate Guide to PCB silk screen printing process
What is the surface finish process in PCB?
What are the most common methods of PCB testing?
Sonic Yang
As a major of Electronics and Mechanical Automation, Sonic has been engaged in PCB design, R&D, manufacturing of eletronics for around 22 years, as engineering director and coordinates with supply chain(components&CNC parts), providing professional supports and consults for global customers.
en
WhatsApp