How to Choose the Right Photoresist: Dry Film, Liquid Resist, and How Exposure Works

Photoresist selection is often treated as a small process detail in PCB manufacturing. In reality, it directly affects circuit definition, finished trace width, etching accuracy, production yield and overall manufacturing cost.

Before deciding between dry film and liquid photoresist, it is useful to understand what photoresist actually does.

How does a continuous copper surface become a pattern of fine PCB traces?

Ultraviolet light does not directly cut or remove the copper. Instead, it changes the chemical structure and development solubility of selected areas of the photoresist.

For a typical negative-working dry film, the basic reaction is:

UV light → Photoinitiators → Free radicals → Polymerization → Crosslinking → Developer resistance

This photochemical reaction creates a temporary protective pattern on the copper surface. Development, etching and stripping then convert that pattern into the required copper circuit.

Understanding this principle makes it easier to choose the right photoresist for a specific PCB design and manufacturing process.

What Is PCB Photoresist?

PCB photoresist is a light-sensitive material used to transfer circuit data onto a copper surface.

It acts as a temporary protective mask during processes such as:

• Copper etching
• Pattern plating
• Hole tenting
• Selective metal processing
• Fine-line circuit formation

After exposure and development, selected areas of the photoresist remain on the board while other areas are removed.

The remaining resist controls where subsequent chemical or electrochemical processing can occur.

After the circuit has been formed, the photoresist is stripped away.

Photoresist should not be confused with solder mask.

Circuit photoresist is normally a temporary process material. Solder mask is a permanent coating that remains on the finished PCB to protect conductors and reduce soldering shorts.

What Does Photoresist Contain?

Exact formulations vary according to the supplier, product family and application. However, a typical negative-working photoresist contains several functional components.

Film-Forming Resin or Binder

The resin or binder forms the basic structure of the photoresist.

It provides mechanical strength, flexibility, adhesion and film-forming properties.

In dry film, it allows the photosensitive material to be produced and handled as a solid sheet.

In liquid photoresist, it helps the coated material form a continuous film after leveling and drying.

Polymerizable Monomers or Oligomers

These molecules contain reactive groups that participate in photopolymerization.

After exposure, they connect to form larger polymer chains and crosslinked structures.

Photoinitiators

Photoinitiators absorb light at a suitable wavelength and generate reactive species.

In common free-radical negative-working dry films, the reactive species are free radicals.

These radicals initiate polymerization and crosslinking.

Adhesion Promoters, Plasticizers and Other Additives

Additional ingredients may be used to adjust:

• Adhesion to copper
• Flexibility
• Photosensitivity
• Development behavior
• Chemical resistance
• Storage stability
• Film color and visibility

The complete formulation determines how the material behaves during lamination or coating, exposure, development, etching, plating and stripping.

How Does Dry-Film Photochemistry Work?

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How PCB dry film photoresist works through UV exposure, free-radical polymerization, crosslinking, development, etching and stripping

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Figure 1: The photochemical reaction of a typical negative-working dry film and a simplified subtractive circuit-forming process. Photoinitiators absorb UV light and generate free radicals, initiating polymerization and crosslinking. The exposed resist becomes resistant to the developer and protects the copper underneath. Unprotected copper is then etched away, and stripping reveals the finished copper traces.

The illustration connects two levels of the PCB imaging process.

The upper section shows what happens inside the dry film:

Light generates free radicals → Polymerization and crosslinking occur → The exposed region becomes resistant to the developer

The lower section shows how this chemical difference is used to form copper circuits:

Development → Etching → Stripping → Finished copper traces

Step 1: Photoinitiators Absorb Ultraviolet Light

Before exposure, the negative-working dry film remains developable.

When the required circuit image is transferred through conventional phototool exposure or laser direct imaging, selected areas receive light at a wavelength that matches the photosensitive system.

The photoinitiators absorb this energy and enter an activated state.

They then undergo decomposition or energy transfer, producing reactive free radicals.

Two conditions are particularly important:

• The exposure wavelength must match the photosensitive range of the resist
• The exposure energy must be sufficient to initiate the required reaction

If the wavelength does not match the material, simply increasing exposure time may not produce a satisfactory image.

Step 2: Free Radicals Are Generated

A free radical contains an unpaired electron and is therefore highly reactive.

The radicals generated by the photoinitiators react with the double bonds in photosensitive monomers or oligomers.

This reaction creates new active centers that continue reacting with nearby molecules.

Free radicals can be understood as the switch that starts the polymerization process.

If insufficient radicals are generated, polymerization and crosslinking will remain incomplete.

Step 3: Polymerization Begins

As the active centers continue reacting, more monomers become connected, forming longer molecular chains.

This process is called polymerization.

The average molecular weight of the exposed region increases, and its mechanical and chemical properties begin to change.

The exposed resist becomes stronger and less soluble in the developer.

Step 4: A Crosslinked Polymer Network Forms

When the photosensitive molecules contain multiple reactive groups, the reaction does not produce only simple linear chains.

Different molecular chains connect at multiple points, creating a three-dimensional crosslinked network.

This crosslinked structure normally provides:

• Higher molecular weight
• Greater mechanical strength
• Lower solubility in the developer
• Improved resistance to development
• Improved resistance to etching or plating chemicals

This is the essential change produced by exposure:

UV light does not directly create the copper traces. It changes the molecular structure and solubility of selected areas of the photoresist.

Why Do Exposed Areas Remain After Development?

For the negative-working dry film shown in the illustration:

Exposed Areas

The photoinitiators absorb light and generate free radicals.

Polymerization and crosslinking then create a stable polymer network. This network does not readily dissolve in the developer, so the exposed resist remains on the copper surface.

Unexposed Areas

The unexposed regions do not receive enough energy to form a sufficiently crosslinked structure.

They remain soluble or dispersible in the developer and are removed.

The principle can be summarized as:

Unexposed resist → Insufficient crosslinking → Removed during development

Exposed resist → Polymerized and crosslinked → Remains after development

This difference in solubility is the foundation of negative photoresist imaging.

From Photoresist Image to Copper Circuit

The lower part of the illustration shows a simplified subtractive process using negative-working dry film.

PCB manufacturers can also use pattern plating and other circuit-forming processes. The areas protected by the resist and the following processing steps may differ.

However, the basic function of photoresist remains the same:

Photoresist creates a selective protective pattern so that chemical or electrochemical processing occurs only in the required areas.

1. Copper Surface Preparation

Before lamination or coating, the copper surface must be cleaned and appropriately prepared.

The process removes:

• Oil
• Fingerprints
• Dust
• Oxides
• Moisture
• Chemical residue

The prepared copper surface allows the photoresist to make uniform and reliable contact with the board.

2. Lamination or Coating

Dry film is laminated onto the copper surface using controlled temperature and pressure.

Liquid photoresist is applied as a coating and then leveled and prebaked to form a continuous photosensitive layer.

At this stage, the copper is completely covered by the resist.

3. Exposure

Circuit data processed by CAM is transferred to the photoresist through a phototool or LDI system.

For common negative-working resist, the areas representing the pattern to be retained receive light and undergo polymerization and crosslinking.

Important exposure factors include:

• Light wavelength
• Exposure energy
• Light-intensity uniformity
• Resist thickness
• Phototool contact
• Registration accuracy
• LDI compensation
• Material expansion and contraction

4. Development

The developer removes the unexposed photoresist while the exposed and crosslinked regions remain.

After development, the required protective pattern is visible.

However, the copper underneath is still a continuous layer. The actual circuit has not yet been formed.

5. Etching

In the simplified subtractive process shown in the image, copper without resist protection is removed by the etchant.

Copper beneath the remaining photoresist is protected and becomes the required circuit pattern.

Etching does not proceed only downward. It also moves laterally beneath the resist to some extent. This effect is called lateral etching or undercut.

The thicker the copper, the longer the required etching process normally becomes, and the more difficult it is to control undercut.

This is why the following requirements must be considered together:

• Copper thickness
• Minimum line width
• Minimum spacing

For example, producing 3/3 mil traces with 0.5 oz copper is not equivalent to producing the same geometry with 2 oz copper.

6. Stripping

After etching, the photoresist has completed its protective function and is removed.

The finished copper traces are then revealed.

The complete process can be summarized as:

Copper preparation → Lamination or coating → Exposure → Polymerization and crosslinking → Development → Etching → Stripping → Copper circuits

Dry Film vs. Liquid Photoresist

Dry film and liquid photoresist perform the same basic task: they form a photosensitive layer on copper and create a temporary protective pattern through exposure and development.

Their most obvious difference is how the photosensitive layer is applied.

Dry Film Photoresist

Dry film is supplied as a solid photosensitive sheet with a relatively consistent thickness.

It is laminated onto the prepared copper surface using controlled heat and pressure.

The basic process is:

Copper preparation → Lamination → Exposure → Development

Dry film is widely used for PCB inner-layer and outer-layer circuit production because its thickness is predictable and its manufacturing process is well established.

Liquid Photoresist

Liquid photoresist is coated onto the copper surface in liquid form.

It requires leveling and prebaking so that the solvent evaporates and a continuous photosensitive film forms.

The basic process is:

Copper preparation → Liquid coating → Leveling and prebaking → Exposure → Development

The clearest difference therefore occurs before exposure:

• Dry film forms the photosensitive layer through thermal lamination
• Liquid photoresist forms the layer through coating and drying

After the layer has been formed, both rely on light-induced chemical changes and development to create the required pattern.

For a more detailed comparison, read:

Comparison of Liquid Photoresist and Dry Film

Negative vs. Positive Photoresist

“Dry film or liquid resist” describes the material form and application method.

“Negative or positive” describes which areas are removed during development.

Negative Photoresist

Exposure causes polymerization, crosslinking or another chemical change that makes the exposed regions less soluble.

Therefore:

• Exposed areas remain
• Unexposed areas are removed

Positive Photoresist

Exposure causes a chemical change that makes the exposed regions more soluble.

Therefore:

• Exposed areas are removed
• Unexposed areas remain

Liquid photoresist is not automatically positive-working, and dry film does not describe one universal chemical formulation.

The correct material depends on the imaging method and complete production process.

Why Exposure Control Matters

Exposure energy must be sufficient to complete the necessary photochemical reaction, but more exposure is not always better.

Insufficient Exposure May Cause:

• Incomplete activation of photoinitiators
• Insufficient free-radical generation
• Incomplete polymerization and crosslinking
• Poor resistance to the developer
• Damage to narrow resist features during development or etching

Excessive Exposure May Cause:

• Unwanted reaction around image boundaries
• Expansion of the protective pattern
• Reduced spacing between traces
• Bridging between adjacent features
• Difficulty during development
• Loss of fine-image accuracy

Exposure parameters must be matched with:

• Photoresist type
• Resist thickness
• Photosensitive wavelength
• Circuit geometry
• Exposure equipment
• Development conditions
• Production speed

PCB manufacturers normally establish a controlled exposure window using exposure scales, test panels, first-article inspection and production records.

Why Development Is Equally Important

Exposure creates a chemical difference between selected areas of the photoresist.

Development converts that chemical difference into a physical circuit pattern.

If the unexposed resist is not completely removed, a very thin and difficult-to-see residue may remain on the copper.

This residue can prevent the etchant from contacting the copper, leaving unwanted copper or creating shorts.

If development is excessive, small resist features that should remain may be damaged, reducing their ability to protect the copper during etching.

Exposure and development must therefore be treated as one controlled imaging system.

How to Choose the Right Photoresist

The correct photoresist is not necessarily the most expensive or technically advanced product.

It is the material that provides a sufficiently stable manufacturing window for the actual PCB.

Selection should consider the complete process rather than material price alone.

1. Circuit-Forming Method

First determine whether the resist will be used for:

• Inner-layer subtractive etching
• Outer-layer pattern plating
• Hole tenting
• Fine-line imaging
• Special metal processing

An etch resist must protect copper reliably during etching.

A plating resist must withstand cleaning, activation, plating chemistry and repeated rinsing while maintaining stable feature walls.

2. Minimum Line Width and Spacing

Finer geometry requires better imaging resolution, adhesion, development control and dimensional stability.

However, the material supplier’s stated resolution is not the same as the PCB factory’s stable production capability.

Actual results also depend on:

• Copper thickness
• Exposure equipment
• Registration
• Development
• Etching capability
• Pattern distribution
• Production quantity
• Finished tolerances

3. Copper Thickness

Thicker copper normally requires a longer and more aggressive etching process.

This increases the challenge of controlling lateral etching and finished conductor geometry.

For heavy-copper boards, the main question is not only whether the circuit can be imaged. The resist must continue protecting the copper throughout the longer etching process.

4. Resist Thickness

A thicker resist may be useful for certain plating processes or when greater chemical resistance is required.

However, thicker resist does not automatically provide better fine-line capability.

Greater thickness can make exposure, development and sidewall control more difficult.

Resist thickness should be selected according to:

• Copper thickness
• Circuit dimensions
• Hole structure
• Plating requirements
• Subsequent chemical processing

5. Exposure Equipment and LDI Compatibility

When LDI is used, the photoresist must match the equipment’s wavelength and exposure-energy range.

A material developed for conventional phototool exposure may not provide the same photosensitivity, resolution or development window with an LDI system.

Important factors include:

• Photosensitive wavelength
• Required exposure energy
• Resolution
• Development conditions
• Registration stability
• Compatibility with production speed

6. Surface Condition and Adhesion

The resist must remain attached to the copper throughout development, etching or plating.

Surface preparation and adhesion become especially important for:

• Fine traces
• Large copper areas
• Thin boards
• Uneven surfaces
• Difficult hole structures
• Longer chemical processing cycles

7. Development and Stripping

The developer must completely remove unwanted photoresist without damaging the features that need to remain.

After etching or plating, the resist must also strip efficiently without leaving residue between fine features or around holes.

8. Total Manufacturing Cost

Photoresist purchase price is only one part of the real cost.

The complete cost also includes:

• Lamination or coating efficiency
• Exposure time
• Developer consumption
• Etching or plating yield
• Rework
• Scrap
• Stripping efficiency
• Equipment capacity
• First-pass yield

A slightly more expensive resist with a wider and more stable process window may reduce total PCB manufacturing cost.

A Practical Selection Guide

PCB or Process Requirement	Main Photoresist Consideration
Standard inner-layer etching	Stable lamination, development and etch resistance
Fine-line circuits	Resolution, adhesion, exposure window and development control
Heavy copper	Strong adhesion and resistance during longer etching
Pattern plating	Plating-chemical resistance and stable resist sidewalls
Hole tenting	Film strength and reliable coverage across openings
LDI production	Wavelength sensitivity and suitable exposure energy
High-volume production	Repeatability, throughput and total process cost
Thin or flexible material	Lamination behavior, flexibility and dimensional stability

The selection should always be validated through actual production conditions rather than based only on a product datasheet.

Why Photoresist Selection Affects Final Trace Width

The customer’s Gerber data defines the intended circuit geometry.

The actual copper trace must still pass through:

• CAM compensation
• Photoresist imaging
• Development
• Etching or pattern plating
• Stripping
• AOI inspection

Final conductor width may be affected by:

• Starting copper thickness
• Finished copper thickness
• Resist thickness
• Resist resolution
• Exposure energy
• Light spreading
• Development conditions
• Etching undercut
• Pattern-plating uniformity
• Circuit distribution
• Equipment and chemical stability

Being able to image a particular line width does not necessarily mean that the manufacturer can consistently produce a compliant finished copper trace.

Meaningful capability is:

The ability to manufacture finished conductors consistently under the specified copper thickness, circuit distribution, production quantity and acceptance tolerances.

For multilayer PCBs, circuit imaging must also be coordinated with layer registration, material movement and lamination structure.

Learn more about:

Multilayer PCB Manufacturing

What Should PCB Designers and Buyers Confirm?

PCB customers normally do not need to specify a particular dry-film brand.

They should instead clearly define:

• Starting and finished copper thickness
• Minimum line width and spacing
• Finished conductor-width tolerance
• Impedance requirements
• IPC class
• Production quantity
• Reliability requirements

When evaluating a PCB manufacturer, useful questions include:

1. What copper thickness applies to the stated minimum line width and spacing?
2. Does the factory use conventional exposure or LDI?
3. Are fine-line circuits inspected by AOI?
4. Are published capabilities maximum values or stable production values?
5. Does the factory perform a dedicated DFM review for heavy-copper fine-line designs?
6. Will the manufacturer communicate before production if the design approaches the process limit?

A reliable PCB manufacturer should not simply say, “Yes, we can make it.”

A more useful answer explains:

• Under what conditions the design can be manufactured
• Which features create production risk
• Whether copper thickness, line width or spacing should be adjusted
• Whether prototype and volume production can follow consistent control requirements

Conclusion

Choosing the right PCB photoresist begins with understanding how the material works.

For the negative-working dry film shown in the illustration:

Photoinitiators absorb UV light → Free radicals are generated → Polymerization begins → A crosslinked network forms → Exposed areas resist the developer

The subsequent process is:

Development removes unexposed resist → Etching removes unprotected copper → Stripping reveals the finished circuit

Dry film and liquid photoresist differ mainly in how the photosensitive layer is formed:

• Dry film is laminated onto copper using temperature and pressure
• Liquid photoresist is applied through coating, leveling and prebaking

The correct choice depends on the complete manufacturing system:

• Circuit-forming method
• Copper thickness
• Minimum line width and spacing
• Resist thickness
• Exposure equipment
• Development conditions
• Etching or plating process
• Production volume
• Required yield

SQPCB uses circuit LDI, AOI and controlled PCB manufacturing processes to support standard FR-4, multilayer, fine-line, heavy-copper, controlled-impedance and high-reliability products.

If a project includes fine traces, heavy copper, a special stack-up or strict finished-conductor requirements, a DFM review should be completed before production.

For projects requiring shorter manufacturing schedules, learn more about:

Quick-Turn PCB Manufacturing

Frequently Asked Questions

1. What is the main difference between dry film and liquid photoresist?

Dry film is supplied as a solid photosensitive sheet and laminated onto copper. Liquid photoresist is coated onto the copper surface and dried before exposure.

2. Does UV light directly create the copper traces?

No. UV light changes the chemical structure and development solubility of selected resist areas. The copper traces are formed later through etching or plating.

3. Why do exposed areas of negative dry film remain after development?

Exposure initiates polymerization and crosslinking, creating a polymer network that does not readily dissolve in the developer.

4. Is liquid photoresist always positive-working?

No. Liquid or dry describes the material form and application method. Positive or negative describes which areas are removed during development.

5. Is dry film better for fine-line PCBs?

Dry film can provide reliable fine-line imaging when the material, copper thickness, exposure, development and etching processes are properly matched. The complete manufacturing process is more important than the material name alone.

6. Why is heavy copper difficult to combine with fine traces?

Thicker copper generally requires longer etching, which creates greater lateral undercut and makes finished conductor width more difficult to control.

7. Is photoresist still required when LDI is used?

Yes. LDI changes the exposure method but does not eliminate the photoresist. The digital circuit image still needs to be transferred to a photosensitive material.

8. Does higher exposure energy always improve resist strength?

No. Insufficient exposure causes incomplete crosslinking, while excessive exposure can enlarge features, reduce spacing and make development more difficult.

9. Do PCB buyers need to specify the photoresist brand?

Normally, no. Buyers should specify finished copper thickness, minimum line width and spacing, conductor tolerances, IPC class and reliability requirements.

10. How should a PCB manufacturer choose the right photoresist?

The manufacturer should evaluate the circuit-forming method, copper thickness, geometry, resist thickness, exposure equipment, chemical compatibility, production quantity and required yield.