Photoresist PCB Making at Home: UV Exposure and Developer

The photoresist PCB making with UV exposure method produces higher-resolution circuit boards than toner transfer, making it ideal for fine-pitch SMD pads, tight trace widths, and professional-quality prototypes. By coating a copper board with a UV-sensitive photoresist film, exposing it through a printed transparency to UV light, and developing away the unexposed resist, you can achieve traces as fine as 0.2 mm reliably at home. This guide explains the complete process, including materials sourcing and tips specific to Indian makers.

Table of Contents

• Photoresist vs Toner Transfer: When to Use Each
• Materials and Equipment
• Creating the PCB Artwork Film
• Applying Photoresist Film
• UV Exposure Process
• Developing the Photoresist
• Etching and Finishing
• Frequently Asked Questions

Photoresist vs Toner Transfer: When to Use Each

Both methods produce home-made PCBs but have different strengths. Choose the photoresist method when you need:

• Minimum trace width below 0.5 mm (photoresist can reliably do 0.15–0.2 mm)
• Fine-pitch SMD pads (TSSOP, QFP, 0402 passives)
• Better repeatability across multiple identical boards
• Smoother trace edges (less jagged than toner transfer)

Use toner transfer when: you need quick one-off prototypes, you don’t want to invest in photoresist film, or minimum trace widths are 0.5 mm or larger.

Materials and Equipment

Photoresist Film or Spray

Two forms of photoresist are available to Indian hobbyists:

• Dry film photoresist (DFR): Laminated onto the copper board using a laminator. Available in rolls (width 30 cm × length 5–10 m, ₹400–800). Brands: Riston, Ordyl, Chinese generic. This is the most reliable method for home use.
• Liquid photoresist spray: Sprayed onto the board in a darkroom, then baked. More uniform for very flat boards but harder to apply evenly. ₹200–400 per can.

UV Light Source

You need a UV source with output in the 365–405 nm range (near-UV). Options:

• UV LED array panel (30×30 cm, 365 nm): ₹600–1,200, exposure time 60–120 seconds. Most consistent results.
• UV nail lamp (9W, 365 nm): ₹200–400, works for small boards but exposure is less even.
• Sunlight: Works in India during peak summer (11am–2pm), exposure time ~20–40 seconds. Highly variable, requires experimentation.

Developer Solution

• Sodium carbonate (washing soda): Mix 10g per 1 litre of warm water. Available at grocery stores as “washing soda” (Nirma brand, ₹20–30). This is the standard developer for dry film photoresist.
• Sodium hydroxide (NaOH, caustic soda): Stronger developer, 5g per litre — develops faster but can strip too much if over-developed. Available at chemical suppliers, ₹30–80 per 500g.

Other Equipment

• Laminator (for dry film application): ₹800–1,500
• Transparency film for laser printer: ₹15–30 per sheet
• Glass plate for UV exposure contact printing
• Plastic developing tray
• Ferric chloride etchant (same as toner transfer method)

Creating the PCB Artwork Film

The artwork film is a transparency (clear film) with your circuit printed in opaque black ink or toner. Light passes through the clear areas but is blocked by the black circuit pattern.

Printing the Artwork

1. Design your PCB in KiCad or EasyEDA
2. Export the bottom copper layer as a PDF or Gerber, then convert to a print-ready file
3. Do NOT mirror the artwork for the photoresist method — print it normally (correct orientation), because you will place the printed side face-down on the board (the film then acts as a correct mirror)
4. Print on transparency film using a laser printer at maximum toner density
5. Print two films and align them perfectly (stack two prints) to double the opacity — this gives much crisper UV blocking and sharper trace edges

Artwork Quality Check

Hold the artwork against a bright light. The black areas should be completely opaque — no pinholes visible. Any light leakage through the black areas will expose the photoresist in trace areas, causing those traces to develop away. Touch up pinholes with an opaque black marker.

# KiCad: Export transparency artwork for photoresist
# File > Plot > PDF
# Layer: B.Cu (bottom copper)
# Options:
#   - Mirror: OFF (do not mirror - the film will be flipped face-down)
#   - Negative plot: OFF (for positive-acting dry film resist)
#   - Line width: 0 (use pad/track widths as-is)
# Print PDF on transparency film at 100% scale

Applying Photoresist Film

Darkroom Requirements

Dry film photoresist is sensitive to UV, blue, and green light but not red light. Work under a yellow or red safety lamp (or in a dimly lit room away from windows) when handling exposed film. Normal indoor incandescent or warm-LED lighting is generally safe for short durations.

Laminating Dry Film Photoresist

1. Clean the copper board with fine sandpaper (800-grit) and acetone — the surface must be absolutely clean
2. Preheat the laminator to 110–120°C
3. In subdued lighting, peel the protective polyethylene cover from one side of the dry film
4. Position the sticky side of the film onto the copper board, starting from one edge
5. Pass the board through the laminator slowly — the heat bonds the film to the copper
6. Allow to cool for 30 seconds before handling
7. Keep the clear Mylar protective top layer on the film until after UV exposure

Storing Coated Boards

Once laminated with photoresist, boards must be used within a few hours if stored in normal light, or within 24–48 hours if stored in a light-tight bag. India’s high humidity (particularly during monsoon) can affect dry film adhesion — use silica gel packets when storing.

UV Exposure Process

UV exposure hardens the photoresist where light strikes it. Unexposed areas remain soft and are washed away during development, revealing the copper to be etched.

Setting Up the Exposure

1. Place the laminated board (Mylar protective layer still on) under a clean glass plate
2. Position the printed transparency film PRINTED SIDE DOWN directly on the Mylar surface
3. Add another glass plate on top to press the artwork flat against the board — good contact is critical for sharp edges
4. Position the UV light source 10–15 cm above the sandwich

Exposure Times

• UV LED array (30W, 365 nm at 10 cm): 60–90 seconds
• UV nail lamp (9W at 5 cm): 3–5 minutes
• Direct sunlight (12pm India, summer): 20–40 seconds

Exposure time is highly dependent on your specific UV source and resist type. Run a step-wedge test: expose strips of the board for 30s, 60s, 90s, 120s and see which gives the best developed result.

After Exposure

The exposed areas of the resist will appear slightly darker or more matte than unexposed areas. Remove the artwork film carefully. Now proceed quickly to development — prolonged exposure to room light will gradually harden unexposed areas too.

Developing the Photoresist

Development removes the unexposed (soft) resist, leaving only the hardened circuit pattern on the copper. The remaining hardened resist protects those traces during etching.

Developing Procedure

1. Prepare fresh developer: 10g sodium carbonate in 1 litre of warm water (30–35°C)
2. Peel the Mylar protective top layer from the board
3. Immerse the board in developer solution
4. Gently agitate — within 30–90 seconds, the unexposed resist will begin to dissolve away
5. Watch carefully — the copper in cleared areas will become bright and visible
6. Remove the board immediately when all unexposed resist is gone — over-development will lift even the hardened areas
7. Rinse thoroughly under running water for 2 minutes
8. Inspect: the hardened resist should cover your circuit traces perfectly; the areas to be etched should show bright copper

Troubleshooting Development

• Resist not coming off: Developer too weak, temperature too low, or over-exposed. Try stronger solution or warmer water.
• Resist coming off in traces too: Under-exposed, developer too strong, or developer too warm. Reduce temperature or dilute developer.

Etching and Finishing

Once developed, etch the board using ferric chloride exactly as described in our DIY PCB Etching guide. The hardened photoresist is acid-resistant and will protect your traces during etching.

Removing Photoresist After Etching

After etching and rinsing, remove the hardened photoresist using acetone or a strong sodium hydroxide solution (100g/litre). The cleaned copper traces will be bright and ready for soldering.

Quality Checks

Step 1: Visual inspection under magnification
  - Check for shorts (resist bridges between traces)
  - Check for open traces (breaks due to pinholes)

Step 2: Continuity test with multimeter
  - Test each net for continuity end-to-end
  - Test adjacent traces for shorts (should be OL/open)

Step 3: Isolation resistance
  - Between nets should be >1 MΩ for signal lines
  - Between power planes should be >10 MΩ before component assembly

Frequently Asked Questions

Is the photoresist method harder than toner transfer?

Yes, it requires more steps and more careful control of UV exposure time, but the results are significantly better for fine-pitch work. Once you’ve done it two or three times, the process becomes straightforward. The additional equipment investment (UV lamp, dry film, laminator) totals approximately ₹1,500–3,000, which pays off quickly if you make PCBs regularly.

Can I use sunlight as the UV source for photoresist PCBs?

Yes — and in India this is very practical! Sunlight contains plenty of 365 nm UV. Exposure times are shorter in summer (15–30 seconds at noon) and longer in winter or overcast weather (1–5 minutes). The disadvantage is variability — use sunlight only when you’ve calibrated your exposure time with test strips.

Where can I buy dry film photoresist in India?

Dry film photoresist is available from electronics component dealers on Indiamart, Amazon India, and specialised PCB material suppliers. Search for “dry film photoresist 30cm” or brands like Ordyl, Riston, or Eternal. Prices range from ₹400–800 for a 5-metre roll, sufficient for approximately 25–40 standard 10×15 cm boards.

What minimum trace width can I achieve with photoresist at home?

With good UV contact printing (glass pressed firmly on artwork), a quality UV LED source, and careful development, photoresist reliably produces 0.15–0.2 mm traces in home conditions. Fine-pitch 0.5 mm pitch QFP ICs are achievable. For anything below 0.15 mm (BGA, 0.4 mm pitch), professional PCB fabrication is necessary.

How does humidity affect photoresist PCB making in India?

India’s high humidity (70–95% during monsoon) can cause dry film photoresist to absorb moisture, reducing its UV sensitivity and development quality. Store unopened rolls in sealed bags with silica gel. Work on calm, drier days when possible. During monsoon, use the film within 30 minutes of opening in humid conditions.

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