Have you ever held a flexible LED strip and wondered how such a thin, delicate piece of film can carry current, resist bending, and survive soldering temperatures? The answer is a surprisingly complex manufacturing process that transforms raw copper-clad film into precision flexible circuits.
Understanding the FPCB manufacturing process isn’t just factory trivia — it helps you design better products, troubleshoot quality issues, and communicate more effectively with your FPCB suppliers.
Let’s walk through the process step by step.
Step 1: Cutting (Sheeting)
The process starts with raw material in roll form — typically copper-clad PI film (copper foil bonded to polyimide film with adhesive).
What happens:
- The roll of raw material is cut into individual sheets of the required size
- Large sheets may be further cut into smaller panels to maximize material utilization
- The size is determined by the manufacturer’s equipment and the panel layout efficiency
Why it matters: Panel layout directly affects your unit cost. Better nesting = more pieces per panel = lower cost per unit. This is why odd-shaped FPCBs can be more expensive — they waste more material.
Step 2: Drilling
Next, holes are drilled through the panel. These serve two purposes:
- Tooling/registration holes — used to align layers and fixturing throughout the process
- Component holes / via holes — for through-hole components or for connecting top and bottom copper layers (on double-sided FPCBs)
Two drilling methods:
| Method | How It Works | Best For |
|---|---|---|
| Mechanical drilling | Physical drill bit spins and cuts | Larger holes, thicker materials, lower cost for standard holes |
| Laser drilling | Laser beam ablates the material | Very small holes (microvias), fine-pitch designs, thinner materials |
For double-sided FPCBs, this step is critical — the vias are what allow electrical connection between the top and bottom copper layers.
Step 3: Copper Plating (PTH — Plated Through-Hole)
After drilling, the holes are just bare holes through the film. They need a conductive copper coating to connect the layers.
This happens in two stages:
Electroless Copper (PTH / Desmear + Electroless)
- First, the hole walls are cleaned and etched to create a rough surface for adhesion
- Then a thin layer of copper (typically 1–2 µm) is chemically deposited on all surfaces — including the hole walls — using an electroless (no external current) chemical reaction
- This creates a continuous conductive path from top copper to bottom copper
Electroplating
- The panel goes into an electroplating bath where additional copper is deposited using electric current
- This builds up the copper thickness on the surface and in the holes to the required specification
- Typical plating adds 20–30 µm of copper, depending on the design requirements
The purpose: Make the through-holes electrically conductive so the top and bottom copper layers are connected. This is the foundation of any double-sided or multilayer FPCB.
Step 4: Dry Film Lamination
Now we’re ready to create the circuit pattern. First, a photosensitive dry film is applied to both sides of the copper-clad panel.
What happens:
- The dry film (a photoresist material) is laminated onto the copper surface under heat and pressure
- The film adheres tightly to the copper
- It will act as a photographic mask — after exposure and development, it protects the copper we want to keep during etching
This is the first step in the photoimaging process — essentially using light to “print” the circuit pattern onto the board.
Step 5: Exposure
The circuit pattern is transferred from the design file to the dry film using photographic exposure.
What happens:
- A phototool (film artwork) is created from the Gerber design files — it’s essentially a photographic negative of the circuit pattern
- The phototool is aligned with the panel (using those registration holes drilled in Step 2)
- UV light shines through the phototool onto the dry film
- Areas of the dry film exposed to UV light polymerize (harden) — these become resistant to the developer chemical
- Areas not exposed to light remain soluble
Think of it like developing film in a camera — except the “picture” is your circuit layout, and the “film” is a thin plastic layer on copper.
Step 6: Developing
After exposure, the panel goes through a developer solution (usually sodium carbonate or potassium carbonate).
What happens:
- The unexposed (unpolymerized) dry film is washed away
- The exposed (hardened) dry film stays in place, protecting the copper underneath
- What you’re left with is copper covered by a dry-film “stencil” that exactly matches your circuit pattern — the copper that will become your traces is protected, and the copper you want to remove is exposed
At this point, you can actually see the circuit pattern forming on the panel.
Step 7: Etching
This is where the real magic happens — the unwanted copper is removed, leaving only the circuit traces.
What happens:
- The panel is sprayed with etching solution (typically ferric chloride or an alkaline etchant) at a controlled temperature
- The etchant chemically reacts with the exposed copper (the copper not protected by dry film) and dissolves it away
- The copper under the hardened dry film is protected and remains
- The result is the actual copper traces of your circuit
Etching is one of the most critical process steps — too much etching and your traces become thinner than designed (affecting current capacity and impedance); too little and you get shorts between traces.
Step 8: Stripping (Dry Film Removal)
After etching, the dry film has done its job and needs to be removed.
What happens:
- The panel is treated with a strong alkaline solution
- The hardened dry film is dissolved and stripped away
- What’s left is the bare copper circuit pattern on the flexible substrate — your FPCB traces, fully formed and visible
At this stage, the basic circuit is complete. But we’re not done yet — we still need to protect it, add surface finishes, and test it.
Step 9: Coverlay Lamination (Layup + Pressing)
For FPCBs, the equivalent of solder mask on rigid PCBs is usually a coverlay (cover film) — a layer of PI film with adhesive, laminated over the circuit traces.
Two sub-steps:
Layup (False Bonding)
- The coverlay film (PI + adhesive, with openings cut for pads) is carefully positioned over the panel
- It’s temporarily tacked in place with light heat and pressure
Lamination / Pressing
- The panel with coverlay goes into a hot press (vacuum lamination press)
- Under high temperature and pressure, the adhesive flows and the coverlay is permanently bonded to the circuit
- The result is a protective dielectric layer over all the copper traces, with only the pads exposed
Why coverlay instead of solder mask? Coverlay is much more flexible and resistant to bending fatigue — critical for FPCBs that will be flexed. Solder mask ink can crack when bent, which is why true flexible circuits use coverlay.
Step 10: Solder Mask Printing & Silkscreen
In addition to (or instead of) coverlay, some FPCBs have solder mask and silkscreen printed on them:
- Solder mask — a liquid ink (often white for LED applications) printed and cured to protect copper and prevent solder bridges. For LED strips, white solder mask under the LEDs improves light reflectivity.
- Silkscreen / Legend printing — text, logos, part numbers, polarity marks, and other markings printed onto the board for identification and assembly guidance
This step uses screen printing or inkjet printing technology, and the ink is cured with UV light or heat.
Step 11: Surface Finish
The exposed copper pads need a protective coating to prevent oxidation and ensure good solderability. This is the surface finish step.
Common surface finishes for FPCBs:
- OSP (Organic Solderability Preservative) — thin organic coating, low cost, flat surface. Most common for LED strips.
- ENIG (Electroless Nickel Immersion Gold) — nickel + gold, excellent solderability, supports wire bonding. Higher cost.
- Electrolytic Gold Plating — for edge connectors / gold fingers.
- Tin Plating — for solderability or etch resist.
- HASL (Hot Air Solder Leveling) — less common on FPCBs due to high temperature.
The surface finish is applied only to the exposed copper pads — not to the areas covered by coverlay or solder mask.
Step 12: Final Inspection & Shipping
Before the FPCBs leave the factory, they go through quality control:
- FQC (Final Quality Control) — visual inspection, dimensional check, electrical test (continuity / insulation resistance), AOI (Automated Optical Inspection)
- Sampling inspection — statistical sampling to verify overall quality
- Packaging — FPCBs are carefully packaged (often on reels or in trays) to prevent damage during shipping
- Shipping — delivered to the customer or sent to the SMT assembly house
The Full Process at a Glance
- Cutting — raw roll material cut to panel size
- Drilling — tooling holes and via holes
- Copper plating — PTH + electroplating to make holes conductive
- Dry film lamination — photosensitive film applied to copper
- Exposure — UV light transfers circuit pattern from phototool to dry film
- Developing — unexposed film washed away, revealing the pattern
- Etching — unwanted copper dissolved, leaving circuit traces
- Stripping — dry film removed, exposing bare copper circuits
- Coverlay lamination — protective film laminated over circuits
- Solder mask & silkscreen — printing and marking
- Surface finish — pad protection (OSP, ENIG, etc.)
- Inspection & shipping — QC, packaging, delivery
Why This Matters for Your LED Products
Understanding the manufacturing process helps you:
- Design for manufacturability (DFM) — know what’s easy vs. expensive to produce
- Troubleshoot quality issues — if you see trace undercutting, you know it’s an etching problem; if vias fail, it’s likely a plating issue
- Negotiate with suppliers — speak the same language as your FPCB vendor
- Estimate lead times — know which steps add time and cost
In the next and final article of this series, we’ll look at the key performance parameters of FPCBs — including voltage drop — and how to calculate the right trace width and copper thickness for your LED design.
Version 1.0,Content by LED Product Manager,Time Sept-10-2026,Space Dongguang
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