Semi-Flex FPC: What The Flexi Circuit Is All About

Semi-flex FPCs enable large circuits to fit in compact installation areas. 

Also, they give engineers the flexibility to build commercial products without adhering to the spatial requirements of rigid boards. 

But these electrical engineers have the option of picking flex or semi-flex FPCs. So how do they differ?

And how do you design a semi-flex FPC? Read on to find out more!

What Is a Semi-Flex FPC?

Semi-flexible printed circuits (FPCs) are thin, bendable circuit boards that you can fold without damage.

The composition of these boards is different from the rigid type.

But the flexibility usually depends on the chosen material’s ability to maintain high electrical conductivity while avoiding mechanical damage.

So most PCB fabricators build these boards using a blend of fiberglass (FR4) and epoxy.

But that is only the substrate layer. These boards have the following composition.

Semi-Flex FPC Composition

A basic FPC consists of the following layers.

Insulating Layer

These insulators are high-density polyethylene layers fabricated by forcing resin through a nozzle before layering it on the substrate.

The film aids the circuit base material by preventing capacitive coupling from affecting the core.

On top of that, it provides an electrical shield, separating the upper layer circuits from other internal connections.

Lastly, it creates a vapor barrier that shields the substrate from moisture when curing.

Electrostatic Adhesion Layer

This layer sits above the insulator and helps attach electronic devices to the flexible circuit board.

Usually, it consists of acrylic or polyimide. And it enables LEDs and other electronic components to contact the flex circuit.

Overall, this layer cuts the time and materials required for assembly.

Flexible circuit boards

Conductor

The conductive layer lies above the adhesion plane, and it helps prevent circuit warping.

This conductive film can consist of polyamide, epoxy resin, or the board panel.

But for effective warping prevention, you must spray the conductor layer with a 100°C solvent solution.

Board Enhancements

This secondary adhesion layer sits above the conductor to enhance the board’s integrity.

It can consist of cellulose or acrylic, and it minimizes the chances of flexural or cracking issues.

Coverlay

The final layer in an FPC underside is the Coverlay. It is an acrylic or polyimide panel that doubles up as an insulator.

So it protects the other side of the flex circuit from moisture while ensuring high-temperature resistance.

Therefore, you can use the flex circuit board in heaters or ovens.

Semi-Flex FPC vs. Flex PCB

Since both are flexible, what is the difference between semi-flex and flex PCBs? The former is partly bendable.

You can flex it a limited number of times before it gets damaged.

So most semi-flex FPCs are ideal for one-time folded installation.

An orange flex 

But flex PCBs are fully flexible and won’t get damaged even if you bend them repeatedly.

Another crucial difference is that flex PCBs can bend at any spot on their surface.

But semi-flex FPCs can only flex at specific areas. So some parts are rigid, while others are flexible.

Semi-Flex FPC Fabrication Process

The board’s fabrication process can differ depending on the manufacturer, but these are the general steps.

• Substrate cutting
• Dry film coating
• Automated optical inspection
• Brownization
• Lamination
• X-ray inspection
• Electroplating
• Graphical conversion
• Etching
• Silkscreen printing
• Surface finishing
• Milling (depth control)
• Electrical evaluation
• Quality control testing
• Packaging

A milling cutter working on a PCB during fabrication

Challenges and Solutions During FPC Fabrication

The primary fabrication challenges when making FPCs occur when doing depth control milling.

You must consider the resin material, board thickness, and milling tolerance to avoid quality issues emanating from the following.

• Oil peeling
• Resin fractures

Here are some factors to consider.

Test A

Manufacturers use the mapping approach to handle depth milling at 0.3mm, 0.275mm, and 0.25mm.

After milling, the panel gets tested to determine if it can withstand a 90° bend.

And it is critical to consider fiberglass thickness, dielectric condition, and panel thickness when doing depth milling.

Why? Because fiberglass gets damaged if its remaining thickness is 0.283mm.

Test B

The copper thickness between the second layer and the solder mask usually ranges from 0.188mm to 0.213mm.

So the manufacturer must consider the following.

• The warping that can occur when drilling
• How the drilling can affect the overall thickness consistency (0.245mm - 0.213mm thickness tolerance)

Rigid circuit board processing on a CNC machine

Test C

Size shrinking creates a forward effect on board warpage and machine uniformity.

So after prototyping, the fabricator can do depth control milling according to the size (6.3 x 10.5 inches).

Next, the mapping point measurement process ensures a consistent vertical and horizontal spacing of 20mm (machine uniformity).

Designing Bending Areas in Semi-Flex FPCs

Flexing is the primary defining feature of FPCs, but you must lay out the bending areas carefully to prevent mechanical damage.

Here are some guidelines to help you design these areas.

• While it is possible to design a board that bends along the X and Y planes, let the flexing occur in one direction. It will be cheaper and more durable.
• The flexing area should have equal-width tracks running parallel to each other. And the traces should have the same insulation resistance, which should be perpendicular to the flexing line.
• Split wide conductive tracks multiple narrow lines at the bending area.
• Fill open regions in the flexing area with blind conductors to enhance mechanical strength.
• Ensure the bending area has traces running perpendicular to the bending axis.
• Avoid placing plated-through holes in these spots.

Plated-through holes on a circuit board

• Leave at least 1mm spacing between any component hole on the rigid PCB and the flexible section.
• Use curves instead of corners when designing conductor traces in this area.
• If the spot or entire board has two or more flex layers, shift the conductive film placement on the front and back sides.
• Ensure the board design allows the manufacturer to produce the flexible area with a single milling router run. Thinning the FR4 using multiple passes will create weak points that can snap quickly.

Also, remember to optimize the bending area to suit the specific application.

For instance, FPCs built for stable flexings, such as once during installation, need the following optimizations.

• Contour narrowing
• Drill slots/holes
• Stabilization (more copper)

On the other hand, FPCs built for frequent bending need the following.

• Copper edges around the bending area
• Drill slots/holes
• Bending aids (copper stiffeners)

Designing Pads and Vias in Semi-Flex FPCs

Copper adhesion on flexible PCBs is not as good as on rigid FR4 boards.

So make the copper pads as broad as possible to increase the surface area for bonding.

And you can use teardrops or anchors to improve adhesion.

We recommend using the following measures to enhance Via stability on FPCs.

• Use teardrops to bind the Vias
• Maximize the size of annular rings
• Avoid placing Vias on the flexing area
• Use anchors to boost film adhesion

Pads and vias on a printed circuit board

Semi-Flex FPC Advantages

• Space saving (bends and folds to fit in tight spaces)
• Reliable if built correctly
• Cheaper than flex PCBs (reduces FR4 thickness through controlled-depth milling)
• Flexible

Semi-Flex FPC Applications

• Automotive (PCBs for engine control, airbag, and ABS systems)
• Medical devices (endoscopes, hearing aids, and mobile blood pressure monitors)
• Consumer electronics (smartphones, laptops, wearable electronics, etc.)
• Gaming consoles
• Aeronautics (avionics, communication, etc.)
• Military systems
• Industrial electronic devices
• Telecommunication (satellites and GPS trackers)

Wrap Up

Although there are rigid-flex circuits in the market, semi-flex FPCs give the advantage of having a continuous board. 

However, take care of the thinned section because it is a weak point that can easily snap.

Also, get a manufacturer with the required expertise and equipment to build this board for your project. 

At OurPCB, we have the expertise, experience, and equipment to deliver high-quality semi-flex FPCs.

So contact us today with your designs to begin fabrication.