Single-Sided vs Multilayer Flex PCB: Which to Choose

The Choice That Sets the Whole Project

A flexible circuit is specified for a reason, usually that the product has to fold, bend or conform to a shape. Once that is decided, the next question is the layer count, and it is the single decision that most affects the cost, the lead time and the mechanical behaviour of the finished part. A single-sided flex circuit and a multilayer one are made on the same materials and assembled on the same lines, and they behave very differently.

Roughly speaking, a single-sided circuit is a thin, cheap, extremely flexible ribbon that carries a simple circuit. A multilayer circuit is a stack with a ground plane, controlled impedance and interconnections between the layers, which is stiffer, more capable and considerably more expensive.

Single-Sided Flex Construction

One copper layer bonded to one polyimide base film, with a coverlay laminated over the traces to protect and insulate them. The finished thickness can be as little as 0.05 millimetres, which is thinner than a sheet of paper, and the bend radius is correspondingly small.

The advantages follow from the simplicity. It is the cheapest flexible construction, the easiest to make and to assemble, and the most tolerant of tight bends. The limitations are the absence of a second routing layer and of a ground plane, which means no impedance control, no shielding and no double-sided component placement. It suits an LED strip, a simple sensor tail, a printer head connection or a ribbon that replaces a wire harness.

Typical volume pricing is in the region of 0.10 to 0.30 US dollars per piece for simple circuits, with a prototype setup cost of roughly 80 dollars plus a per-piece charge, which makes it accessible for high volume consumer products.

Multilayer Flex Construction

Three or more copper layers laminated with dielectric between them, and interconnected with blind and buried vias. A typical multilayer flex design has a ground plane, sometimes a power plane, and one or two signal layers, and it can be folded into a three-dimensional shape with components mounted on the rigid or stiffened regions.

What the extra layers buy is significant. The ground plane provides a controlled impedance for high speed signals and a return path that contains the emissions. Components can be mounted on both sides. The routing density rises, so the flexible section can carry more signals in the same width, and the whole assembly can replace several separate cables and connectors. The price is a stiffer, thicker part with a larger minimum bend radius, longer lead times and a much higher cost.

Typical volume pricing for a four layer flex circuit is around 2.50 to 5.00 dollars per piece, with prototype setup from about 300 dollars.

Manufacturing Differences

The single-sided process is a simple sequence: etch the copper pattern, laminate the base and the coverlay, and finish and test. The multilayer process adds several steps that only exist because there is more than one layer.

• Sequential lamination. The stack is built up in stages rather than in one press cycle, because a buried via between inner layers has to be drilled and plated before the outer layers are added.
• Laser drilling. Microvias in thin polyimide are drilled with a laser, because a mechanical drill cannot produce the small diameter without damaging the material.
• Layer registration. Each layer has to be aligned to the ones below to a tolerance measured in tens of micrometres, and polyimide moves during lamination, so the registration is compensated rather than assumed.
• Plating and surface preparation. Polyimide absorbs moisture and is difficult to bond, so each lamination stage needs a controlled bake and a surface treatment.
• Inspection. The inner layers are inspected before they are buried, since a defect hidden inside the stack cannot be repaired.

Our notes on PCB manufacturing describe these processes, and our notes on flex PCB assembly cover the handling and support the finished parts need.

Electrical and Mechanical Comparison

• Signal speed: moderate on a single-sided circuit, because there is no reference plane; high on a multilayer circuit with a controlled impedance and a ground plane.
• EMI: weak on a single-sided circuit, which radiates from both faces of the traces; strong on a multilayer circuit, where the plane provides a return path directly under each trace.
• Bend radius: very small on a single-sided circuit, and larger on a multilayer one, because the stack is thicker and the copper on the outside of the bend is being stretched.
• Dynamic bending: a single-sided circuit tolerates millions of flex cycles if the copper is a rolled annealed foil and the bend is designed properly; a multilayer circuit is normally used where the part is folded once and then fixed in place.
• Thickness: 0.05 millimetres and upwards for single-sided, typically 0.1 to 0.3 millimetres for multilayer.
• Component density: single-sided placement only, against both sides and more layers on a multilayer circuit.

Cost, Lead Time and Volume

Indicative figures for a four layer flex circuit against a single-sided one, in moderate volume, are 2.50 to 5.00 dollars per piece against 0.10 to 0.25 dollars. The gap is not just the material; it is the sequential lamination, the laser drilling, the tighter registration and the lower yield.

Lead times follow. A single-sided prototype is typically three to five working days and volume production seven to ten. A multilayer prototype takes seven to ten working days and volume production twelve to eighteen. Expedited routes exist, with a single-sided board possible in about two days and a multilayer prototype in about five.

Design Rules for Flexible Circuits

Bend across the traces, not along them. A trace should run perpendicular to the bend line, and the bend area should be free of vias, plated holes and solder joints, all of which are the first things to crack.

Use a generous radius. The practical minimum is a multiple of the stack thickness, and a multilayer stack needs a larger multiple than a single-sided one. Keep the copper balanced on both sides of the neutral axis where the layer count allows, so that the stress is shared.

Anchor the pads. A pad on a flexible circuit peels far more easily than on a rigid board. Use a coverlay opening rather than a solder mask, add a stiffener behind the pads where a connector is attached, and avoid placing a connector at a point that will bend.

Plan the stiffeners. Component areas are stiffened with a bonded FR-4 or polyimide plate, and that stiffener is part of the mechanical design; our notes on PCB design and layout cover the layout implications, and our notes on PCB assembly cover how the stiffened areas are handled in production.

Design the thermal path. A flexible circuit has little copper and little area, so a power component needs a copper pour, thermal vias to the other side and often an aluminium stiffener to spread the heat. Our notes on prototype PCB assembly describe how to validate the design on a first article before volume production.

Assembly and Repair

A single-sided circuit is easy to assemble and easy to inspect visually, and it is often used in products that are not repaired. A multilayer circuit needs precise placement, support during reflow and X-ray inspection of the hidden joints; it is difficult to rework, and the usual approach is to use a rigid flex construction so that the components sit on rigid sections that can be handled and reworked normally. Pre-testing the circuit before assembly, and building a functional test early, reduces the risk considerably.

FAQ

How many layers can a flexible circuit have? Multilayer flex circuits commonly reach four to eight layers, and rigid flex constructions can go higher, though the cost and the lead time rise quickly with each layer.

Which one is more reliable? Both are reliable when used as intended. A single-sided circuit is better in a continuously flexing application, and a multilayer circuit is better where a ground plane, impedance control and density are needed.

How much does a multilayer flex board cost? Typically 2.50 to 5.00 US dollars per piece in volume for a four layer design, against 0.10 to 0.25 dollars for a simple single-sided circuit.

Can a single-sided flex circuit carry high speed signals? Not well. Without a reference plane the impedance is uncontrolled and the emissions are high, so high speed signals need a multilayer construction.

Conclusion

Single-sided and multilayer flexible circuits are made on the same materials and serve different purposes. A single-sided circuit is thin, cheap and extremely flexible, and it is the right answer for a simple ribbon or a sensor tail. A multilayer circuit adds a ground plane, controlled impedance, density and two-sided assembly at several times the price and a longer lead time. Choose from the electrical requirement and the bend requirement together, and design the bend area before the routing rather than after it.