What Are Flexible Printed Circuits

A printed circuit board (pcb) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. pcb's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.

The simplest definition for flexible printed circuit: A pattern of conductive traces bonded on a flexible substrate. A better definition would be: The perfect solution to your electronic packaging needs.

Flexible printed circuits are also known as flexible circuits, flex circuits, flexible pcbs and sometimes just flex. Flex circuits are sometimes regarded as a printed circuit board that can flex. In reality there are significant differences when it comes to design, fabrication and functionality. One common mistake that designers make is to design a flexible circuit using the same rules as a circuit board.

Flexible Printed Circuitry (FPC) offers a tremendous opportunity for the packaging engineer and electronic designer. These versatile electronic wiring systems can be shaped, bent, twisted and folded into endless dimensional configurations...limited only by an engineer's origami creativity. In this regard they offer significant design advantages over a two dimensional and inflexible rigid printed circuit board (PCB). This added dimension can make flex circuits a designer engineer's dream, but with the addition of flexibility come some "rules" that need to be followed (sounds like an oxymoron??) to make certain a robust design is achieved.

Different manufacturing methods and material sets are used for FPC's and an immediate difference is the dimensional properties. Rigid printed circuits are generally more dimensionally stable vs. the standard polyimide film used as the building block in 98% of the flex circuits produced. This increased dimensional variability means a flexible circuit requires different design rules than its rigid printed circuit board relative. Unfortunately, much of the design software available uses rigid PCB design rules and this can create manufacturing and functional problems for the flexible circuit. Getting a flexible circuit design ready for fab is referred to some in the industry as "flexizing" the design.

The list below details five of the more common ways "flexizing" makes a design more robust, more producible, and ready for fabrication.

1.Solder mask or coverfilm openings: During fabrication flexible circuitry can demonstrate dimensional change after exposure to processes like pumice scrubbing, copper plating, and/or etching. While some change can be accounted for, flexible circuitry design rules generally require larger tolerances to accommodate subsequent registrations for coverfilm, stiffeners, or die cutting. Additional consideration is required for the adhesive squeeze out that occurs during lamination of the coverfilm dielectric. Complicating the prediction of compensating design features is the myriad of processes and sequences required to produce a custom flexible circuit. The bottom line is the openings in the coverfilm generally need to allow more room in a flex circuit design.

2.Spacing between solder pads and adjacent traces: Here is the tradeoff, i.e. design compromise, which will be made based on item #1. When the coverfilm or soldermask openings are made larger, the edges of the adjacent conductor traces could be exposed if they were routed too close to a solder pad. This can cause shorts if solder bridges between connector pins or pads. Physical size of the circuit is another factor that can affect registration capability. In general more space is needed between a solder pad and an adjacent conductive trace to accommodate the coverfilm or soldermask placement tolerance.

3.Stress points in conductors: Because flex circuitry is used in both fold to install and dynamic flexing applications, trace configurations that are acceptable in a rigid PCB may create problems in a flexible circuit. Conductor traces with sharp corners and acute junctures at the base of solder pads become natural "stress points" when the area near them is flexed. This can result in trace fracture or delamination. A good flexible circuit layout will have a smooth radius for conductor turn points (instead of sharp corners) and a gentile radius from the trace to the pad fillet instead of a sharp angle. Selective attachment of stiffeners will prevent bending in soldered regions and is a common design practice.

4.Stacked traces: Traces on opposite sides of the dielectric should not directly "stack" on each other. Traces in tension (on the outside of the bend radius) may crack when the circuit is bent if they directly align in parallel with a trace on the opposite side. The traces in tension are forced farther from the neutral axis of the folded region and can fracture, especially with repeated bending. A good design practice is to keep the copper in the neutral axis of a bend by designing this region as a single conductive layer. When this is not possible, a proper design will "stagger" the traces between top and bottom copper layers to prevent top and bottom alignment.

5.Soldered joints too close to bend point: A solder joint is formed by an intermetalic bond of the solder alloy to the copper trace. While the copper trace is normally flexible, regions that have been soldered become very rigid and inflexible. When the substrate is bent near the edge of the solder joint, the solder pad is either going to crack or delaminate. Either situation will cause serious functional issues.

The bottom line is that designing a flex circuit with standard PCB software can result in some serious manufacturability and reliability issues. It is best to work with your flexible circuit supplier or a flexible circuit design expert to either "flexize" the design prior to beginning fabrication or create the layout directly from a net list. This will assure that the design can be manufactured to meet your needs.

The word “printed” is somewhat of a misnomer as many of the manufacturing processes today use photo imaging or laser imaging as the pattern definition method rather than printing.

A flexible printed circuit consists of a metallic layer of traces bonded to a dielectric layer. Copper is a very common metal, but there are other forms of conductive materials used. Thickness of metal layer can be very thin ( <.0001") to very thick (> .010"). The dielectric layer is usually polyimide or polyester, but other materials can be used. Dielectric thickness can vary from .0005″ to .010". Often an adhesive is used to bond the metal to the substrate, but other types of bonding such as vapor deposition can be used to attach the metal.

Because copper tends to readily oxidize, the exposed surfaces are often covered with a protective layer. Gold or Solder are the two most common materials because of their conductivity and environmental durability. For non-contact areas a dielectric material is used to protect the circuitry from oxidation or electrical shorting.

The number of material combinations that could go into a flexible printed circuit are nearly endless; current, capacitance, chemical and mechanical resistance, temperature extremes and type of flexing are just some of the criteria that impacts the material selections that best meet the functional needs. An experienced All Flex design engineer takes the critical requirements into consideration when designing a circuit to meet your needs.

Basic Types of Flexible Printed Circuits
There is a wide range of circuitry configuration, sizes and functionality, but printed circuits can be classified as one of the following types.

Single Sided Circuit: Consists of a single layer of metal traces on one side of a dielectric layer.
Double sided Circuit: Metal layers are on both sides of a single dielectric layer. Metal layers are often connected by metalized thru- holes.
Multi-layer Circuit: Several copper layers separated and encapsulated by dielectric layers. Metal layers are often connected by metalized thru- holes.
Rigid Flex Circuits: This is a multi-layer circuit where some of the layers are hard board and some are flexible circuitry.

Flexible Printed Circuit Design Advantages
The fact that a flex can be bent, folded and configured in just about any shape or thickness imaginable gives the designer tremendous options when creating an electronics package. Size and space limitations are far less of an issue than traditional packages using hardboard circuits. Assembly and handling costs can be significantly decreased because the entire interconnect system can be built as one integrated part. Add All Flex’s ability for component assembly and testing and the supply chain management becomes greatly simplified.

This tremendous flexibility in design choices leads to electronic packages being smaller, lighter and more functional.

The benefits of flexible circuits, which are explained below, are plentiful. From solving packaging problems to amazing thermal management, All Flex flexible circuits are designed with the customer in mind. See how you can benefit from a flex circuit click here to receive a FREE sample!

1.A Solution to a packaging problem.
•Flexible allow unique designs which solve interconnection problems.
•The formability of a flexible circuit enables a package size reduction.
•flexible circuit makes installation and repair practical and cost effective.

2.Reduce assembly costs.
•Flex circuits can be tested prior to assembly of components.
•Elimination of connectors and solder joints reduce costs.

3.Replacement for a circuit board and wires.
•Flex circuits simplify system design.
•Flex circuits reduce the number of levels of interconnection required in an electronic package.
•Flexible circuits eliminate human error common in wire assemblies as routing is determined by artwork and repeatability is guaranteed.

4.Reduce weight and space.
•Considerable weight reduction is a benefit over wire harnesses.
•Thickness can be as thin as .004 inches (.10mm) in total.

5.Dynamic flexing.
•The thinness of the material makes flex circuits the best candidate for flexible applications up to millions of flexures.

6.Thermal management/high temperature applications.
•Flex circuits dissipate heat at a better rate than any other dielectric materials while providing the added benefits of vastly improved flexibility.

7.Aesthetics.
•Flexible circuits improve the internal appearance of an electronic package, which can have an influence on the decision making process of prospective users of the product.

Fabrication
There are two basic categories of processes for manufacturing a flexible printed circuit: Subtractive and Additive.

In a subtractive process, one starts with a solid area of metal, and the unwanted areas of metal are removed to form the traces. Screen printing and photo imaging are the two most common processes used for defining the circuitry pattern.

In an additive process, one starts with a bare dielectric layer and the metallic traces are added only where needed to form the circuit. The conductive layer can be printed, plated or deposited in a variety of manners.

The subtractive processes are much more common because of they are more robust, cost effective and allow greater choices in final product configuration. The circuits created by the additive process have less current carrying capability and environmental resistance than circuits created by the subtractive processes.

Information on Dielectric Coverings for Flexible Circuits
The circuit traces of a flexible PCB often needs a dielectric covering to selectively protect the traces from shorting against a conductive surface or to prevent select areas from subsequent metal coating or plating operations. The dielectric will have openings that are aligned to specific features of the flexible circuit such as a component pad. This dielectric covering is normally referred to as a coverlay, cover coat or solder mask.

There are a variety of materials and methods that can be used to create this dielectric, criteria for selecting the best options include cost, registration accuracy, environmental requirements, copper thickness and post processing requirements. The most common types of coverings are:

•Selectively screen printed cover coat
•Pre-punched or drilled dielectric film
•Photo imaged solder mask
•Laser skiving

Screen printed cover coat is the least expensive method to create a dielectric insulator, but it also has the poorest mechanical, electrical and chemical resistance properties. Additionally, the alignment of cover coat openings to exposed pad is not as accurate as the other methods. There are a wide variety of cover coat materials available; common curing methods include UV, IR or convection heating.