Six Tips for Flexible PCB Design

flex pcb

Flexible printed circuit boards have become essential in modern day electronics where size, weight and motion provide a challenge. Medical devices and wearables, automotive sensors and aerospace systems — flex circuits are enabling designers to replace bulky wiring with thin, reliable, bendable interconnects. Still, a successful flex PCB is not the result of simple thinning down a rigid board. It is a matter of design considerations, process knowledge and working together with a Flex PCB manufacturer right from the beginning of your development. Here are 6 practical rules that will allow you to design flexible PCBs that are manufacturable, reliable, and cost efficient.

Know the Actual Conditions of Operation

Layout When layout do you need to ask yourself just one question is: how is the flex going to be used? A flex PCB needs to be bent once on installation, bent over and over during operation or stay in a bent position for decades. These aspects impact material selection, copper thickness, bend radius, adhesive selection and stackup design. Professional flex PCB manufacturers usually distinguish static flex applications from dynamic flex applications. Flex circuits Static flex circuits are only flexed during assembly, dynamic flex circuits are in the motion. Applications that are dynamic controlled necessitate more stringent design control, as copper fatigue, delamination, and coverlay cracking may result from an inadequately optimized design. Designers should specify:

• Minimal bend radius
• Anticipated number of bending cycles
• Exposure to temperature and humidity
• Conditions of vibration or mechanical shock
• Limitations on assembly and installation With this information, the flex PCB manufacturer can suggest the most appropriate polyimide films, rolled annealed copper (RAC), adhesive systems, and reinforcement techniques.

Choose Materials for Flexibility and Reliability

Selecting a material is the key to determining the performance of a flex PCB. The most popular substrate material now is polyimide because it has good thermal resistance, good dimensional stability and good flexibility. Copper may be electrodeposited or rolled annealed, but dynamic bending favors rolled annealed copper due to its grain structure which is able to better tolerate repeated flexing. An experienced flex PCB manufacturer will also evaluate if your design would better be served with adhesive-based or adhesiveless laminates. Adhesiveless films are thinner, more flexible and better suited for high-density circuit, high- temperature application and dynamic flexing. Adhesive based materials are in many instances more cost effective and they can be used for less demanding applications. Coverley is also a critical material. As opposed to solder mask applied to rigid PCBs, flex circuits typically contain polyimide coverlay that is bonded with an adhesive. It shields the copper traces and still is flexible. The coverlay holes must be processed accurately in order to expose the pads but without generating any stress concentration. Selecting the material should be a trade off of:

• Flexibility
• Cost
• Copper adhesion
• Thermal performance
• Dimensional stability
• Long-term reliability If you are not sure about what to do, getting advice from a flex PCB manufacturer on the front end may save you a lot of money on the back end.

Stack-Up for Manufacturabilty Design the Stack-Up for Manufacturability

A flex PCB stack-up describes the physical layup (in terms of stacked layers, including copper layers, dielectric films, adhesive, coverlays, stiffeners, surface finishes, etc.) of a flex circuit. A simple single-layer flex circuit can be straightforward to produce, but multilayer flex and rigid-flex configurations need far more organization. Material Preparation The manufacturing process usually starts with preparation of material. Copper clad polyimide is cleaned and primed for imaging. The circuit pattern is then applied by means of photoresist exposure and development. Then, etching is employed to remove the unwanted copper, and only the desired traces are left. Drilling, plating, coverlay lamination, surface finishing, profiling, electrical testing and final inspection are also required. A reputable flex PCB manufacturer will also be highly attentive to registration precision in the imaging, drilling, and lamination stages. Since flexible substrates can be stretched or shrunk during the process, it is more difficult to control dimensions than with rigid FR4 PCBs. The following are our stack-up suggestions:

• All symmetrizability Is good.
• Don’t make the copper too thick.
• Fewer layers in areas of dynamic flex.
• Bring copper to the neutral bend axis.
• Don’t step changes in material thickness too.
• Add stiffeners only to where mechanical support is needed The best flex PCB designs are straightforward where they’re bending and reinforced where they’re stiff.

Control Trace Routing in Slightly Bend Areas

Routing is one of the most important design considerations for flex PCB longevity. If possible, copper traces should be routed perpendicular to the bending direction, and avoid sharp corners. When you curve your traces, you can distribute the mechanical stress more evenly than with a right-angle bend. A knowledgeable flex PCB manufacturer will often suggest wider trace widths and wider spacing in areas subject to bending. Thin traces are good for saving space, but they are more prone to fatigue when subjected to repeated movement. Balancing copper is also important. Large solid copper planes may cause a flex circuit to be overly stiff and lead to stress concentration. Good practice routing should would be to  include: -Use curved trace transitions -Don’t place vias in bend areas -Do not put pads on the bend line -Stagger traces rather than stacking them directly -If you need flexibility use hatched copper, not solid copper •Make exhibits of bending areas thin and uniform as possible

For dynamic application, the flex PCB manufacturer can also recommend special bend testing prior to mass production. This will allow you to confirm the design can outlast the anticipated motion cycles.

Begin Consideration of the Manufacturing Process Sooner Rather than Later

The rigidity of flex PCB manufacturing is minimal as compared to that of rigid PCBs manufacturing. Every operation needs to maintain accuracy of dimensions and integrity of the material. Minor process deviations in the production process can influence the end flexibility, impedance, solderability and mechanical strength.

The following steps are contained in a general flex pcb fabrication process:

• Engineering evaluation and manufacturability analysis
• Material slicing/scoring and cleaning
• Dry film lamination
• Circuit imaging
• Copper etching
• Hole drilling or laser drilling
• Plated through-holes if they’re required
• Coverlay lamination
• Apply surface finish
• Stiffener bonding
• Profile cutting (by routing, laser or die punching)
• Electrical testing
• Visual and mechanical inspection
• Packaging for shipment

Temperature, pressure, and time should be strictly controlled during coverlay lamination. Poor lamination may cause bubbles, wrinkles, adhesive bleed-out, or bonding inadequacy. When cutting profiles, edges should be clean because burrs could cause the material to crack. A professional flex PCB manufacturer will conduct design for manufacturability reviews prior to production. This analysis could identify hazards like lack of annular rings, spacing that is too tight, poor support for the pad, coverlay misregistration, or inappropriate bend design.

Include Stiffeners and Finishes with Purpose

Despite the fact that flex PCBs are meant to be bent, not every point has to be flexible. Elements, connectors, solder joints, and test points commonly require mechanical support. Polyimide, FR4, or stainless steel stiffeners can be applied to certain areas to enhance handling and assembly yields. An expert flex PCB manufacturer will assist in selecting suitable stiffener material and thickness according to the application. Polyimide stiffeners are thinner and more suited for lightweight designs, while FR4 stiffeners are typically used for connector areas. In harsh conditions, stainless steel may offer excellent mechanical support. It is critical to consider the surface finish as well. ENIG is most popular since it has good solderability, oxidation resistance, and is suitable for fine-pitch parts. Other finishes may be chosen based on bonding, soldering, cost, environmental, etc .. Designers need to avoid terminating stiffeners suddenly in bend areas which are subjected to high stress. This kind of smooth transition can help to avoid the cracking. The manufacturer of the flex PCB should also verify that the selected finish is compatible with assembly processes such as SMT, wire bonding or connector insertion.

Final Thoughts

However, the design of flex PCBs is not just a simple matter of tight packaging. It involves full knowledge of materials, the nature of bending, stack-up construction, process limits, and assembly requirements. The above six rules are practical ones and they give basic guidelines: define the operating environment, choose materials wisely, simplify the stack-up, route carefully in the bend zone, plan manufacturing early and employ stiffener and finish with clear purpose.

The best results are achieved when the design team collaborates closely with a flex PCB manufacturer from the very start of the layout. Early communication leads to less risk, shorter production time and higher yield.

In the highly competitive electronics market, a reliable flex PCB manufacturer can be more than just a supplier, but a partner in engineering. Choosing the best flex PCB manufacturer will help you to have the most reliable products with the very best quality and the fastest delivery. A proven flexible circuit manufacturer has the process know-how to transform flexible circuit ideas into robust, manufacturable products.

EFPCB is the market leader and expert in the flex PCB design industry. Interested to know more about flex PCB, just write us now at sales@efpcb.com.