Designing a Flex PCB Prototype: PCB Design Tips and Considerations

In order to produce a flex PCB prototype, there are several considerations related to the fabrication and the end use of the PCB circuit, including the design of the copper pattern itself. Before you start routing circuits in your Flex PCB, use the following design tips and considerations to maximize durability and yield. 

These tips help you balance durability in flex PCB prototypes with component placement and route traces in advanced PCBs. 

But first, let’s briefly go over a flex circuit prototype.

Understanding a Flex PCB Prototype

Flex PCBs are a special type of circuit board that consists of metallic traces of copper bonded to a dielectric layer such as polyimide. The circuit boards can bend into any shape needed, making them useful for high-density applications. The concept of flex PCBs first originated in the 1900s and found mainstream success because of the near-limitless applications and possibilities.

 

Modern electronics would not be possible without using flexible PCB prototypes, including smartphones, computers, smartwatches, keyboards, and various industrial applications that power technology.

The Components of Flex PCB

Flexible PCBs may look like they are fragile and will crumble under heavy use, yet their demand has skyrocketed because of their reliability and longevity. The main components of a flexible PCB are as follows:

 

Electrical conductors: They are made of copper, traces spread throughout the circuit.

Dielectric substrate film: This serves as the base material of the flex PCB. The most popular material for the dielectric substrate film is polyimide (PI), owing to its resistance to temperature and traction.

Adhesive Material: Adhesive is used to connect different parts of the circuit together.

Protective coating: A thin coating of protective material that is applied to the flex PCB board. It is made with a cover coat or cover lay.

Design Considerations for Flex PCBs

Flex PCBs require a unique approach compared to rigid PCBs. The design process for flex PCBs should provide major savings in manufacturing cost, lower weight, and reduced real estate on the circuit board. The board must be optimized for their applications and materials.

A properly designed flex PCB will be durable, easy to use by technicians, suitable for the intended use (such as wearables devices or computers, and lightweight.

Here are a few things to consider when designing your first flex PCB prototype.

 

The Bendability and Flexibility of the Flex PCB

Your flex PCB board will be subject to varying degrees of flexibility depending on its application. You will need to know how many times it will be flexing and the severity of the flexing. The amount of times it can flex will play a role in determining if the board is to be static or dynamic. Static boards are designed to flex less than 100 times in their life time and are considered bend-to-install. 

The design considerations for a dynamic board are much more different because of their nature. Dynamic boards need to be more robust because they will be subject to more flexing and will need to tolerate thousands of bends. These boards are intended for applications without room for failure, such as military and spacecraft.

Flex PCB Bend Radius

Bend radius, also known as the minimum amount of bendiness for flex areas of the circuit, must be identified in the earliest stages of flex PCB design. This will ensure that the design can accommodate a certain number of bends without compromising the structural integrity of the copper. In general, the standard minimum bend radius for flex PCBs is ten times the thickness.

The following table shows the relationship between the number of layers and bend radius. 

Number of layers	Bend radius in mm
Single-sided	Flex thickness by 6
Double-sided	Flex thickness by 12
Multi-layer	Flex thickness by 24

 

Flexible PCBs have thinner copper layers and insulating layers that allow them to occupy a minimal surface area and fit into tighter spaces. It is worth noting that 90-degree bends can increase the chances of impedance discontinuity which can affect functionality. For this reason, designers often avoid plated-through holes in the bend area.

Staggered vias are a preferable option when designing the PCB because they ensure the circuit’s longevity in multilayered circuits. Small conductors (less than ten mils) are better utilized in the neutral bend axis where there is no compression or tension during flexing. This ensures that the circuit’s features are not affected when flexing results in compression and tension.

Pad and Via Placement

When you design a flexible PCB circuit, the focus should be on the bend. It is advisable to not place pads and vias within the bend areas. It is also recommended to use teardrops on plated holes to maximize their durability and ruggedness. The flex PCB prototype may be subjected to different angles of bending, so the pads and vias should be sufficiently larger than drilled holes.

Routing in Flex PCBs

A good rule of thumb when designing flex PCBs is making sure that traces are always routed at a 90 degree angle to bend lines. If you are creating a board for high speed signals, such as PCB bus routing, make sure that both the top and bottom traces are alternatively staggered instead of being on top of one another.

This will evenly distribute the mechanical stress on all traces. In general, all sharp corners (such as right angles) should be avoided on flex PCB circuits. A better alternative is to use curves to maximize the prototype’s lifespan.

Component Placement

It is recommended to isolate low-frequency components from high-frequency ones. This ensures that the high-frequency components do not disrupt the performance of the low-frequency components through noise induction. Doing so could affect the signal quality, which is highly undesirable, especially in military applications.

It is recommended to shield clock signals with the help of a material enclosure, usually made out of aluminum.

The Flex PCB prototype should undergo various tests before it is finally used. The tests should check for basic performance and the essential characteristics of the flex PCB board. The above tips should help designers build a prototype with minimal errors. However, the perfect flex PCB prototype depends on the application that it is designed for.

For more information on design considerations for flex PCB prototypes, get in touch with the experts at Hemeixin.