The Comprehensive Guide to Principles and Practices of Flex PCB Design and Layout

It is not easy to connect the dots when it comes to PCB design. The future of technology is flexible, and manufacturing these flex PCBs is becoming complex as new technologies emerge.

A flex circuit board is a specially laminated material chosen for flexibility, superior electrical characteristics, and stability over a long period. There are many different ways by which designs can be installed on printed circuit boards (PCBs), but these particular circuits are designed in such a way that they can be bent to enable the incorporation of mechanical flexibility and ultimate usability. By being able to bend and flex, these printed circuits are ideal for use in specific applications where standard PCBs would not be usable.

Taking the time to understand the principles of flex PCB design and layout can be overwhelming. To help level the playing field and remove any complexity, let us start with a few basic concepts and review some basics.

Principles and Practices of Flex PCB Design and Layout

Flex circuits are typically used to connect components and devices in electronic systems. Their design and layout is a complex process that requires a great deal of knowledge and experience. There are many factors to consider, including material selection, mechanical design, electrical requirements, and environmental conditions. However, the most important factor when designing flex circuits is the ability to withstand repeated bending.

The following are some considerations for creating your flex PCB design and layout:

Bend Ratio

Bend ratio refers to a trace's maximum permissible bend radius on a printed circuit board (PCB). It is calculated by dividing the trace width by its thickness. For example, a 1-mil thick trace with a width of 250 mils has a bend ratio of 250/1,000, or 0.25:1. The smaller the bend ratio, the more rigid the board will be and less likely to crack during bending or flexing; however, it also results in higher costs due to increased material thicknesses and larger copper traces.

Materials with higher strength tend to have lower bend ratios, while those with lower strength tend to have higher bend ratios. The thickness of the copper layer also affects the bend ratio because thicker copper layers tend to break more easily than thinner ones.

The most common practice for determining bend ratios is the 3:1 rule, which states that you should use the bend radius for each layer no more than three times your trace width.

Stiffener Placement

Stiffener placement on a PCB is an important practice in the design process. The goal is to place them in areas where they will provide the most resistance without interfering with functionality. Stiffeners are used to support the copper traces on a flex PCB.

Stiffeners should be placed at regular intervals to support the copper foil and prevent it from bending too much. Stiffeners can be made from FR-4, FR-5, or other materials, depending on the application requirements. The spacing between stiffeners should be at least twice their thickness to avoid any stress concentration points along their length. The stiffeners must be positioned, so they do not interfere with adjacent parts or electrical connections.

Via Construction

Another important principle affecting the flexibility of a PCB is via construction. Through-hole vias are usually used on rigid boards, while surface mount (SMT) vias are typically utilized on flexible boards. The reason why through-hole vias cannot be used on flexible boards is that they can easily deform during bending or warping processes due to their large size and high thermal stress during soldering processes. Surface mount vias are smaller than through-hole ones and can reduce the thermal stress during soldering processes by dissipating heat from underneath them with solder paste or solder alloy paste layers covering the surfaces that prevent them from overheating quickly like with through-hole vias.

Via Underfill Away from High-Stress Areas

To avoid cracks or breaks in the via underfill, it needs to be kept away from high-stress areas like sharp bends or corners where there may be an increased mechanical strain on the substrate due to bending or flexing of the board during use.

For example, when designing a flex PCB with multiple layers and through-hole components, it may be necessary to add underfill around each via to prevent any short circuits between layers. However, if these vias are located close together or near a corner of the board where it could bend easily during use, this added stress could cause damage to the underfills and possibly cause them to break off. In this case, adding an additional solder mask over these areas may help protect against damage from flexing during use.

Signal Splitting

When designing a flex PCB, it is important to remember that the traces on one side of the board will not be perfectly aligned with those on the other. This means that if you have signal traces on both sides of your board, it is important to design them so that they do not interfere with each other when bent or flexed during use. For example, if you have a signal trace going from one end of your board to another that passes through one or more layers, then it would be advisable to split this trace into multiple shorter traces to avoid any interference between them.

Moreover, the best way to avoid interference between signal splits is by adding a ground plane under each layer of your PCB layout. This way, when you bend or flex your PCB during use, there will be no shorting between the different layers, and their corresponding signals should remain intact.

Stackup Considerations

Stackup considerations are critical when designing a flex circuit. The stackup is the order and thickness of each layer in your PCB design, which can ultimately affect how flexible it is. For example, having two copper layers followed by one silkscreen layer will make your board more rigid than if you had two copper layers followed by two silkscreen layers.

Also, if you have a rigid substrate material like FR4 or Rogers 3400 with a thicker core layer and thinner plating layers, this will make your board more rigid compared to a thinner substrate material with thinner core and plating layers.

The most critical aspect of the stackup that affects flexibility is the choice between having thick copper traces or thin copper traces on your PCB. Due to its larger cross-sectional area, a thicker copper trace will provide better conductivity and add more stiffness to your board. A thinner copper trace will have less conductivity but offer greater flexibility due to its smaller cross-sectional area and lower stress.

Pad Size and Pad Area

The pad size and area are important factors when designing a flexible circuit board. The larger the pad size and area, the more likely they will be damaged during use.

Smaller pad size and pad area can help prevent damage to pads during bending or flexing of the board during use, as well as help reduce the amount of underfill needed around each via to prevent short-circuits between layers.

However, with smaller pads comes higher electrical resistance, reducing signal quality and increasing power loss due to heat generation within the circuit. Also, if there is no underfill added around vias located close together or near a corner of the board where it could bend easily during use, then this added stress could cause damage to pads and possibly cause them to break off. In this case, adding a solder mask over these areas may help protect against damage from flexing during use.

Solder Mask

A solder mask protects the copper traces on a PCB from damage during use. It also provides insulation between different layers of the board and prevents shorts between them. A solder mask can also protect high-stress components like through-hole vias and other sensitive board areas where increased mechanical strain may occur during bending or flexing.

When designing a flex PCB, it may be necessary to add a solder mask over certain regions of the board to prevent shorts between layers or around high-stress components such as through-hole vias.

However, if these vias are located close together or near a corner of the board where it could bend easily during use, this added stress could cause damage to the solder mask and possibly cause them to break off. In this case, adding additional underfill around these areas may help protect against damage from flexing during use.

Final Word

Whether you are designing a PCB for the first time or have been doing it for decades, this guide will serve as an excellent review of the basics and a jumping-off point to learn some advanced design principles. With the rapid maturation of flexible PCB technology, it is important to understand what flexible PCBs can do and how they work, too.

Learn about the basics, target applications, and design considerations when using flex circuit boards from Hemeixin to get started on your next project.