Key Parameters to Consider During Flex Circuit Design
Flex Circuit Design
A flex circuit is a type of printed circuit board that has conductive copper layers etched on both sides of the board. This enables it to bend and twist in various ways without compromising electrical performance. It is used in many different types of products from printers to airplanes and can provide significant weight savings and other benefits over rigid PCBs.
A flex board’s key parameters to consider during its design include the material it is made of, its thickness, the number of layers, and its specific application. It is also important to understand how much it can be flexed, and how far its copper should extend beyond the edge of the board. This is determined by its minimum bending radius, and can be found by using IPC 2223’s formula.
The material used for a flex circuit board is crucial, as it has a direct impact on its durability and reliability under repeated stress. Typically, it is formed from a thin polyimide film. This material is very strong and durable, and can withstand multiple solder reflow cycles. It is able to expand and contract with temperature fluctuations, which helps it cope with the frequent flexing that a flex circuit must undergo.
Key Parameters to Consider During Flex Circuit Design
Depending on the application, there are many different thickness options available for a flex circuit. While there are some applications that require a stiffer circuit, most can be achieved with a standard 1–2 layer construction. For more demanding applications, it is possible to use up to six or even 10 layers in a single-sided flex circuit.
Another factor to consider is the etching process used. It is recommended to avoid ED (electrodeposited) copper for flex circuits, as it is not suitable for constant bending and can cause the material to crack and fracture. Instead, rolled annealed copper is preferred as it has better ductility and allows for the circuit to flex more freely. It is also essential to call out the grain direction of the flex material on the fabrication drawing so that the manufacturers can plan ahead for it during plating.
Drill-to-copper distance is also a critical consideration for a flex circuit. As a general rule, a gap of 8 mil or less is optimal for a flexible PCB. The gap will prevent solder from melting and shorting out through the hole, reducing potential failure points.
Other considerations for a flex circuit are the size of its conductor traces and where they run on the board. Conductors traversing a flex circuit should be placed as close to the bend axis as possible, and should be staggered in order to reduce strain on the circuit. Also, any holes that are needed for mounting devices should be positioned outside the flex area, as this will help protect them from abrasion and moisture.
Finally, the placement of stiffeners is an important factor to consider when designing a flex circuit. Stiffeners are small pieces of material used to support and reinforce a flex circuit during its operation, and are required for dynamic flex circuits to withstand the stresses of continual bending.