Used in Flex Circuit Boards
A flex circuit board is a printed circuit board that uses flexible materials in the core and conductor layers. The materials that are used will depend on the number of layers in the board and whether it is intended to be a rigid-flex or a flex-to-install application. The type of materials will also be influenced by the design and size of the conductive paths.
The core of a flex PCB is usually polyimide or polyester. These are non-conductive materials that are able to hold the copper conductors. They are able to withstand the mechanical stresses that will be placed on the flex circuit. Polyimide and polyester are also flame retardant, which is important in a flex circuit board, as they will be exposed to extreme heat and humidity.
A layer of copper foil is the leading conducting material in a flex circuit. This can be electro-deposited copper (ED) or rolled and annealed copper (RA). RA is preferred as it has a columnar structure, which leads to an even and flat surface that is accessible to roughening and etching processing. Both types of copper foil are suitable for a flex circuit, however, it is important to understand the differences between them in order to make an informed decision about which is best for your design.
Materials Used in Flex Circuit Boards
To insulate the copper traces, a coverlay is used. This layer protects the copper from damage and corrosion, similar to the way a solder mask is used on a rigid-flex or a rigid-board. Coverlay is typically made of a layer of PI or PET, although there are some manufacturers that offer adhesiveless coverlay materials. Dupont, Panasonic and ThinFlex are a few of the manufacturers that produce adhesiveless products.
As a rule, the coverlay layer should be thicker than the copper in order to provide adequate resistance. It should also be thick enough to withstand the mechanical stress of bending. Choosing the right thickness for your design is essential, because thinner covers will not be as strong and may not be able to resist the flexing that the flex circuit board will undergo.
Depending on the design of your flex circuit, you might want to use a pad-only plating process, which is also known as button plating. This type of plating allows for higher yields as well as more precise control over the thickness, width and spacing of copper pads. It can also help reduce signal reflections and achieve controlled impedance at high speeds.
It is important to know that a flex circuit will experience more expansion and contraction when it is being manufactured than a rigid PCB, which means that the drill-to-copper distance should be larger. This will prevent holes from being misaligned with the traces, and can improve the overall quality of your flex PCB. To check if your flex circuit board will be able to withstand the intended amount of movement, you can use a manufacturer calculator. This will give you an idea of how much your design will cost to manufacture, and will also let you know if it needs to be produced in a rigid-flex format.
