Flex Circuit Support
In flex circuits, copper foil is etched to create the conductor pattern. The etching process is usually done using an electrolytic solution, but it can also use chemical vapor deposition or plasma etching. The type of copper used varies depending on the application. In general, a thinner layer of copper is more flexible than a thicker one.
A flex circuit can be made up of three or more layers of conductors. The layers can be laminated together or bonded to rigid parts. If they are laminated, openings are typically plated through to provide access to lower circuit level features. The openings can be plated using blind or buried vias. In most cases, buried vias are preferred to blind vias because they allow for more flexibility in the circuit.
Can a flex circuit handle high voltage? A flex circuit can support up to 5kV. The maximum rated voltage depends on the layer stack and the conductive material used. A 1 mil Kapton flex substrate can easily handle this voltage. In some applications, a flex circuit will need to be placed on a rigid PCB. In this case, it is important to stagger the locations of the buried and blind vias to prevent stress at critical points.
How Does a Flex Circuit Support Fine Pitch Components?
The most basic flex circuits are single sided flex. This variety has a thin copper sheet bonded to a dielectric film, which is then laminated into a rigid form. The conductor pattern is etched into the copper sheet per precise design specifications. In some cases, a flex circuit will need to have access to component termination features from both sides of the board. This is known as back bared flex and requires specialized processing to achieve.
In addition to etching, copper circuits are plated with gold to improve their performance in an electrical environment. This is particularly true for applications where a flex circuit will be bent. In these cases, a layer of gold is applied to the top and bottom of the copper layer to provide an extra degree of protection.
The flex-to-rigid joins and interfaces of a flex circuit must be strong enough to resist the repeated bending and stretching of the product assembly process. They must also be tolerant of solder reflow cycles. Silicones and hot-melt glues are commonly used to strengthen these areas of the layer stack.
As a final quality control step, a skilled inspector will visually examine the finished flex circuit for contaminants and scratches. This inspection is crucial for the successful operation of a flexible circuit board. Once this inspection is complete, the flex circuit can be mounted on a rigid PCB by gluing it in place with silicones or hot-melt glues. It can then be pasted with solder and positioned for component placement. It can then be submitted to a preset profile oven for reflow and assembly. This process dramatically reduces assembly time and costs, especially when compared to wire or cable assemblies. It will also reduce the potential for human error during assembly.
