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Flexible Circuit Board Part
Flexible circuit board material properties and selection methods 06 Dec 2018

Substrate:

Flexible Printed Circuit is a highly reliable and excellent flexible printed circuit made of polyimide or polyester film. Mostly the material for flexible circuit board is polyimide, which is a high temperature resistant, high strength polymer material. It is a polymer material invented by DuPont, and the polyimide produced by DuPont is called KAPTON. In addition, you can also buy some polyimide made in Japan, which is cheaper than DuPont.

double layer flexible printed circuit


It can withstand temperatures of 400 degrees Celsius for 10 seconds and tensile strengths of 15,000-30,000 PSI.


The 25 μm thick substrate is the cheapest and the most popular. If the board is required to be harder, a 50μm substrate should be used. Conversely, if the board is required to be softer, a 13 μm substrate is used.


The transparent glue of the substrate:

Divided into epoxy resin and polyethylene, both are thermosetting adhesives. The strength of polyethylene is relatively low, and if it is desired that the board is relatively soft, polyethylene is selected.


The thicker the substrate and the clear adhesive on it, the harder the board. If the board has a relatively large bent area, a thinner substrate and a transparent adhesive should be used to reduce the stress on the surface of the copper foil, so that the chance of microcracking of the copper foil is relatively small. Of course, for such areas, single-layer boards should be used whenever possible.


Copper foil:

Divided into two types: rolled copper and electrolytic copper. Calendered copper has high strength and is resistant to bending, but it is more expensive. Electrolytic copper is much cheaper, but has poor strength and is easily broken. It is generally used in applications where there is little bending.


The thickness of the copper foil is chosen according to the minimum width and minimum spacing of the leads. The thinner the copper foil, the smaller the minimum width and spacing that can be achieved.


When using rolled copper, pay attention to the rolling direction of the copper foil. The direction of rolling of the copper foil is consistent with the main bending direction of the board.


Protective film and its transparent plastic:

Similarly, a 25μm protective film makes the board harder, but at a lower price. For a circuit board with a large bending, it is preferable to use a protective film of 13 μm.


Transparent plastic is also divided into epoxy resin and polyethylene. The circuit board using epoxy resin is relatively hard. When the hot pressing is completed, the edge of the protective film will be extruded with some transparent glue. If the pad size is larger than the opening size of the protective film, the extruded rubber will reduce the size of the pad and cause irregular edges. At this time, a transparent glue of 13 μm thickness should be used as much as possible.


Pad plating:

For boards with large bending and partial pad exposure, nickel plating + electroless gold plating should be used. The nickel layer should be as thin as possible: 0.5-2μm, chemical gold layer 0.05-0.1μm.


Learn more about flexible circuit board manufacturing process from www.flyheaters.com.


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