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News Release from: UK CAM | Subject: Flex circuit design
Edited by the Electronicstalk Editorial
Team on 28 July 2005
Design issues for flex circuit
manufacturers
Nik Vyas from UK CAM looks at some of the constraints in the "black art" of flexible circuit design.
It has been known for many years and noticed more recently (due to the more complex and technologically advanced printed circuit boards) that PCB manufacturers will invariably need to "reverse engineer" a PCB design so that it can be manufactured to any degree of efficiency and/or reliability This is particularly the case for flexible (flex) circuit boards, primarily because it is seen as a "black art" by many
This article was originally published on Electronicstalk on 8 Aug 2005 at 8.00am (UK)
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In all cases the rules applied to rigid board design should be applied to flex circuit design, but with the following simple additions.
The use of a paper doll or mockup is helpful to determine flex lengths, minimum bend radius, and service loops.
This can locate any problem areas in the early stages of the design rather than later.
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Mylar is a good material to use for this.
Equal trace and space routing will aid in the etching process as well as making optical inspection easier and faster.
Curved trace routing is needed to relieve stress along the curve.
This is especially important in dynamic applications.
Whenever possible, traces and annular rings should be made large.
This will create a more robust circuit, reducing the chance of micro-fractures and cracking of the copper features.
This also helps gain stability of the material.
One area of stress occurs where the trace meets the pad.
The use of filleted or teardropped pads is required and mounting tabs are also used.
This increases the copper area at the pad, which reduces the chance of the pad lifting during the soldering process.
Radiused corners, with the addition of copper tear stops, are used to reduce ripping or tearing of the flexible circuit.
Diagonal cross-hatching of shields and ground planes is used to gain flexibility.
Silver epoxy coatings are also used to gain greater flexibility.
A coverlay is a single piece of polyimide with adhesive on one side.
Openings are predrilled and routed and this coverlay is laminated to the copper after etching.
The coverlay protects the copper features, leaving openings for through-hole and surface mounted devices.
Coverlay serves the same purpose as a solder mask on a rigid board.
For single-sided flex circuits, coverlay openings are typically 0.010in smaller than the pad size, ie 0.060in pad diameter would have a 0.050in diameter coverlay opening.
This 0.005in encroachment acts as an anchor to hold down the pad.
Coverlayer openings for double-sided flex circuits can be the same size as the pad diameter.
The barrel of the plated through hole helps hold the pad down.
However, it's a good idea to have the 0.005in encroachment whenever possible.
For surface mounted devices a 0.005in encroachment of coverlay is needed.
For fine pitch parts (0.050in or less), the opening is gang routed.
Typical stiffener material is polyimide and FR-4.
These stiffeners can be installed to the top or bottom side of the flexible circuit.
These provide specific areas of rigidity.
FR-4 stiffeners are usually installed with pressure sensitive adhesive (PSA).
Polyimide stiffeners are usual installed with an acrylic adhesive, known as a thermal set adhesive (TSA).
With these measures in place, the flex circuit will behave in the way intended as well as considerably reduce the associated manufacturing costs.
In addition to holding electronics engineering degrees, the engineers at UK CAM have spent time working in the engineering departments of flex circuit manufacturers, assuring customers of successful, efficient and economical designs.
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