Mechanical Mock-Ups

Employing a paper or Mylar mock-up can be useful in determining adequacy of flex lengths, minimum bend radius, and service loops. Lending physicality to the three-dimensional design, the mock-up can help to pinpoint problem areas early in the design process.

Trace Routing

Flex circuits require curved trace routing to relieve stress along curves and bends. This is especially essential in dynamic applications. Copper usage should be maximized whenever possible, and traces and annular rings should be made large. This will create a more robust circuit, minimizing the chance of copper cracking and enhancing material stability.

Filleted Pads/Tear Stops/Tie-Downs

Radiused corners, with the addition of copper tear stops, are useful to reduce ripping and tearing of the flex circuit.

Crosshatched Shields and Planes

Diagonal crosshatching of shields and ground planes is a technique used to gain flexibility.

Copper Grain and Trace Direction

The copper used in flex circuits is a rolled annealed variety and is more flexible in the direction of the grain. The direction in which the majority of traces run should match that of the copper grain. This can increase the life of the flex.

Coverlay

A coverlay is a single piece of polyimide with adhesive on one side. Openings are predrilled and routed out. The coverlay is laminated to the copper after the etching process. This layer protects the traces, leaving openings for through-hole and surface-mount devices. It serves the same purpose as solder mask on a rigid board.

Coverlay Openings

With single-sided flex circuits, coverlay openings are typically 0.010 in. smaller than the pad size; that is, a 0.060-in. pad would have a 0.050-in.-diam coverlay opening. For double-sided circuits, this opening can be the same as the pad size. The barrel of the plated through-hole helps to keep the pad down: however, it is always a good idea to have a 0.005-in. encroachment whenever possible. Surface-mount devices need a 0.005-in. encroachment of coverlay. For fine-pitch parts (0.005 in. or less), the opening is gang-routed.

Stiffeners

Typical stiffener material is polyimide or FR-4. Stiffeners can be installed at the top or bottom of the circuit to provide specific areas of rigidity as required. An example would be an area where particular components or connectors require enhanced rigidity for assembly or installation. Holes in stiffeners usually are 0.015–0.020 in. larger than the flex circuit hole size. This aids in registration of the stiffeners and allows clearance for soldering and inspection.

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