The designing difference between rigid circuits and flexible boards

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Most design rules of rigid board are widely used forflexible boards,but there still some newsneed to be noted.

1. Current carrying capacity of the circuit
Compared with rigid board, we have to ensure enough circuit’s width because of the FPC’s bad heat radiation.

2. Outline
Using rectangle as outline will beyour first choice of saving money and keep enough distance from the edge of board, but that subject to the remaining laminate.
For the shape, inner corner should be round, because the board with sharp one will easy to be break off.
Minimize the circuit’s width and gap and layout more wider circuit If room permitted, and place tear drips on the PAD where plated through hole or component’s hole located.
As a popular standard, every circuit should smoothly change the direction, otherwise, the circuit can’t be passed through because sharp corner will put together all of strength.

3. Flexibility
As a general standard, corner radius should be designed as large as possible. And better use more thin laminate and wider circuit to afford more continued corners. So the single sided FPC can show their best property while lots of continued corners here.

4. PAD
Conductor is easy to be broken because there has a change from flexible to rigid around the PAD. So PAD can’t be placed at where easy to bend. Tear drips are usually used for PAD shape and make sure no gap between the two adjacent PADs with cover lay.

5. Stiffener
Stiffened flexible circuit with rigid laminate is widely used in small electronic device and the price also become more and more cheap.

Flexible cable was assembled in a rigid board with right slot so that it can be departed in future. In order to enfold it to the shape what we want, rigid board will be cutting into different parts after SMT components and wave soldering.

Above special requirement show that the complexity of flex PCB’s designation is less than rigid board. But you can’t forget part of important differences.
1) The 3D room is very important for flex circuit because which’s flexibility can save room and layers for you.
2)Compared with rigid board, there are more relaxed tolerance requirements for flexible board.
3)It will be designed longer than requirement because the two wings can be bent.

To minimize the cost, below skills should be considered:
1)Always considering how to assembly the circuit to the board.
2)Keep circuit small and flexible, complex circuit is not a good idea but small one.
3)Follow suggested tolerance at anytime.
4)Design connecting area only if it is really needed.
5)Stiffened flexible circuit will cheaper than rigid flex board except for multilayer PCB.
6)Designed 0.1mil adhesive for 1oz copper (include plating copper).
7)Sometimes, circuit and PAD without any cover will be cheaper.

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