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Flexible Circuits

Flexible (Flex) and rigid-flexible circuits (Rigid / Flex) have recently become popular. This type of printed circuitry brings many benefits. The basic ones are:

  • no need to connect several printed circuits with connectors, cables and wires
  • the possibility of connecting and making two or three printed circuits in one technological process
  • chance to minimize the dimensions of printed circuits
  • the ability to test all application components in one process
  • the possibility of assembling electronic components in one technological process instead of two or three.
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DProduction documentation of flexible and rigid-flexible circuits is created according to the same principles as rigid circuits. However, additional technological limitations resulting from the production process should be taken into account.

The long-term reliability of the printed circuit depends largely on the design of the distribution of layers in the PCB cross-section. Especially in the case of multilayer rigid / flexible laminates, symmetrical constructions are recommended in which the polymidic elastic layers are stacked up in the middle.
Sample stack-up for a rigid / flexible printed circuit:

  • final laminate thickness 1.0
  • Cu thickness on all 35um layers
  • 6 Cu layers in the rigid part
  • 4 Cu layers in the flexible part

There is not recommended the stack-up construction, where the elastic band is glued to the outer layers of the rigid part. Such a solution does not ensure the long-term reliability of the printed circuit, especially when the flexible-flexible elements are working dynamically. When designing rigid-flexible circuits, it should be additionally taken into account that:

– the suggested length of the elastic part depends on the bending radius and is calculated according to two models:

  • • for polyamide with double-sided Cu layer, the minimum recommended length of the elastic part is 12 x the thickness of the tape, the width of the tape is 20 x the thickness of the tape
  • • for a polymide with a one-sided Cu layer, the minimum recommended length of the elastic part is 6 x the thickness of the strip, it is scanned 10 x the thickness of the tape

– the minimum recommended distance of the SMD component from the joining edge of the rigid and flexible part should be 1.0mm

– the minimum recommended distance of the metallized hole from the joining edge of the rigid and flexible part should be 0.5mm

Technological limitations for rigid-flexible printed circuitsh

Technological limitations for flexible printed circuits