Semi Flex - Semi Flexible Circuits

Semi Flex / Semi Flexible Circuits / Semi-Flex / Semi Flexible PCB / Semi Flexible PWB

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Since 2002 we have seen a growing request for flexible-, flex-rigid circuits. And over the latest years, an increast request for semi-flex circuits.

The basic material request is for copper cladded polyimide foil (PI), being used in computers, cell phones, cameras, medical, military and avionics applications.

From the custom base at Elmatica mostly of what we have seen has been delivered on PI material. This has been the most advanced circuits with high demands on flexibility, temperature and reliability. The PI material can  withstand the high temperatures of typically +260 degrees Celsius.

The low end of the market uses polyester or other low temperature dielectric films, that are generally not soldered in assembly.

Polyimide material is quite costly, and interest for the semi flex solution has been growing for some years. Especially for applications that are price sensitive and do not need a dynamic flexibility.  The semi flex solution is mostly based on standard FR4 material. One area within a rigid pcb can be made thinner and thus be flexible in this area. The bending of the thinner part will be limited, but for typical "bend and stay" solutions it will function quite well. And most of all it is less costly than using PI.

Single and double sided polyimide dielectric is either bonded with copper foil using an adhesive or alternatively, polyimide is cast on copper foil or copper sputtered, then plated directly onto polyimide without the adhesive layers.

The adhesiveless versions are more expensive products, but the homogeneous dielectric materials allow for higher temperature and better dynamic flex performance, and do have a better processability at PCB manufacturing. 

According to ISOLA.DE, approximately 90% of the flex circuit market is served using the polyimide materials. The industry is looking for alternatives for using polyimide, and there can be several reasons for this.

1. A number of designs using polyimide for the flexible part, do not need the performance regarding multiple bendings which is given with the PI material.

2. A disadvantage of PI-based foil is the high material cost resp. the price. Especially in those cases, where the performance of the PI material is not needed.

3. In some cases the moisture pick up of polyimide could generate a problem. In most cases, additional temperature processes could be required prior to
    assembly and soldering. ( A defined baking process)

4. Sometimes the availability of the material can be a problem. Especially if material that is not standard has been specified.  Most common or standard
   dielectric thickness is 25um and 50um.

 

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