Solderable electronic film opens FPC niche

While polyimide (PI) is presently the standard material for most FPCs, this thermosetting material cannot be processed by film extrusion. Additionally water absorption of PI is very high, and its chemical resistance to alkaline materials is quite poor. PEEK is an environmentally friendly, recyclable polymer. Films from this material can be manufactured by continuous extrusion processes. 

PEEK offers the following benefits:
 

• a high continuous use temperature,
• excellent chemical stability,
• superb mechanical properties, and
• low water absorption, and
• an intrinsic non-flammability 

With these properties, the semicrystalline, thermoplastic polymer offers excellent potential as an alternative base material for FPCs. However, as a consequence of its outstanding chemical resistance, PEEK's adhesive properties are poor.

The low metallization adhesive strength achieved with PEEK has prevented this polymer from being used as the base material for FPCs. Erlangen researchers found, however, that surface activation by either of two different pretreatment methods could improve the adhesion strength of PEEK/metal laminates.

The standard method of laminating PEEK and copper films involves the use of glue. Alternatively, direct metallization by physical vapor deposition (PVD) can be used. The adhesive strength demanded for this application (0.6 N/mm) was measured by a peel force test. This was in accordance with DIN 60249. This can be achieved and/or exceeded by an appropriate surface pre-treatment of the PEEK film, either by surface etching with chromic sulfuric acid or oxygen plasma treatment.

When glue is used, the failure mode changes from interfacial failure at the interface between the adhesive and the polymer for the untreated sample to an interfacial failure between the adhesive and copper. Both plasma treatment and surface etching with chromic sulfuric acid resulted in the incorporation of oxygen groups. This created an increase in secondary bonds between the polymer surface and the adhesive.

With its outstanding properties, the PEEK FPC substrate can be soldered in standard reflow processes using lead-free solder alloys and can also be used under extreme conditions (high temperature, aggressive media, and dynamic load). Aged PEEK-copper-laminate also showed excellent results. 

Even after storage for 250 hours at 200 °C in an air atmosphere, sufficient adhesive strength with the metallization was maintained (0.7 N/mm). The long-term stability of the adhesive strength proved to be better than that of a commercial PI-based FPC material.