: Aurotech Flex process sequence

Aurotech Flex / Aurotech Flex-M: ENIG process with capability to withstand a high number of bending cycles

Flexible PCBs are offering more design options and application possibilities with regard to integrating assembled printed circuits into the final product. Their employment is gaining fast. There is a growing demand to improve reliability for assembled flex PCBs. In order to achieve this, alternative surface finishes, such as electroless nickel/immersion gold (ENIG), are increasingly used.

However it is well known that common electroless nickel/phosphorus layers are lacking of a reasonable ductility and can easily crack when bent. From this point of view the classical ENIG processes are not really suitable for being applied on flexible printed circuit boards (FPCBs). Nevertheless end-users did not give up on ENIG due to its favorable characteristics and therefore the FPCB manufacturers had to follow.

: Test conditions: Load: 0.5 kgf; angle: 135°; speed: 175 rpm

And they did it with a compromise. With increasing thickness the internal stress of a Ni-P layer also rises. With this in mind the Ni-P thickness plated on FPCBs was reduced to below 2 µm as compared to usually 5 µm. This was for some time accepted. In reality it did not provide a significant improvement towards bending performance but increased the risk of nickel corrosion as thin Ni-P layers are more susceptible to corrosive attack by immersion gold baths. It became necessary to improve the situation.

Atotech accepted the challenge and started developing an electroless nickel bath, which could provide a Ni-P layer with significantly improved bending performance, Aurotech Flex.

The results achieved with the MIT Folding Endurance Tester could proof the outstanding bending performance of Aurotech Flex versus a standard nickel layer even with a thickness of 5 µm.