PTFE or polytetraflouroethylene, commercial name Teflon, has an inert molecular structure that makes it an excellent material for use as non-stick coating. PCB fabricators are increasingly using PTFE laminates compared to conventional FR4 materials, because of the unique properties of PTFE that allow it to be used for high frequency applications. Although fabricating PCBs made of PTFE is very similar to those followed for conventional PCBs, fabricators need to be careful in handling the rather soft material, and fine-tune their processes with special emphasis on those areas where PTFE differs from traditional materials because of its unique properties and chemistry.

For instance, PTFE laminates are soft and the surface is able to bend, wrinkle, or dent more readily than their FR4 counterparts. While such surface imperfections are acceptable in consumer electronic circuits, they can significantly affect functional performance in high frequencies. Therefore, PTFE laminates need a flat support while storing, to prevent them from sagging or drooping, as this can set over time.

Surface Preparation for Metallization, Marking, and Multilayering

It is not advisable to prepare the copper surface of a PTFE laminatemechanically. Equipment such as bristles, pumice scrubbers and composite brushes suitable for conventional rigid material should not be used as the soft PTFE substrate could stretch to absorb the stresses leading to unpredictable dimensional results.

To prepare the PTFE surface, the standard process that the PCB industry uses is Sodium Etchants or Plasma Gas Cycling. These processes strips or removes the fluorine from the PTFE surface making it suitable for metallization, marking, and multilayering.

To avoid problems associated with registration resulting from dimensional stretching, fabricators use soap or a degreasing bath to remove the potential organics. They also use chemical cleaning to remove anti-tarnish coating on the copper foil. This typically removes about 40 millionths of an inch from the surface of the foil to promote photoresist adhesion.

Lamination

PTFE and copper films can bond without the use of bonding films and/or prepregs. Usually, fabricators use temperatures of 700F and pressure of 450-500 psi as a starting point for the lamination process. The temperature and pressure changes with ceramic filling and other compositions of the PTFE laminate.

Fabricators also use bonding films with lower melting point for reducing the processing temperatures to about 250-425F. Others may use ceramic filled bonding plies as woven glass reinforced prepreg, requiring process temperatures of 550F.

Drilling

Although there are no hard and fast rules while drilling copper laminated PTFE substrates, it is essential to employ new tools at all times. Typically, slow infeed and higher chiploads are preferred for eliminating spurious laminate fibers or PTFE tailing.

Fabricators achieve additional benefits such as easier drilling and cleaner holes with ceramic-filled laminates, as this material has a modified dielectric constant, and lower CTE. However, ceramics filling increases drill wear by 25-50%.

Metallization and Copper Plating

As pure PTFE laminates have a very high Z-axis CTE, it is necessary to use high tensile plated copper on the walls of through holes. Copper of high ductility reduces the chances of pad lift, barrel cracks, and blistering, as PTFE has an inherently low modulus.

Soldermask

Fabricators use a standard PTFE plasma cycle process prior to application of soldermask to enhance the SMOBC adhesion to the copper. For best results, application of soldermask should preferably be completed within 12 hours of circuit etching.

Conclusion

At PCB Global, we currently fabricate Teflon PCB’s for microwave applications and the defence industry and have the knowledge, experience and capability to advise our customers on the use of Teflon base material PCB’s and how this can be implemented for their custom PCB design for their intended application. For more information or any inquiries of Teflon PCB’s, please proceed to contact us or simply email your Teflon base design file for a fast quotation to sales@pcbglobal.com