Introduction

The high frequency material the PCB fabrication process is using can affect the circuit performance for the end user. That means there must be interaction between the fabricator and the designer, while the end user should understand the concerns of the PCB fabricator for these materials. This is important for achieving good manufacturing yields along with a highly reliable and quality finished product, as each type of high frequency material has its own unique fabrication concerns.

The most common high frequency materials are PTFE (Teflon), PTFE with ceramic fillers, and non-PTFE thermoset resin systems with ceramic loading. Less commonly used are LCP or Liquid Crystalline Polymer materials.

Characteristics of PTFE Materials

The PCB manufacturing industry has long been using PTFE materials for high frequency circuits. Apart from pure PTFE, some of the substrates may also consist of a small amount of micro-fiber glass impregnated into it. Others can be PTFE with woven glass reinforcements. Still others may possibly be PTFE with ceramic filling. However, compared to all other types of materials used for high frequency circuits, the nearly pure PTFE is the most challenging type of circuit material faced by PCB fabricators. The reasons for this are:

• PTFE has a high CTE
• PTFE does not allow other materials to adhere to it easily
• PTFE is a soft substance, easily able to distort

However, from the perspective of electrical performance, PTFE substances are the best to use. PCB fabricators find the ceramic filled PTFE substrates the easiest to handle.

Working with PTFE Materials

When working with PTFE materials, PCB fabricators must be careful in not creating smears when drilling, not altering the substrate with scrubbing or other mechanical processes, fine tuning dimensional stability issues, and using best practices for minimizing handing damage to the soft substrate. PTFE substrates need a special through-hole preparation process to allow copper plating to adhere to the wall of the hole, and another special process for laminating PTFE materials with other bonding materials.

As there are no known methods or processes for de-smearing PTFE, it is very important to minimizing heating during the drilling process, since heating is responsible for causing smearing. The cleanest possible drilling for pure PTFE requires a new drill tool to ensure no smearing, but ceramic filled PTFE substrates can tolerate a re-sharpened tool.

Pure PTFE substrates need to undergo a wet-chemistry process prior to the copper plating process, to ensure good adherence. Typically, sodium naphthalene or a derivative can remove a fluorine atom to make the PTFE substrate accept the copper plating. Ceramic filled PTFE must undergo an additional process of baking to remove moisture the substrate has absorbed during the wet-chemistry phase. An alternative is to use a special plasma cycle using helium, which avoids the baking process.

Lamination on the PTFE surface requires a bonding medium, and most bonding materials that the PCB industry conventionally uses are applicable for PTFE as well. However, fabricators must be careful to not alter the exposed substrate surface after the copper etching process, as any scrubbing will polish the soft PTFE surface, and hinder further bonding.

Conclusion

The application of PTFE is unique due to the common use in high frequency application. If you are looking to improve the efficiency of your PCB with the use of Teflon, please contact the team at sales@pcbglobal.comfor more information and recommendation for your project and design.