Flex standard focuses on high speeds, fine lines

Updated IPC standard provides guidance for those moving to high speeds and fine lines.

A growing number of designers and manufacturers are facing complex challenges as chip speeds skyrocket while system sizes diminish. This evolution is forcing companies to look closely at every nuance of a design to ensure that products meet performance and reliability requirements.

More development teams are turning to flexible circuits as more functions are crammed into portable products and other small systems. Using thin substrates brings benefits even when these substrates don't need to flex.

To respond to this growing need, IPC has revised IPC-4204A, Flexible Metal-Clad Dielectrics for Use in Fabrication of Flexible Printed Circuitry. It now details materials that let designers and manufacturers move to far higher frequencies.

"We need to know that each material will still work at these high frequencies, that there won't be any crosstalk or any other problems," said committee chairman Clark Webster of All Flex Flexible Circuits.

The standard is used by materials manufacturers, who test their materials to ensure that they meet IPC requirements and by flex circuit board manufacturers, who use the standard and its specification sheets to demonstrate that their assemblies will meet vendor requirements.

"We've written specifications for 1 and 10 GHz," said committee vice chair Michael Beauchesne of Amphenol Printed Circuits. "Within the past two years, a lot of raw material suppliers have done much more to extend their testing to provide data on permittivity (aka dielectric constant). They've also provided data on loss tangent (aka dissipation factor). The new terms, which were changed to better suit high speed environments, highlight the challenges that face companies as semiconductor speeds soar. At the same time, developers and manufacturers must also improve precision.

Along with higher frequencies, the revised standard focuses on improved dimensional stability, particularly for adhesiveless assemblies. Enhanced dimensional stability is very important in environments like cell phones, where flex circuit must match up with traces on glass that can be smaller than 12 microns, Webster noted.

"In the past, designers knew the dimensional stability and the shrinkage and other factors that impacted stability, and they could modify the artwork to meet those changing parameters," Webster said. "It's a lot easier to meet today's requirements if you have better stability and don't have to guess how much the material will change during processing."

The standard, which addresses the basic aspects of flexible circuit board design, also includes more focused information. Specification sheets at the end of the document provide users with information for classes of materials. Currently, they address three categories: polyimides, liquid crystal polymers and polyesters.

"Within these families of materials, people can see the requirements, parameters and test methodologies. Specification sheets make it easy for people to see the best material for their applications," said Beauchesne, who has worked on this standard since the 1970s when it was IPC-FC-241.

Now that IPC-4204A is completed, committee members are moving on to tackle another flex technology. Members are revising IPC-4203, Adhesive Coated Dielectric Films for Use as Cover Sheets for Flexible Printed Circuitry and Flexible Adhesive Bonding Films. That document is often used by flex manufacturers in conjunction with IPC-4204A, Beauchesne noted.