IPC-6018B Sets Standards for Microwave Technologies

Estimated reading time: 4 minutes

Microwave technologies have existed in a limited number of fairly lofty applications. But that’s changing as communications evolves and chip speeds rise higher every year. These trends are driving an expanded usage of microwave technologies, increasing the number of engineers who must deal with the tricky challenges of designing and manufacturing systems that run in the microwave ranges.

Printed boards that utilize high-speed chips and microwave circuitry have many parameters that are quite different from more mainstream rigid and flexible printed boards. These differences are explained in IPC-6018B, Qualification and Performance Specification for High Frequency (Microwave) Printed Boards. High frequency is one of IPC’s three primary bare printed board classifications (rigid and flex are the other board classifications addressed by similar standards).

That makes IPC-6018B something of a companion document to IPC-6012, Qualification and Performance Specification for Rigid Printed Boards, and IPC-6013, Qualification and Performance Specification for Flexible Printed Boards.

Typically, IPC tries to update these three qualification and performance specifications somewhat simultaneously. However, while both of those documents have gone through multiple revisions over the past decade, IPC-6018 had remained at Revision “A” from January 2002.

“This document lagged, and as a result it had gotten out of synch with the others in terms of requirements for copper wrap plating, thermal stress testing, hole fill and microvia structures,” said John Perry, IPC’s technical project manager.

The standard wasn’t updated as frequently partially because there are fewer users in this market than in more mainstream technologies. There are only a small number of suppliers of PTFE, the Teflon material that’s often used for microwave substrates. That’s a sharp contrast to the many companies that produce FR-4 laminate based printed boards. However, when it comes to usage, small is a relative term in the enormous electronics industry. Many microwave boards are now in service.

“This technology is more widely used nowadays in commercial applications like cell phone towers and in military products,” said Michael Luke, chair of the IPC D-22 subcommittee that developed IPC-6018.

Microwave technologies will be needed in other areas as chip speeds continue to rise. “More rigid boards are beginning to adopt lower dielectric constants for impedance control,” Luke said.

Perry noted that there have been many technical changes over that lengthy period. “When an IPC-6010 series performance standard has new material, the text or figure and table headers are highlighted with gray shading. There’s a lot of gray shading in Revision B,” Perry said.

Those additions address a number of changes in both board materials and the traces on these substrates. The lines of a microwave section have significantly different performance parameters than those used for more conventional printed boards. Many of the conductors on a typical microwave printed board can be designed using IPC requirements for rigid and flex boards. But the conductor width, thickness and line spacings are much different in the sections that handle high-speed microwave signals, so using another standard for procurement of microwave printed boards is out of the question.

There are also differences in substrates. Most microwave boards are made with PTFE, or Teflon, materials rather than conventional FR-4 materials. PTFE laminates have distinctive properties when layers are laminated together. Dimensional stability is quite different, so designers and manufacturers have to consider that when they’re laying out boards and positioning buried and blind vias or other elements that require drilling.

 When those holes are drilled, resin debris can be left behind following the formation of the hole wall, resulting in what is termed as “resin smear.” “IPC-6018B has specific criteria for the removal of such resin debris, referred to as “smear removal,” given the unique properties of high frequency printed board laminates.  This is a big issue for PTFE boards,” Perry said.

A number of other changes have occurred since Revision A was completed early in 2002. Developers incorporated additions to Section 3 adding references to embedded passive resistors and capacitors. The new version also better defines hole breakout requirements, which occur when holes aren’t drilled in the center of a pad. Another topic, thermal stress, has also been revised to address advances in convection reflow process for thermal stressing of microsection or production printed board samples.

“Now we’ve got three techniques for measuring thermal stress. In the past, there was only one,” Luke said. “We’ve also add two new reflow profiles.”

The document also expands the number of finishes that can be used. Electroless nickel/electroless palladium/immersion gold (ENEPIG) is one of them, Luke said. He also explained that the revision has a couple other improvements.

“At the end of the document, we have updated a table for inspection criterion,” Luke said. “We’ve also added an appendix for the space and avionics industries, which have tighter requirements and are often utilized for performance of military product.”