Developing and implementing a rigid/flex printed wiring board form factor for VME 6U applications

Engineers have used rigid/flex printed wiring board (PWB) constructions successfully for many years in a variety of high-performance, high-reliability military and commercial applications

May 1st, 2000

By Greg Bartlett
Teledyne Electronic Technologies

Engineers have used rigid/flex printed wiring board (PWB) constructions successfully for many years in a variety of high-performance, high-reliability military and commercial applications. This technology has advantages over conventional hardboards that include reduced weight, assembly complexity, and inventory carrying costs, as well as greater reliability, thermal dissipation, and electrical performance.

Still, long lead times, the perception of high cost, a relatively small supplier base, and designer unfamiliarity have led to the pigeon-holing of this technology into highly specialized or niche uses. As packaging and performance requirements continue to test the limits of conventional hard-board technology - especially in the realm of commercial off-the-shelf (COTS) equipment - the advantages of rigid/flex technology can provide designers with attractive alternatives.

Teledyne Electronic Technologies in Hudson, N.H., a leading supplier of flex and rigid/flex technology for more than 37 years, has proposed a new approach to make this technology more accessible to potential users. The cornerstone of this approach is to standardize rigid/flex form factors (mechanical and electrical) for use in otherwise standardized hard-board applications. This approach is appropriate for motherboard and daughtercard suppliers.

Perhaps the greatest advantage that rigid/flex technology can provide to COTS applications is ease of ruggedization. Historically, connectors tend to be the weak link for mechanical shock and vibration environments, as well as climatic stresses (e.g., corrosion induced by airborne pollution, and temperature/humidity related effects). Elimination of the majority of connectors increases resistance to these mechanical and environmental stresses. An added benefit is the simplification of signal routing in the absence of plated through holes and vias associated with these connectors.

The challenge of designing in three dimensions has led to a wide range of construction methods, materials, and tooling approaches - many of which are protected as intellectual property by the flex and rigid/flex fabricators. As a result, there are few examples of commonality within the industry, which in turn has led to confusion within the design and user communities, uneven support by tooling and equipment suppliers for the specific technologies, and the resulting fragmentation of the overall rigid/flex fabrication supply base. The combination of these effects solidifies this technology as a solution to specific, complex problems, but it restricts its growth as a viable, enabling technology that could better serve the broader industrial marketplace.

Teledyne's efforts to develop industrial standards for popular form factors and applications, coupled with initiatives to work more closer with computer-aided-design service bureaus, electronic design automation tool suppliers, and potential high-volume fabrication licensees should help to make this more of a reality. Whenever engineers imagine new ideas and concepts, they are tempted to protect them with patents. Yet in this case the benefits of keeping these standards open and available to the general marketplace outweigh the drawbacks.

There are several potential industrial applications that could be well-served by this approach. Yet the one that Teledyne engineers chose initially was conventional VME in a 6U form factor because of its undeniable popularity.

There are other noteworthy candidates for future work, including CompactPCI, which could also benefit. Teledyne experts reviewed VME 6U standards, and evaluated potential rigid/flex form factors to meet these standards. Criteria for selecting the best rigid/flex form factor included:

  • maximizing component packaging density;
  • improving airflow through the slot envelope;
  • ensuring impedance control throughout the board;
  • maximizing the number of layers which could be accommodated within the slot constraints;
  • compatibility with CAD/CAM tools;
  • ease of assembly and test; and
  • maximizing fabrication yields and reducing fabrication costs.

Experts developed tools and programs to build proof of mechanical concept boards using the best rigid/flex form factor, which has been designated VME Flex. These first boards were 12 layers total, 10 of which were flex layers, and were constructed with Teledyne's REGAL Flex technology. Designers assembled pertinent hardware and DIN connectors to these mockups, and demonstrated plug-in compatibility with a VME 6U chassis. The designers implemented information gathered from these boards into the next iteration, which incorporated an additional level of sophistication. They also fabricated the boards in several configurations to represent Teledyne proprietary and non-proprietary constructions.

Following Proof of Mechanical Concept, designers targeted vehicles for proof of electrical concept through the efforts of several companies. A CAD service bureau, using the standardized rigid/flex outline, incorporated the netlist and component footprints from an OEM's functioning VME design, and a contract manufacturer was engaged to assemble the final product. This entire operation has not yet been completed.

Once the entire concept has been demonstrated, and technology and business implications are understood, Teledyne officials anticipate that a multi-pronged marketing effort will be necessary to make this product available to users.

The first phase will be to lobby relevant industrial organizations (such as VITA) to recognize this rigid/flex form factor as a standard. Publication of papers, applications notes, data sheets, and all relevant data will be necessary to accomplish this task.

The next phase will be to assure that the supplier base has access to all necessary design and tooling packages. This concept needs to be non-proprietary in order to succeed, so it is important that competitors and licensees alike have access to the appropriate templates. The final aspect of the marketing effort is to approach strategically selected customers and target individual product designs.

Greg Bartlettis director of engineering and research and development at the Teledyne Electronic Technologies Printed Circuit Technology business unit, in Hudson, N.H. Prior to joining Teledyne, he held technical positions at Mercury Computer Systems Inc. in Chelmsford, Mass., and Digital Equipment Corp. in Maynard, Mass. He holds engineering degrees from Johns Hopkins University in Baltimore and the University of Virginia in Charlottesville, Va. He can be reached by e-mail at Gregory_Bartlett@Teledyne.com or by phone at 603-889-6191 x239.

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