Great Connector Inventions: Compliant Press-Fit Pins

In the first of a new series that focuses on key achievements in the development of backplane connector technology, we look at the role of compliant press-fit pins.

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Compliant pins, waferized connector designs, modular connectors, egg-crate shielding, air-dielectric designs – we can now look back on inventions like these and put them in perspective. It is interesting to see how a real understanding of the technology and the problem, as well as a vision toward the future, can lead connector design engineers, and now signal integrity engineers, to outside-the-box solutions that fundamentally improve connectors and change the industry in the process.

One of the most fundamental connector technologies, compliant press-fit, did not exist until the 1970s when Robert Knowles of Winchester invented the C-Press compliant pin and convinced Western Electric, the manufacturing arm for the great AT&T monopoly, to qualify this new technology. Up until then, backplane pins were either soldered or rigid square or rectangular pins were forced into plated through-holes. For this process to create a gas-tight joint, the hole tolerance had to be +/- 0.002″ (5%), which was very challenging, especially for large boards. The dominant technology at that time used large 0.040″-diameter holes with 0.025″ square pins.

The telco operating companies at the time designed nearly all of their equipment for 25- to 40-year life. That sounds like a lot until you realize that the junction box on your street corner is quite likely to be that old. You will also note that we are always adding capacity to telco networks, but if the old lines still work, they remain in place. Today, that model has been overturned by the internet, massive data-center-centric networks, wireless, and ubiquitous video, but this rapid change really has occurred in just the last five years or so.

Anyway, back to the importance of compliant pins. The C-Press pin was fantastic in service, but difficult to stamp. This pin contacted >70% of the diameter of the plated through-hole. The conservative operating companies tested these compliant pins to ensure they could last for 40 years through accelerated life testing. They learned that electrical performance was comparable to soldered pins and the compliance made these much easier to use in manufacturing, opening up PCB hole tolerances to +/- 0.003″ (7.5%). The effect of this was profound:

  • Cheaper PCBs resulted because they could be drilled at faster speeds with fewer drill replacements.
  • Thicker boards were much more practical and allowed for higher layer counts, more density, and more power.
  • Press fit pins are repairable, even in the field. (Imagine trying to remove and replace a broken soldered pin.)
  • Pin-stitching or mass insertion processes allowed for custom configuration as the backplane was made. Frequently the header shrouds were then placed over the pin field.
  • Pins could be left out, allowing OEMs to only pay for the pins they needed.
  • Multiple pin lengths were possible, facilitating sequenced mating, an important enabler for hot swapping of cards.

OEMs would often use wire wrapping for small-volume production, common for prototyping and development. Keep in mind that CAD was just coming to be and much of the PCB layout was done by hand, and therefore was expensive, error-prone, and extremely time-consuming. Wire wrap allowed simple machines to place wires to connect pins. Press-fit pins with no solder were very friendly to this process.

Cross-section through the C-pin-compliant section in the largest and smallest holes

Cross-section through the C-pin-compliant section in the largest and smallest holes

The C-pin was the first broadly used design, but over the years, the eye-of-the-needle (EON) design has proved to be the most adaptable and durable design, especially as the contact pins and plated through-holes shrank. Compliant pins are now used in holes as small as 0.012″ in diameter.

Eye-of-the-needle compliant Pin (illustration from Cleaver Brinkerhoff)

Eye-of-the-needle compliant Pin (illustration from Cleaver Brinkerhoff)

Nearly all of the fine-pitch compliant pins use the EON. All the inventors of this technology are very old or gone. The first patent I can find for EON pins is from William Robert Evans, then with AMP in Harrisburg, Pa. Initial designs used a pin that was nearly the size of the hole with a rectangular cross section. Later, improvements coined the portion of the pin that mates with the hole, which reduced insertion forces and protected the hole plating. Further improvements involved refinements to the outer surfaces, as well as refinements to the shape and position of the hole. Woody Wurster of Custom Stamping has tooled EON pins for every conceivable form factor, including backplane and edge-card connectors. Wurster also has EON patents for enhancements to the EON shape and has provided pins for many of the top connector companies over the years.

Molex M-Pin and TE Connectivity Action Pin are examples of compliant sections.

Molex M-Pin and TE Connectivity Action Pin are examples of compliant sections.

To get around these patents, other connector companies quickly designed a number of competing designs that used different shapes of the compliant zones to achieve a gas-tight joint. AMP, now TE Connectivity, invented the Action Pin that it still uses today. Other shapes like V-press, sigma pin, tri pin, and the like proliferated quickly.

When the EON patent ran out in 1991, all vendors were free to adopt and tool the EON pin that is the dominant design today.

So, raise your glass in a cheer to William Robert Evans, an individual whose invention made a major contribution to the quality of life we have today.

This article is the first of a series that will focus on important inventions that have enabled connector technology to keep pace with the dramatic improvements in semiconductors and electronic systems. David Brearley will explore a number of developments that changed the industry direction, created companies, and enabled whole families of connectors such as waferized connector designs, 2mm hard metric connectors, modular connectors, two-shot molding, air dielectric designs, and much more. The focus of this series is backplane connectors, a specialty of the author. Please contact him regarding other fundamental breakthroughs worthy of future articles at dbrearley@bishopinc.com.

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David Brearley

David Brearley

Market Development Manager, at Bishop & Associates Inc.
Dave has been in the connector industry for more than 30 years. He began his connector career with Berg Electronics, a division of DuPont in Harrisburg and the Netherlands, then in 1990 he joined Molex to build its backplane connector business. Dave has also managed products at Amphenol-TCS, FCI, and was midwest salesperson for Neoconix. Prior to joining Bishop in 2015, he was contributing editor for Connectortips.com. Dave is active in IEEE, and was draft editor for the IEEE 1301.3 standard for metric backplane connectors. He can be reached at dbrearley@bishopinc.com.
David Brearley

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4 Responses to Great Connector Inventions: Compliant Press-Fit Pins

  1. tom cohen May 25, 2016 at 1:11 pm #

    The original EON pin was developed by Teradyne in the early 70’s by Len Johnson and Roy Barnett. I would have to check the dates but I think it may pre-date the C press pin. Teradyne developed the “solderless” backplane pins for NAFI fork and blade connectors under contract from the US Navy. As it was developed under contract there was never a patent application.

  2. Peter Roche May 25, 2016 at 8:47 am #

    Great article.. hard to belive press fit is nearly a half century old technology.

    Still today many find it hard to trust press-fit connections in spite of all evidence to the contrary.

    • David Brearley
      David Brearley May 30, 2016 at 3:33 pm #

      Thanks Peter,

      You can imagine the resistance to this dramatic new technology back then. You are right, the 50 years went fast.

      Dave