Micros Go Commercial
ver since the Microway Systems Division became a part of Glenair, we've been encouraging our customers and sales force to "think Glenair when you think Micro-D". The exercise has certainly paid off: Glenair now has more employees dedicated to Micro-D manufacture than the rest of the industry combined. From our Glendale and Chicago factories to our Miles Roystone Division in Mansfield, England we're increasing capacity. And while much of our output is dedicated to military and defense applications, a growing percentage of micro product is going to commercial customers who need the high-reliability and ease of customization the TwistPin micro provides.
The ability to put our MIL-DTL-83513 style products to work in commercial and industrial settings represents a huge opportunity for continued growth in our TwistPin manufacturing and assembly divisions. This quarter's QwikConnect outlines some of the reasons why commercial customers are interested in high-reliability interconnects in the first place, and how our products stack up against the competition in terms of the commercial customer's toughest performance requirements.
But First a Little History
Every element of the MIL-DTL-83513 style Mi-cro-D is exactly controlled, from terminal spacing to approved wire termination methods. The MIL-DTL-83513 performance specification provides explicit standards to insure qualified connectors are intermateable and perform to acceptable levels and ratings. The MIL-DTL-83513 standard defines contact resistance, dielectric withstanding voltage, corrosion resistance, shock and vibration tolerances and a wide range of other electrical, mechanical and environmental performance standards. Standardized measurement and test methodologies ensure consistent, predictable performance throughout this broad family of ruggedized rectangular connectors.
The evolution of commercial-off-the-shelf (COTS) versions of MIL-DTL-83513 connectors has contributed to the increased specification of these components in large scale application programs. Today, standardized MIL-DTL-83513 Micro-D connectors are produced in aluminum or plastic shell versions for #24 through #30 AWG wire sizes. Available in both .100 inch and .075 inch spaced terminals with 9 through 100 contacts, these high-density COTS connectors provide high caliber performance without the cost of designing and qualifying a made-to-order interconnect.
But despite the performance standardization provided for in the MIL-DTL-83513 spec, and the evolution of standard COTS components, the business of designing and building these rugged, high-density interconnects has historically been a highly customized, made-to-measure enterprise. From initial concept to finished parts, the production of high-reliability micros is more a process of accommodating diverse layout and wire selections, and a
host of other custom requirements, than it is of mass producing standard Mil-Spec parts. Modifications in grounding, shell plating, mounting hardware and O-ring configuration only scratch the surface of the range of customizations typically specified in many military applications. Other design customizations address such requirements as hybrid power, fiber, signal and coax integration, moisture proofing and hermetic sealing, modular or individually keyed shell configurations and other one-of-a-kind features.
Not surprisingly, the ability to fine-tune the 83513 style Micro-D to meet the exact requirements of each application has led to their increased use in a wide range of commercial products. Manufacturers of satellite communications systems, geophysical exploration devices, medical diagnostics and industrial equipment face many of the same packaging requirements for reduced size and weight as do their military counterparts. And the ability to design-in a wide range of modifications is a distinct advantage over mass-produced rectangular connectors - making the 83513 style Micro-D the connector of choice for many unique or small quantity applications.
Costs and Benefits
The purpose of any interconnect is of course to provide a convenient means of connecting and disconnecting the power, signal, filter or ground wires servicing an electronic device. The challenge for the electrical engineer is to effect this interconnection as reliably as possible within the performance constraints and cost structure of the complete system. Despite the obvious performance benefits, no one would argue that the specification of a ruggedized, Mil-Spec connector is called for in the majority of commercial applications. The connectors would not, for example, be justified in a low-cost consumer product such as a cellular telephone or a personal computer - despite the rough handling these products often receive.
In more complex applications however, such as industrial robotics, engineers must carefully weigh the risks of using lower-cost, mass-produced interconnects versus the benefits of specifying heavier duty Mil-Spec style connectors. Taken alone, the need to downsize the electronics package would not be reason enough to justify the move to a Mil-Spec interconnect. But any major performance factor, such as the need to provide better electromagnetic shielding or better environmental sealing, could tip the scales in favor of the high-reliability Micro-D.
The decision becomes much more straightforward in applications which demand high levels of reliability due to the mission-critical nature of the electronic system the interconnect serves. For applications where interconnect failure is simply not an option, the high-reliability MIL-DTL-83513 style micro offers a wealth of performance benefits which far outweigh any potential cost savings realized by specifying a lesser caliber connector. The cost-benefit of selecting a high-reliability connector for use in a $200 million communications satellite is an apt example. The use of precision micro interconnects in the telecommunications switching gear for a medical emergency center is another. While standard SCSI-2 connectors with stamped shells and "tuning fork" contacts may be adequate for use in systems where "downtime" is not a critical concern, they cannot match the long-term durability and performance advantages of the MIL-DTL-83513 style Micro-D, which include:
Minimum Acceptable Performance
The MIL-DTL-83513 spec defines the minimum acceptable performance levels for high-density rectangular connectors. While all connectors approved to this specification must meet the performance criteria detailed in Table 1, manufacturers are allowed considerable leeway in the exact materials and manufacturing processes they choose to produce approved connectors. Independent testing shows that differences in materials selection and manufacturing processes can affect peak performance levels - particularly operating temperature range, contact retention and crimp strength.
Some of the differences between manufacturers are relatively unimportant. For example, 6061 aluminum is generally preferred for the fabrication of shell bodies due to its good plating characteristics and because there is no need to heat treat the material after machining. But 7075 aluminum is recognized to be the stronger of the two materials and is also used in shell production due to its outstanding machinability. The MIL-DTL-83513 specification qualifies both materials and leaves it to the manufacturer to make the best choice for each application - as long as the final product meets the threshold performance of the 83513 spec.
Other accepted variances in 83513 connector design and manufacture are much more significant in terms of their affect on long-term performance and durability. Four such areas - contact design, crimp fabrication, contact finish, and insulation material selection - are particularly significant and can combine to measurably affect the overall performance and reliability of the connector - especially in rugged application environments.
Contact System Performance
If reliability and performance were the only active considerations in the design of a micro contact system, everyone would opt for a twist pin style contact and a machined socket and crimp sleeve. But cost and ease of manufacture are significant issues as well, which is why stamped and formed contacts are still widely used - even in approved MIL-DTL-83513 contact systems. Glenair's Microway Systems division has been producing its unique TwistPin contact since 1986. The Glenair TwistPin contact has passed both the MIL-DTL-83513 and the stringent British standards BS9523F0002 and BS9524F0013. The development of the TwistPin contact has allowed a wide range of military and commercial engineers to design-in downsized interconnects in applications where the use of a stamped contact would not meet peak performance requirements.
Not only is the TwistPin more rugged than a stamped pin, it also provides a superior wire attachment, which translates to lower contact resistance. Glenair's TwistPin spring contact features a unique low force, rugged spring pin recessed into the insert to prevent contact damage. Extreme misalignment of the pin does not result in mismating or pin damage due to the hemispherical profile of the pin. The rugged design and construction of the pin and socket mating interface provides outstanding electrical continuity under extreme conditions of shock and vibration.
The spring properties of the TwistPin create a multi-point contact system while under compression in the mating socket. The contact socket is precision machined to ensure mating and unmating forces stay well within acceptable values. Heat from electrical or environmental sources can soften mated contacts over time and reduce contact retention force. Under extreme conditions of shock and vibration this loss of normal retention can result in unstable resistance across the interconnect. This is the case for all types of contacts - machined, drawn, stamped and twisted. But materials selection and advanced fabrication and heat treating techniques enable Glenair's TwistPin contact to resist high temperature stress relaxation for up to 1000 hours at 125o C and thus perform at levels unmatched by other contact designs.
In high-reliability interconnect systems, contacts should outperform conductors. Contacts should not, in other words, degrade overall current carrying capacity, nor introduce impedance or discontinuity to the transmitted signal. Likewise, contacts should never be the weak link in the system in terms of tensile strength: Wire conductors should break before pulling free of the contact in pull strength tests.
Wire crimping takes place at both the pin sleeve as well as the socket. In stamped contact designs, the process of folding the stamped contact sleeve around the wire provides reasonably good results, and is approved by the MIL-DTL-83513 spec. But several industry alerts have been issued which indicate both high and inconsistent resistance levels are common in stamped contacts - especially in solid wire printed circuit board applications where the crimp cannot achieve adequate pressure against the hard wire conductor. To address this problem, some stamped contact manufacturers augment the crimping process with soldering to effect a more reliable crimp joint. But soldering can introduce other problems, including corrosion and wire brittleness. And even with the addition of solder, the .004 wall thickness of the stamped contact sleeve does not provide the same reliability as the thicker, machined sleeves used in twist pin contact fabrication. In the final analysis, stamped contact crimps are simply unable to match the pull-strength of the twist pin contact.
The Glenair TwistPin crimp has proven to be completely reliable and to result in a consistent gas tight crimp. The eight indent crimp process used by Glenair provides superior wire strain relief when compared to stamped contact crimp methods, as well as improved environmental sealing. Most importantly, the eight indent crimp provides optimal electrical performance as the multiple points of contact are arranged in a tiered configuration which ensures electrical continuity regardless of conductor material. The simplicity of both the pin bundle sleeve and the static socket contribute to the reproducibility and reliability of both automated and manual TwistPin crimps, as no special expertise, such as spot welding, is required to complete the wire termination.
For their small size, micro connectors are very robust in terms of temperature cycling. This is a critical attribute, as current carrying capacity in connectors is not so much a function of conductor size as the overall temperature rise of the interconnect while in use. Virtually all electrical and mechanical performance properties of an interconnect are affected by temperature. Normal retention, or the tendency of the mated contact to stay engaged under stress is seriously affected by long-term temperature rise over 125oC. Damage from repetitive mating cycles, vibration and shock is also exacerbated by extremes in temperature.
The MIL-DTL-83513 spec requires connectors to withstand 96 hours at 125oC. The more rigorous British Standards require 1000 hours at 125oC. While every aspect of Micro-D materials selection and manufacturing affects operating temperature levels, the performance of the contact insulator has the greatest individual affect on operating temperature.
Various materials, from polyesters to glass filled nylons are used as insulators in Micro-D fabrication. Glenair has chosen to use a liquid crystal polymer insulator which, combined with our machined aluminum connector bodies, is capable of sustaining up to 200oC continuous operating temperature. The Glenair LCP insulator provides optimal structural integrity and outstanding temperature stability when compared to other insulation materials. LCP is amorphous (non-crystalline), nontoxic, and facilitates higher current ratings due to its superior insulating properties.
One of the biggest challenges in the manufacture of microminiature interconnects is the reduction of galling action between gold plated contact surfaces. The issue is critical for long-term durability, especially for applications which require frequent mating and unmating of connector pairs. Manufacturers typically prepare contact plating surfaces with a suitable underplating to clean and activate the surface in preparation for the final gold finish plating. As gold is a relatively porous material, the underplating also serves to prevent oxidation of the copper contact. Various approaches are used, including copper flash, nickel underplating and multiple finish platings of soft and hard gold.
As the 83513 specification does not specifically detail contact underplating, care should be given in the selection of connectors for use in applications where long-term durability is a critical concern. Copper flash, for example will oxidize through the porous gold finish plating to form CuO2 which can act as an insulator and result in higher circuit resistance. In humid application environments, such contact corrosion can lead to the premature replacement of connectors and assemblies.
Glenair has perfected a nickel underplating combined with a proprietary duplex gold overplating which provides optimal contact lubricity (anti-galling) and effectively eliminates the oxidation common to copper flash underplating. Glenair's advanced plating process contributes to the overall durability of the connector by reducing contact engaging and separating forces. 100 contact Glenair connectors have been successfully tested to 25,550 mating cycles (ask for test report ER1010) proving the durability of the Glenair plating process.
A Few Final Words
MIL-DTL-83513 connectors provide a wide range of performance benefits when compared to low-cost commercial connectors such as SCSI-2s or other mass-produced products. The selection of high-reliability MIL-DTL-83513 style microminiature interconnects for use in commercial and industrial settings is justified for mission-critical applications which cannot afford performance problems or outright failure. The ability to modify standard designs within reasonable purchase quantity limits is a further advantage over mass-produced interconnects.
The MIL-DTL-83513 specification defines the minimum acceptable performance for all approved connectors. But differences in materials selection, contact design and manufacturing processes between approved manufacturers can translate to significant differences in peak performance and long-term durability. Glenair's unique TwistPin contact, and the company's in-depth understanding of all aspects of microminiature fabrication, has resulted in a product which truly meets the need for a high-reliability microminiature interconnect for military as well as commercial applications.
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