Small yet perfectly formed
Although connectors are often overlooked, having a sturdy, high-quality connector in a design can mean the difference between years of reliable operation and premature failure. Lindsay presents a few of the considerations involved in choosing a connector, using PC/104 as a general example that can apply to all Small Form Factors (SFFs).
Connectors are often the last components to be considered in new equipment designs. However, a connector isn’t just a connector. There’s more to making these engineering marvels than many engineers realize, especially when working at a miniature level.
Embedded systems start with one major advantage: Being standards-based, many of the choices that normally cause headaches are predetermined. In the case of the PC/104 Consortium’s PC/104 platforms, which are based on the IEEE P996.1 Standard for Compact Embedded-PC Modules, connectors are specified in considerable detail. Making off-the-shelf embedded systems such as PC/104 or EBX interoperable provides confidence that the connectors will work the first time when one board is plugged into another.
Connector use in PC/104 families
PC/104 uses the 104-pin ISA (PC/AT) bus connector, which is actually a pair of connectors stacked side by side. The XT bus is a 64-position (dual-row 32) board-mount socket on 0.100" x 0.100" pin-to-pin spacing, designated J1. The AT bus connector is a 40-position (dual-row 20) board-mount socket on 0.100" x 0.100" pin-to-pin spacing, designated J2. (Sockets are shown in Figure 1.) These buses may be stack-through, whereby pins extend to the reverse side of the board, or non-stack-through, which is a normal single-sided PCB attachment for connectors.
The higher-speed PC/104-Plus uses the same 104-position connector as PC/104, which is useful for connecting to legacy equipment. For the PCI bus, it adds a 120-pin (four rows of 30 pins) connector, designated J3, which is a high-density board-mount socket connector with 0.079" x 0.079" pin-to-pin hard-metric spacing. This is the only connector specified for PCI-104. All of these connectors use the pin/socket contact design rather than the card-edge or ribbon-style contact. The three specifications detail connector designs, materials, contact finishes, mechanical performance, and electrical performance requirements.
Engineers are essentially left alone to determine the specs for the other connectors they choose to put on the PCB in the areas designated for I/O or in the card’s logic area. In a typical SFF board like PC/104, measuring just 3.6" x 3.8" (90 mm x 96 mm), there can be as many as 12 different connectors across a single side of the card. For example, one manufacturer’s board is advertised as having one GbE and one 10/100 wired Ethernet port, SVGA and dual-channel LVDS flat-panel video, a CompactFlash header, an Ultra-ATA disk interface, a parallel port interface, and a PS/2 port for keyboard and mouse, as well as the standard PC/104 and PC/104-Plus connectors that support off-the-shelf or user-designed specialty I/O modules – truly crowded real estate.
Engineers must consider some general design issues when selecting connectors; these factors are not exclusive but, on the c ontrary, intricately connected (pun intended):
· Intended application
· Operating environment
· Electrical requirements
· User considerations
· Compliance issues
Reliability in harsh conditions is of paramount concern to embedded systems users. Embedded systems intended for mission-critical military, avionics, medical, and industrial applications often operate in extreme cold or hot environmental conditions. PC/104 specifies a -55 °C to +85 °C minimum operating temperature range. Equipment can be built to cope with lower and higher operating limits, but not less than this minimum range. This requirement has major implications for the key materials in connectors, plastic, and metal.
Plastics are used because of their low cost, desirable molding, and use characteristics. “The biggest issue is plastic and how the design uses the plastic,” states Ron Revell, a product engineer at 3M’s Electronic Solutions Division. “At low temps, embrittlement of the plastic can be problematic. At high temps, the material can lose strength.” For example, a system installed at the British Antarctic Survey would endure extremes of cold and if dropped, could suffer damage to connectors inside the box. On the other hand, a military communications system used in the Middle East would experience extremes of heat that could cause its plastic insulators to lose their structural integrity.
Revell points out that dimensional change over the temperature range should also be considered. “The system’s plastic and the features that stress the plastic are of concern,” he remarks. Connector manufacturers use a variety of thermoplastics, from standard glass-filled polyester such as polybutylene terephthalate or polyamide (nylon) to high-temperature plastics such as liquid crystal polyester and polyethylene sulfide. The PC/104, PC/104-Plus, and PCI-104 specs call for “high-temp thermoplastic, UL rated 94 V-0,” which is the norm for electronic connectors.
Several different metals are used in connectors. “Metal is less of an issue, but change in dimensions over time and stress relaxation at elevated temperatures are worrisome,” Revell adds. “Proper alloy selection is needed to keep the interconnection stressed as the temperature increases.”
The base metals most commonly used in connectors are brass, phosphor (phos) bronze, and beryllium copper. Brass is the least expensive of the three, but phos bronze is stronger and has better spring properties. Phos bronze is excellent for applications requiring relatively few insertion/withdrawing (mating) cycles and low contact flexure; it is also the material specified for PC/104 and PC/104-Plus connectors in the specifications. Offering the best combination of mechanical and electrical properties is the more expensive beryllium copper. It has the lowest stress relaxation rate, high resistance to corrosion, extremely high wear resistance, and better electrical conductivity than either brass or bronze, making it ideally suited to high-reliability interconnections such as I/O.
Wear and tear
In a typical PC/104 system where multiple modules are stacked one on top of the other, connector use may be termed “fit and forget.” Unless required for maintenance or an upgrade, the module stack will not likely be broken apart, so wear on the connectors is light. For this reason, the PC/104 and PC/104-Plus specs require a minimum durability of 50 cycles for the J1, J2, and J3 connectors. This is reflected in the choice of 15 microinches minimum hard gold plating. However, for I/O connections to peripherals or external devices such as USB 2.0, RJ-45 Ethernet, or Camera Link Mini Delta Ribbon, connectors may be plugged and unplugged many times during their service life.
In general, the higher the number of mating cycles, the thicker the plating should be. A good example is a header connector for CompactFlash cards such as 3M’s N7E50 Series (shown in Figure 2). In this case, the nonvolatile CompactFlash memory card is used in lieu of a low-capacity rotating disk drive and is unlikely to be removed during the board’s lifetime. However, the CompactFlash Association’s CF+ and CompactFlash Specification Revision 4.1 (February 16, 2007) calls for 10,000 mating cycles because it was originally designed for consumer electronics as digital film for cameras. 3M offers CompactFlash Association-compliant parts in 15, 20, and 30 microinches gold. Even in harsh environments, gold will remain free of oxides that can cause an increase in contact resistance.
Pure gold is too soft for industrial use, so it is alloyed with other elements to increase durability and resistance to wear. Using electroplating, connector manufacturers typically offer products with a wide variety of precise gold thicknesses, including gold flash, low gold (10-15 microinches), and high gold (30-50 microinches) levels. Given that a human hair is 1,500 microinches thick, these are extremely thin layers of protection. Over time, gold plating may be contaminated by copper and zinc atom migration from the base metal. By plating the base metal with a barrier layer (usually 50 microinches) of nickel, gold can be protected from the base metal.
Two-part connector systems are designed for easy connection and disconnection. While the least amount of insertion force is desired for easy insertion, relatively higher withdrawal force is preferred to maximize contact retention and minimize contact resistance. For the 120-position PCI bus connector, PC/104 specifies mechanical performance in terms of insertion force at 2.5 ounces per pin maximum and withdrawal force at 1 ounce per pin minimum. This is consistent with a single-beam gold-plated phos bronze contact with a large number of fine-pitch pins and compares to the 3.5 ounces per pin maximum and withdrawal force at 1 ounce per pin minimum for the ISA AT/XT bus connectors based on thicker .100" grid pins.
The material, surface finish, and surface area touching the lead of both contacts determine contact resistance. PC/104, PC/104-Plus, and PCI-104 specify contact resistance as <30 milli-ohms maximum, which is consistent with a single-beam gold-plated phos bronze contact.
Passing the tests
When assessing different connector designs and suppliers, it pays to look into which tests manufacturers subject their products to and the care they put into assembling the component parts. “3M’s qualification tests use EIA-364 standard test methods,” Revell says. “Thermal shock includes cycles from -65 °C to +105° C (exceeding EIA-364-32B) with about an hour dwell at each extreme and a transition time of seconds or single digit minutes.”
Tolerances also matter. A connector that is manufactured out of tolerance and possibly exacerbated by a placement or handling issue during board processing may cause the connector to be out of alignment relative to the mating part. The result may be what connector manufacturers call stubbing, which occurs when a pin does not wipe against the socket contact at the first pass, but instead strikes some other part of the connector. Avoiding stubbing is particularly important in the PC/104 environment because it is a stack-through architecture.
Poorly made connectors may mean that the boards cannot be stacked, a contact is bent when stacking the modules, or the insulator or solder joint is damaged. The problem may be immediately apparent when the host and add-on modules cannot be plugged together. Of greater concern is if the problem does not become noticeable until later when the board is in service.
Compliance with the current specs for SFF boards will ensure that modules are interoperable. One other aspect of compliance is the regulatory environment of the geographic markets in which products are used. This determines which components are chosen for a bill of materials. The best example of this is Reduction of Hazardous Substances (RoHS), which requires products sold in the European Union to comply with the directive. Connector manufacturers must indicate which of their products comply and provide written validation of levels of compliance. The outcome can impact the processes and solder pastes by which connectors are attached to the PCB.
Engineers must consider many factors when selecting connectors for embedded systems such as IEEE P996.1-based equipment, in particular those systems not covered by industry standards. It pays to consult the connector supplier or distributor early in the design process to ensure proper form, fit, and function. A small yet perfectly formed connector, functioning quietly and unobtrusively as part of the system design, will provide end users with years of reliable service.
P. Lindsay Powell is global business development manager at 3M’s Electronic Solutions Division, based in Austin, Texas, where he supports new product introductions into the distribution channel. He has worked in the electronics industry for more than 20 years in Europe and the United States. Born in Wales, Lindsay is a graduate of the University of Aston in Birmingham, England.