Many industrial electronics OEM designers use the word “rugged” to describe their board-level interconnect needs. While rugged can mean different things to different people, it usually includes the ability to withstand high shock and vibration applications, maintain mechanical and electrical integrity after exposure to both harsh environments and high mating cycles, and provide EMI shielding attributes, to name a few.
Designers in the mil/aero, high-performance computing, telecom, and datacom industries face the challenges of ever-increasing data rates, denser systems, and shrinking product footprints. Fortunately for designers in the industrial sector, interconnect requirements don’t change as quickly. Reasons for this include: longer product life cycles, the absence of constant pressure to reduce PCB real estate, and lower bandwidth requirements, among others.
Several design elements contribute to connector ruggedness, but three especially important factors are: contact design, plating, and insulator design.
Contact base materials and design are among the most important parameters for rugged applications. Common base metals include brass, phosphor bronze, and beryllium copper.
- Brass is the least expensive of the three metals and has excellent electrical properties. However, it’s not often recommended for contacts (receptacles) in a working beam because it could fail due to low yield strengths.
- Phosphor bronze is stronger than brass, has better spring properties, and is excellent for contacts that have relatively few mating cycles and low contact flexure.
- Beryllium copper (BeCu), while more expensive than most contact materials, provides the best combination of mechanical and electrical properties. Once formed and hardened, BeCu will retain its shape under a wide variety of conditions.
Two Samtec contact designs are popular in many industrial applications. The first, the Tiger Eye™ contact, is a multi-finger, heat-treated BeCu contact. Although commonly used in 1.27 and 2.00 mm pitch micro interconnect systems, these contacts are designed for rugged environments. For example, a slot in the tail provides more surface area for solder adhesion. Additionally, connectors with micro tail slots tend to adhere to the wet solder paste prior to reflow better than flat leads. These and other qualities increase the mechanical strength of the connector to the PCB.
Multi-finger, heat-treated BeCu contacts also feature a slot in the transition area between the gull-wing tail and the contact. This slot, while seldom needed, is designed to prevent solder wicking. Although wicking is rare in SMT applications due to the limited amount of solder, if it should occur, the slot disrupts the capillary action so the solder doesn’t migrate into the contact area.
The second of Samtec’s most popular industrial contact designs is a dual-wipe, phosphor bronze tuning fork design, called Tiger Buy™. This design is popular in rugged applications due to its contact geometry. Specifically, the length of the two mating beams (fingers) enables firm, consistent normal forces and is less likely to take a permanent set after exposure to numerous cycles.
Contact Design for Bandwidth
While system speed isn’t a concern for most industrial electronics OEM designers, it may become a concern for some in the future. Industrial Ethernet is usually the maximum bandwidth requirement; although, industrial suppliers are following the speed advancements as ruggedized components make improvements viable. Likewise, applications for micro-pitch interconnects (<2.00 mm centerline) are relatively rare, but they are required for rugged handheld devices, like those used in data collection.
Contact systems can be designed to meet both rugged and higher bandwidth requirements, though. Samtec’s popular Edge Rate® design incorporates BeCu to maximize spring properties, while the contact geometry and orientation in the insulator optimizes signal integrity. Specifically, the surface of the contact is milled, creating a smooth mating surface area, rather than stamped, which mates on a cut edge. This smooth mating surface reduces wear tracks on the contact, increasing its durability and life cycle. It also lowers insertion and withdrawal forces, which allows the connectors to be zippered when unmating.
These contacts are also positioned in the plastic insulator so that the narrow edges of the pins are parallel to each other. This minimizes the parallel surface area, reducing both broadside coupling and crosstalk. The video below shows the design of the Edge Rate contact.
Designers frequently ask which plating finish is best. The best plating finish is whichever material combination meets your requirements at the lowest cost. Gold is generally specified for high-cycle, high-reliability applications and low-voltage or low-current applications. Even in extremely hostile environments, gold will remain free of oxides that could cause an increase in contact resistance.
Tin is a lower-cost alternative and has excellent solderability. It’s frequently used in connector systems with fewer expected cycles, but is also used in high normal force contacts, as these cause sufficient wiping action during lead insertion to help break up the tin oxide surface film.
For these reasons, selective gold-tin plating, which provides designers with the best of both worlds, is Samtec’s most popular plating option. This option provides the critical contact area with the reliability of gold, and the tail area with the lower-cost and solderability benefits of tin.
Plastic insulator design features that are popular with industrial product designers include:
- Board locks on connectors that mechanically lock two PCBs together.
- Positive latching systems on discrete wire and IDC cable systems. These manually activated latches can increase unmating force by up to 200%.
- Screw downs, which mechanically secure the connector to the board.
- Weld tabs, which significantly increase shear resistance of the connector to the PCB.
- IP ratings, such as IP67 and IP68, since dust and water protection are often a concern.
Beyond these few features, there are innumerable insulator design permutations, such as: insulator material, heat deflection temperature, maximum processing temp, RoHS compliance, and dielectric strength.
In conclusion, interconnect systems don’t meet rugged industrial performance requirements by chance. Contact systems, plating materials, and insulators, amongst other critical features, provide connectors with the rugged, high-reliability performance characteristics required to withstand high mating cycles and harsh environment applications, and even (most often on a smaller centerline) to meet higher bandwidth requirements.