With RoHS compliance the talk of the industry, the process of choosing printed-circuit-board connectors now includes issues of added costs and redesign.
By Bill Carson
The mission of the connector system is to make a foolproof, reliable electrical connection with little or no distortion that can withstand vibration, contamination, or extremes in temperature. To do this successfully, several factors must be considered, based on the application. What type of clamping system should be used? What are the material requirements? Are there environmental factors to consider? Is board density a factor? What manufacturing process will be used? What is the real cost of the connector? Will the end product comply with the E.U.’s Restriction of Hazardous Substances (RoHS) Directive? With so many PCB connector options available, many customers simply think in terms of cost and delivery, but much more is going on with those little connectors on the PCB.
Clamping system
The impact of vibration on a PCB connector has a direct affect on the contact force, which in turn affects the voltage drop across the clamping points. When designing boards destined for high-vibration applications, a connector with a clamping system will resist pullout, and has large contact surfaces to provide adequate contact force. For example, a torque of approximately 0.8 N·m on a 2.5-mm2 clamping yoke system provides a contact force of approximately 750 N, independent of wire gauge. A screw-clamp connector can provide this level of contact because the upper threads overlap and lock the screw. Even changes in the wire diameter caused by temperature fluctuations are compensated by the elastic behavior of the clamping system, so retightening of the screw is not necessary to maintain the required contact force. Well-designed tension clamp connectors can also provide a high level of vibration resistance, as well as shorten the installation time (see Fig. 1).
 FIGURE 1. Before and after aging tests, voltage drop differences remain within limits for screw-clamp, tension-clamp, and insulation-displacement-clamp technologies. |
For low-vibration applications, a range of clamping options exists, including leaf springs, crimp connections, push-on tabs, insulation-displacement connections, spring connections, and push-in connections. While the voltage drop for these techniques may be slightly higher than a screw-clamp or tension-clamp connection, the value should be considerably lower than the limit value required by VDE 0611. If installation time is a factor in the design, opt for a connection type such as an insulation-displacement connection that doesn’t require wire stripping, or a tool-less, push-in connector system.
Material requirements
It’s important to consider the material makeup of connectors when selecting these for specific applications. PCB connectors are manufactured using a variety of materials for both the insulation and the clamping systems. Thermoplastics and thermoplastic polyesters (PBT) are the most commonly used insulating materials because they provide flexibility and resistance to fractures. In addition, the chemical structure of these materials provides inherent flame protection without the use of fire-proofing agents. While a connector made of the thermoplastic polyamide (PA) has a temperature range from -50°C to 100°C and a flammability rating of UL 94 V-2, PBT and PBT with fiberglass reinforcement offer greater dimensional stability at higher temperatures (up to 130°C), and have a UL 94 V-0 flammability rating. For SMT or through-hole-reflow (THR) applications, connectors made from liquid-crystal polymer (LCP) should be considered. This halogen-free material is stable enough for use in all reflow solder processes, including infrared, convection, and vapor phase. In addition, LCP has been tested for double-sided boards at 2 × 290°C for 30 seconds, exceeding the category A standard, EN 61760-1, of 260°C for 10 seconds.
With the deadline for RoHS compliance looming, the material used in the connector clamping system and contact surface is a critical consideration. Because the use of lead-free materials will be essential for all components and soldering processes, the options available should match the needs of the application. Contact surfaces and the soldering zones that are made from pure tin will comply with the RoHS specifications. Yet there are more factors to consider beyond just the lead-free requirements.
Environmental factors
While stainless-steel clamping systems are available for most connector options, zinc-plated-steel components that have the additional surface protection of a yellow chromate layer may be the best choice. This enhanced surface protection increases resistance to corrosion for demanding conditions found in marine, petrochemical, or tropical climates, even if the zinc plating is partially damaged by scratches or pores.
Board space continues to be a priority. In reviewing the total cost of the board layout, now would be the time to consider products that may double the density of connections in smaller areas of the board. Savings can also be found not only on the board, but also with the range of product options available (see Fig. 2).
 FIGURE 2. The use of double density connectors can provide real space-savings on the board. The smaller board contains the same number of connections as the larger board. |
Making the most efficient use of board space is not just about choosing smaller and smaller components. For example, a 3.5-mm-pitch connector that provides a double row of connections can use 10% less board space than a connector with a 2.5-mm pitch, and save up to 30% on the front plate space. Connectors that can be dovetailed together, rather than fixed-pole versions, provide an added level of design flexibility for restricted space applications. Making use of connectors that allow a multitude of different angles between the PCB and field wiring will also make denser connections possible. And applications that make use of double-sided boards will benefit from the use of pin headers with shortened pin lengths.
Manufacturing processes
Trends in PCB manufacturing processes are changing as fewer and fewer PCB components are manually placed on the board. Manufacturers are eliminating wave- and hand-soldering operations. As companies transition to “lights-out” manufacturing to reduce the cost of production time and labor, PCB connectors that can be fitted for pick-and-place robotic applications and meet the demands of SMT through-hole-reflow processes are necessary.
Connectors that are optimized for the SMT/THR process will possess the material characteristics mentioned earlier in this article, and provide the dimensional stability necessary to withstand the higher temperatures required for the lead-free soldering process. Connector packaging is also a factor in selection. Tape-on-reel anti-static packaging designed to fit standard feeders will further reduce labor costs.
Real cost
Is the real cost of the connector the cost of the purchased part, or is it the total installed costs of the PCB? In reality, the answer is the total installed cost. Many of the factors already presented demonstrate how the choices made in the board design relate to the ability to control and reduce the real connector costs (see Fig. 3).
 FIGURE 3. Consideration of required wiring times across clamping technologies can save a manufacturer in installation costs. |
From the correct choice of clamping technology, to the selection of materials, to the manufacturing process used, cost savings can be achieved by assuring the best fit for an application. In some cases, selecting one clamping technology over another may result in a 50% savings in wiring times, or eliminate the need for special installation tools. Savings can also be realized by using two- and three-pole connectors to build up to the needed number of poles, reducing the total inventory of part numbers ordered and kept in inventory. As PCB connectors gain more functionality, the per-pole price of a connector may increase, but the significant savings realized in the bottom-line “real cost” will grow.
The RoHS Directive is an issue in front of the entire industry, and many manufacturers will meet RoHS compliance. The directive should be an impetus to evaluate connection technology to design-in products that address the application requirements and provide a more reliable and cost-effective solution.
BILL CARSON is the North American marketing manager of printed circuit board products for Weidmuller, 821 Southlake Boulevard, Richmond, VA 23236. Tel: (804) 379-6002; Email: bcarson@weidmuller.com.
