Product development requires involvement by the end customer, R&D experts, design engineering, sales and marketing, and manufacturing. Follow the design process of a pluggable terminal block as it follows this integrated approach.
By Paul Silva
In the world of industrial interconnect wiring, density and mechanical security are of paramount importance. Over the past ten years, removable two-piece, European rising-cage terminal blocks have captured most of the growth in the North American terminal-block market. More and more manufacturers of industrial electronics are specifying these two-piece designs into their systems.
This trend, however, has not come without a price. Two-piece terminal blocks are typically larger than their single-piece counterparts, reducing packaging density. Also, leading industrial electronics OEMs are concluding that as two-piece terminal-block systems age in the field, the field failure rate due to the interconnect has increased.
Product density
Increasing the density of a two-piece terminal block over the current generation was relatively straightforward. In most two-piece terminal- block designs, the pin header is completely surrounded by a polarized shroud. This approach demands that the female contact of the mating terminal block must extend away from the wiring clamp and screw to engage into the shroud (see Fig. 1).
Some existing two-piece terminal- block designs are very close in density to single-piece terminal blocks. These products mate with a simple pin strip or "stick" header but unfortunately, these designs offer no polarization and are difficult to mate properly in the field. Extending the pin-strip insulator design into an "L"-shaped configuration allows a polarization slot to be incorporated into the back wall. The female contact of the terminal block is then tucked in behind the wiring clamp and a simple molded tab on the terminal block completes the polarization feature. The benefits of this approach are easily seen when comparing the shrouded two-piece terminal-block design with the "L"-shaped approach and a typical single-piece terminal block.
Robustness and security
The current generation of two-piece terminal blocks approach robustness and mechanical security in a rather old-fashioned way—the two halves of the connector system can be fixed together with an optional screw-and-threaded- insert system. This approach is both cumbersome and costly. One proposed solution was a molded snap system, which would be quick and easy to operate as well as cost-effective to produce.
As terminal blocks are often subjected to 10× the external forces seen by the typical commercial connector, however, the snap approach did not prove simple to implement. Early prototypes incorporated a single latch on the pin-header back wall that engaged with a ledge on the back of the terminal block. These prototypes were subjected to wire loads in multiple axes. These prototypes performed poorly when subjected to wire loads perpendicular to the printed circuit board and in the "up" direction. The "up" wire load could rotate the terminal block slightly which would translate to severe wear of contact plating in a vibration environment.
To address this problem, a "lip" was added to the front of the header (see Fig. 2). This lip has a series of holes that accept "nubs" molded to the lower front of the terminal block at each terminal position. When the terminal block is inserted into the header, the header retains the block at two diagonally opposite points, producing an extremely secure condition when mated. Tests with wire loads in multiple axes produced no motion of the terminal block relative to the header and, therefore, minimal contact plating wear would is expected in a vibration environment.
 FIGURE 3. A screwdriver slot was added to the new two-piece terminal-block design to guide users in the unmating of the connector system. |
In early field deployments, customers identified two areas for improvement: unmating of the connector system was not intuitive and insertion of the terminal block into the latching header required too much force, specifically on devices with many terminal positions. To guide the user in the unmating of the connector system, the design incorporated a slot into the top of the terminal block to identify the proper insertion point of a screwdriver blade. Unmating of the latching system is accomplished by inserting a screwdriver into the slot and simply turning the blade 90° (see Fig. 3).
Design engineers eliminated all but three of the "nubs" on the terminal block, which considerably reduced the force required to mate the terminal block into the header without sacrificing mechanical security. Designers added a second latch to the header to further improve the latching mechanism. Two latches better distribute the latching force, virtually eliminating wear on the plastic insulator exposed to multiple insertions. Addition of the second latch did not appreciably affect the force required to insert the terminal block into the header (see Fig. 4).
Cost and manufacturing
In addition to enhancing performance, this terminal-block system was designed to minimize both unit cost and subsequent assembly cost for industrial electronics OEMs. The components used in this system are common in products produced in high volumes, which drives down the bulk price. The commonality of components also allows the product to be assembled on existing, fully automated assembly equipment, allowing for short lead times.
The female contact design employs select strip plating for applications requiring gold or other precious-metal plating. The pin header insulator is made from a new high-temperature polyamide that allows reflow soldering. This enables the printed circuit board assembler to eliminate a secondary wave-soldering operation for the terminal block.
Systems featuring a latching-mechanism geometry are available with either a traditional rising cage clamp (5.00- or 5.08-mm spacing) or spring-clamp termination technology (5.08-mm pitch). The positive-latching mechanism also prevents plug movement in high-vibration applications, while the high-temperature insulator body is surface-mount compatible for lower applied cost. The positive polarization feature with an audible "click" eliminates mismating and helps facilitate blind mating.
The end result of this integrated development process is a connector system that is up to 50% less expensive than two-piece terminal-block systems using the screw-and-threaded-insert locking system. It also costs 10% to 25% less than other two-piece terminal-block systems.
As with most industries, the one constant in the world of industrial interconnect wiring is change. When experts in manufacturing and R&D collaborate with OEM design engineers and others on product development, it is possible to anticipate changing customer demands and to drive industry trends. In this instance, development of a new two-piece terminal-block system optimizes packaging that advances density and mechanical integrity while reducing overall cost.
PAUL SILVA is product manager for Molex Incorporated, Industrial Division, 4 Aviation Drive, Gilford, NH 03246. Tel: (603) 524-5701; email: paul.silva@molex. com.
