Insulating housing material for connectors should be chosen based on mechanical, electrical and thermal properties.
By Bruce Burroughs
Every printed circuit board (PCB) has a number of different components, depending on the function it performs. Each PCB has some type of terminal block, or connector, that performs the important task of distributing power and signal to and from the board. To complete this function, the connector generally consists of two different components: the insulating housing and the clamping- and current-carrying bodies.
Various materials and technologies are employed to optimize the connector`s performance. In selecting the proper insulating material, several criteria are involved, including the different design and performance criteria. The decisions made by the insulating housing designer must take into account the properties of the base materials, the individual design and the application of the end user.
Mechanical Protection
The insulating housing encloses the clamping- and current-carrying bodies, and the material is chosen based on its mechanical, electrical and thermal properties. Mechanically, the housing supports and captivates the metal parts, insulates them from one another and other components, provides protection from shock, vibration and environmental pollutants, and provides stability and protection to the solder joints.
Well-designed housings also facilitate entry of the clamping tool and conductor. The clamping body opening tool, usually a screwdriver, is guided directly onto the screw head by a cylindrical turret. The turret surrounds the screw, prevents it from slipping off and recesses it in the fully open position. This turret not only keeps the screwdriver on the head but also provides touch safety for live metal parts (see Figure 1).
Ribs molded into the interior of the turret deliver an even higher level of mechanical protection. These ribs protrude into the cylinder of the turret and prevent the screw from backing out, because of vibration, by closing slightly over the head of the screw as it is tightened. This ensures that the clamping body maintains the specified pressure on the conductor.
The housing also serves to absorb the torque from the tightening of the screw, which protects the solder joints. This protection can be enhanced by the use of pins molded into the housing, which fit into matching holes in the PCB.
The tool-guiding turrets are also advantageous for screwless clamping bodies, serving to guide the tool into the correct position for opening the spring-loaded clamps. Mechanical strength is important for these screwless terminal block housings because they must account for the stress of the opening of the spring on the solder joints. This also applies to spring-clamp terminal blocks that do not require a tool, but instead are activated by a lever incorporated into the block itself.
An important balance is sought between mechanical strength and flexibility, which is required to prevent brittleness that leads to breakage. This is particularly important for modular housings, which slide or dovetail together (see Figure 2), as well as for pluggable male and female connectors.
Electrical Properties
The insulating material is also chosen for its electrical properties, including the ability to insulate live-current-carrying parts. General points for consideration are dielectric strength, tracking resistance and creepage characteristics of the insulating material. Dielectric strength is the ability of an insulating material to withstand voltage without breaking down. The evaluation of the material`s dielectric strength is critical to performance. Tracking resistance is the level of resistance the material has to creepage, which is voltage flowing across the surface of the material along creepage paths. Internal resistance is measured to determine conductivity through the material. Air gaps are specific to each product`s design, and refer to the distance between metal parts through open air. Design parameters and environmental conditions can affect all of these electrical properties. For product design and construction, dimensional and environmental issues must also be taken into consideration.
Thermal Characteristics and Testing
The final property to be considered is the thermal characteristics of the material. Thermal characteristics are important in determining how the product reacts to heat, as generated by electricity, and even how the material reacts to open flame. Over long periods of time, when subjected to heat, all plastics go through thermal aging. This process negatively affects the mechanical and electrical properties of the material.
It is important that the material can be stabilized through thermal-aging techniques applied during the manufacturing process. This prevents the loss of mechanical strength, as well as breakdowns in the electrical characteristics of the molded product. It is also important that the material be able to withstand the heat of the various soldering processes and any burn-in testing.
A common flammability test that is applied to insulating materials is the UL 94 rating test. This Underwriters Laboratory test involves applying an open flame at 10 mm from the material for 10 seconds, then measuring the afterflame time. (Afterflame is the presence of an open flame following the application of the test flame; afterglow is the presence of a "red-hot" glow on the part.)
As soon as any afterflame is extinguished, the flame is reapplied for the same interval. Again, any afterflame or afterglow is measured. If any molten material drops from the specimen and ignites the cotton indicator directly underneath, the material is rated UL 94V-2. If the molten material self-extinguishes and does not ignite the cotton, then it is rated UL 94V-0. The superior V-0 rating also has lower afterflame and afterglow time requirements.
Commonly Accepted Thermoplastics
Regarding specific materials, the most commonly accepted and utilized thermoplastic is nonreinforced polyamide. This material has become essential in developing electronics. It carries worldwide approvals and testing certifications by all the major testing agencies.
Polyamide possesses excellent mechanical, electrical and thermal characteristics. Depending on the type used (e.g., polyamide 6.6), it has a temperature range from -40° to 100°C, which is well within the guidelines for the majority of PCB applications. Polyamide can withstand brief peak temperatures of up to 200°C. It remains flexible and resistant to breakage over time and it possesses excellent dimensional stability because of its thermal-aging stabalization. It is also available in a wide variety of colors, in both UL 94V-0 and V-2 ratings.
If a higher operating temperature rating is required, then the polyamide can be reinforced with fiberglass or polyamide GF (glass fiber). This reinforcement also serves to increase the rigidity of the housing while increasing the operating temperature up to 130°C. Because of the difference in the flow and shrinkage rate of the material, a similarly dimensioned GF product may require a different mold than the standard polyamide product. The increased rigidity could possibly lead to problems with two-piece connectors, specifically breakage caused by impact.
Other thermoplastics exist that are also used, including polyester, for specialty applications. Several materials meet the demanding requirements, including thermoplastic polyester (PBT) and polyamide.
Regulatory Agencies
Different regulatory agencies have different indices for determining acceptability of base materials, and also the effects of various environments and applications. The German Institute of Standards (DIN) and the Union of German Electrotechnologists (VDE), for example, consider various voltage categories and environmental conditions (e.g., pollutants, as they relate to conductivity), as well as the location of the device (commercial, industrial, residential, indoor or outdoor) when rating the material. The regulatory agencies provide basic guidelines, but the designer must take care in extrapolating them to the molded products. The end user also has responsibility. It is imperative that the conductors are correctly stripped and fully inserted into the clamping body because exposed conductors can lead to reductions in creepage distances and clearances.
Conclusion
The design of a quality connector housing requires a material that provides mechanical strength, dimensional stability and consideration of the intended application, including the assembly process. The designer`s experience and knowledge of the performance of the material also influence selection of the correct material. Because of the task it performs, a quality connector is crucial to the proper functioning of the PCB.
BRUCE BURROUGHS is product manager, Wieland Electric Inc., 49 International Road, Burgaw, NC 28425-0759; (800) 943-5263; Fax: (910) 259-3691; Web site: www.wielandinc.com.
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Figure 1. Screw-clamp terminal blocks, such as the 8213 Series pluggable connector, incorporate cylindrical turrets over each screw head to render the screws captive.
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Figure 2. Spring-clamp terminal blocks, like those in the modular 8375 Series, use a material with the mechanical strength and flexibility to facilitate the ability to snap together.
SPEC SHEET
End Applications: Terminal blocks or connectors
Related Products: Clamping tools, PCBs, thermoplastics
Main Point: When deciding on an insulating housing material, the designer must take into account the properties of the base materials, the individual design and the application of the end user. In selecting the proper material, several criteria are involved, including mechanical, electrical and thermal properties. Characteristics of the accepted thermoplastics and the standards of the different regulatory agencies are also important.
