By Michel Hodak
Reliability is often the most important factor in a business decision. From a quality or reliability engineering viewpoint, reliability can be defined as, "The duration or probability of failure-free performance under stated conditions," or "The probability that an item can perform its intended function for a specified interval under stated conditions."
The reliability field involves the evaluation and optimization of numerous defined factors using methods that are standardized or are to be developed and agreed upon by the interested parties. Terms such as "derating" and "failure rate" are part of the jargon used in the reliability field. Good sources for this information are the Certified Reliability Engineer and Certified Quality Engineer certifications offered by the American Society for Quality.
Terminal blocks are subjected to a number of environments during their transportation and storage; processing at the manufacturer; and installation, maintenance and use at the end user. These environments can be interpreted as stated conditions by defining them in detail as to temperature, humidity, corrosive conditions, contaminants, interior and exterior conditions and human factors. The duration of exposure to the changing conditions within these environments must also be defined.
Storage and Transportation
The first two environments, storage and transportation, are more understood. They affect all sorts of products and, thus, most procurement, sales, operational and engineering personnel are quite experienced with them.
 Figure 1. Cross-section of a surface-mount solder joint. |
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Storage conditions for terminal blocks can vary from clean, heated or air-conditioned, humidity-controlled warehouses, offices and factories to uncontrolled environments where heat, humidity and corrosive conditions shorten the life of the block substantially. In these uncontrolled environments, the solderability shelf life can be reduced because of the accelerated formation of oxides, corrosion and inter-metallic compounds; the appearance and functionality of metal components can be affected by rust, verdigris and other forms of corrosion; and the plastics used for the dielectric materials can suffer from deteriorations in flexibility, strength and shape. Shelf life times are a mix of short (days, weeks) for terminal blocks that are being made-to-order and long (months, years) for blocks that are built for stock or are rescheduled (pushed out) for shipment at a later date. First-in/first-out inventory handling, date-coded labeling, properly sealed packaging, corrosion protection, improved storage conditions and appropriate terminal block materials and design can all increase reliability and extend shelf life.
Transportation conditions involve the effects of vibration and impact caused by vehicle engines and other machinery, roads and material handling equipment. Temperature extremes can often compound and aggravate vibration and impact effects. Sea or coastal transportation involves exposure to salt air. Transportation times are usually short and predictable, ranging from a few days by land and air to a few months by sea. Terminal blocks and their packaging must be able to survive transportation without dismantling, deforming, corroding or breaking. Using reputable carriers, properly sealed packaging, strong inner and outer cartons, proper dunnage (padding), corrosion protection and appropriate materials and design can all increase the probability that the blocks arrive at their destination in good condition.
Processing Conditions
Processing conditions are remarkably different, depending on the terminal block type and at which manufacturing step it is in the process. Wave-soldered, board-mounted terminal blocks are typically through hole-mounted. The through-hole leads must be correctly positioned, sized, aligned and solderable in order to insert and solder well. The whole assembly must be able to survive the temperature of the preheat zones and the temperature of a solder wave transmitted through the leads into the block. The fluxes must not affect the block and must be easily washed away and not remain in any recesses. The whole assembly must be water-washable. Preheat temperatures can be 100°C or more and solder waves at 280°C are not unusual. Unwanted temperature effects can be melting, warping, blistering and deformation. Processing times include a few minutes for exposure to flux, a few minutes for preheat, a few seconds for the solder wave and a few minutes for washing and drying. Choosing the appropriate metals, resins and shapes improves the reliability of the soldering process.
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Reflow-soldered, board-mounted terminal blocks are surface-mounted or through hole-mounted. The surface-mounted leads must be correctly positioned, sized and coplanar, compliant or floating to meet the board surface at all pads in order to solder with sufficient strength, process yields and electrical circuit reliability (no open circuits; see Figure 1). The through-hole leads must be correctly positioned, sized, aligned and solderable in order to insert and solder well. The whole assembly must be able to survive the full brunt of the heating process where infrared and convective heat sources are used to drive heat into the lead-to-pad interfaces in order to melt the solder paste that was applied in a prior process. Washing may or may not be required but should be expected. Preheat zones step up the temperature until a peak of 230°C or more is attained after a few minutes. Again, unwanted temperature effects can be melting, warping, blistering and deformation. Processing times include a few minutes for preheat, several seconds at peak and a few minutes for washing and drying. Also, choosing the appropriate metals, resins and shapes improves the reliability of the soldering process.
Panel-, chassis- and rail-mounted terminal blocks and ceramic terminal blocks are usually processed manually. Reliability issues focus on the human factors of manual operations where installation involves screwdriving and screw tightening for mounting or wire retention. Overtorquing or uncontrolled screwdriving can severely damage connections in an obvious detectable manner or in a more hidden manner where the effects would only been seen at a later time, perhaps in the field after weeks or months of use.
End Use Conditions
End use conditions are extremely important because they have a profound effect on the material requirements, the assembly configuration and the overall costs of the terminal block. Overall costs involve the purchase, maintenance and warranty costs during the life of the block. Customer satisfaction and manufacturer reputation are also part of the cost equation. The factors to be considered are:
- Application location, such as interior, exterior, tropical, marine, stationary or mobile (in a vehicle passenger or engine compartment)
- Temperature and humidity ranges and extremes
- Temperature within an enclosure during use and proximity to heat sources as electrical circuits cycle on and off
- Exposure to contaminants, dust (conductive or non-conductive), pollution, corrosive gases, liquids
- Exposure to vibration, impact, penetration, voltage spikes, current overloads
- Derating principles — as environmental conditions become more severe, engineers should derate the components
- Human factors involving field wiring and torques, forces and abuse during installation and maintenance
- Laws and regulations that may require Underwriters Laboratories, Canadian Standards Association or other certifications that involve flammability, creepage, clearance, dielectric breakdown, impulse withstand, shock hazard testing and performance to a standard
- Customers that have explicit and implicit requirements
Conclusion
Terminal blocks that perform well in storage, transportation, processing, installation and end use environments do not come from chance. They are the result of dedicated efforts in the design, development and application processes. In an age of globalization, the cost of a product must not only include its purchase price but how well it performs in all its environments.
MICHEL HODAK is Quality Manager, WECO Electrical Connectors Inc., 19900 Clark Graham, Montréal, Québec, Canada H9X 3R8; (514) 457-7257; Fax: (514) 694-0956; E-mail: mhodak@ weco.ca; Web site: www.weco.ca.
SPEC SHEET
End Applications:
PCBs, DIN rails, panels, chassis
Related Products:
Terminal blocks
Main Point:
The reliability field involves the evaluation and optimization of numerous defined factors. Terminal blocks are subjected to a number of environments, which can be interpreted as stated conditions by defining them as to temperature, humidity, corrosive conditions and so on. The duration of exposure to the changing conditions must also be defined.
