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Intro
It is equally important to answer questions such as: Is the application going to be field or factory wired? How much current will the terminal block need to handle? Will the connection need to be easily disconnected for board repair or replacement? What wire size (AWG) will you need? How important is cost? All of these factors and more go into choosing the right interconnection or terminal block.
Continuity
With any connection it is imperative that continuity is maintained. An increase in the impedance at the termination point can spell disaster for many electronic designs. For example, in audio applications an increase in impedance will result in a poor transfer of power and ultimately create distortion as the amplifier output is increased to compensate for the mismatch. Choosing a spring type terminal block will eliminate any chance of a mismatch.
If the application is end user dependant then making terminations must be easy. The use of screw based terminal blocks has for years been the main connectivity device for many consumer products. Unfortunately, unless these connections are properly made, screws can loosen resulting in poor connections, again justifying the use of spring clamp terminal blocks.
Environmental Issues
Environmental issues such as terminations in gaseous atmospheres where deterioration of the termination can occur require terminal blocks that provide a “gas tight” connection. This ensures that the terminations will not degrade. In order for these terminations to be effective positive force must be continually applied so that the risk of a loose connection does not occur.
By using spring clamp terminal blocks a constant pressure is applied to the wire being terminated. This constant pressure assures stable contact impedance and a gas tight connection. Using these types of terminal blocks reduces the chance that a wire will loosen or that contamination at the termination junction is created.
Is the connector you plan to use “Environmentally Friendly”? Does it contain a high percentage of lead that may make it hazardous to the environment? If so and your product is disposable you should consider a low lead terminal block product.
Plating Requirements
The type and thickness of plating used on the contacts of a terminal block becomes an important design issue. When using a pluggable terminal block, the plating should be thick enough to withstand repeated plugging and unplugging of the mated parts. In mated connectors the material used in each piece should be of the same alloy so that low-level voltage (Seebeck Effect) is not produced. Additionally this plating should be such that low-level currents can pass through the connection without loss or degradation.
Size
How much board space is available for a terminal block, or series of terminal blocks? If the current and voltage levels are low enough then circuit spacing of connection points can be dense. With higher voltage levels or current requirements greater spacing will be needed and subsequently the distance between adjacent conductors of the terminal block will increase. Always choose a terminal block for the highest current level it will see. Make certain the correct wire gauge can be accommodated for a given current level. This will help minimize the affect of the terminal block on the circuit.
Agency Approvals
Will the product require any special regulatory compliance? If your product is to be “Field Wired” make certain that the terminal block is rated for field wiring and not just factory wiring. Field wiring allows anyone to connect your device outside of a factory environment and still have agency approval.
As an example, if the product requires FDA approvals then it is imperative that the terminal block be usable in that application. At minimum most U.S. and Canadian manufacturers need Underwriters Laboratory (UL) or Canadian Standards Association (CSA) approvals to market their products. The terminal block you choose should have approval from a recognized agency.
Ease of Termination
Making terminations easily, quickly and securely is imperative. Screw based terminal blocks are an excellent choice for many low cost applications but choosing the lowest cost terminal block may not be in your best long term interest. On the down side using a screw based terminal block within a product typically means backing the screw out, inserting the wire and then turning the screw to secure the wire; very time consuming and not always the best termination technique. Making certain that the screw has been torqued to specification is important to minimize oxidation and potential heat rise at the termination point. Using a spring clamp type terminal block cuts the termination labor by at least 60% and provides a more stable connection. In a factory or field setting, time is money and you can save money by choosing the right terminal block for the application.
Quick Connectivity
If you need to have your product field retrofitable you should design with a method of quickly connecting and disconnecting terminations to, and within the product. Pluggable terminal blocks make removing a printed circuit board or assembly easy and add to the product’s overall marketability. Additionally there is no reason to disconnect wires from the product when two-piece devices can minimize potential rewiring mistakes when the product is put back into service.
Conclusion
In many cases terminal blocks consume the greatest amount of board space. They are the gateway to the circuit boards on which they reside. As such, making the right terminal block selection from the start will insure a cost-effective, trouble free service life.
