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THR compatible terminal blocks and headers.

The components used to populate many of today's printed circuit boards are of the surface mount type. This was not always the case. In the early years, components were placed on the PCB such that their leads passed through the substrate and were soldered on the opposing side. These components are known as thru-hole (sometime spelled through-hole) components, and are still popular today. They are affixed to the PCB via the wave soldering process. This is a process by which a molten bath of solder is formed into a fountain, over which the PCB passes. As the PCB passes over the apex of the solder fountain, the leads are exposed to the solder, and the component subsequently becomes an electrically conductive circuit element. This approach inherently produces a strong bond between the component and the PCB. However, as technology progressed and circuits became more complex, PCB real-estate became more and more valuable. As a result, the surface mount device (SMD) evolved.

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The introduction of surface mount devices and components allowed for increased productivity, while reducing overall board size. Since most surface mount components are small in size and weigh on the order of grams, the inherent strength of the solder pads on which they are affixed was never a serious concern. However, terminal blocks and their plug-in mates, headers, are often times the largest component on a PCB, and subjected to relatively strong rotational and pulling forces. Surface mount Technology (SMT) terminal blocks do exist, but there are limitations as compared to their thru-hole equivalents. As a result, most terminal blocks and headers are manually populated and wave-soldered to the PCB. This makes for a two step soldering process, which inherently costs more and consumes more time to build a completed assembly. A process by which both surface mount and thru-hole mounted components could be soldered in tandem was desired. As a result, the reflow compatible (THR) terminal block and header were realized.

The correct material for the application will make or break a part's ability to withstand the temperature extremes seen during the reflow solder process. This is especially true in lead-free processes, where the temperatures typically reach 260[degrees]C for up to 10 seconds. As a result, thermoplastics used in the construction of a THR terminal block and/or header need to able to withstand elevated temperatures without any deformation or blemishing. Additionally, there are other board level considerations which need to be addressed when employing the use of THR components.

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THR components should be placed such that taller components do not interfere with airflow around the base of the part. Adequate airflow to the base region is required in order to bring the solder paste into the aqueous state. If not, cold solder joints will likely result, and another solder step will be needed to complete the assembly. This defeats the purpose of using a THR compatible component. The PCB holes must be designed such that plated holes have soldering rings on both sides of the board. The diameter of the holes should be 0.10 mm to 0.25 mm greater than the pin diagonal or diameter. The diameter of the rings should be approximately 0.6 mm greater than that of the hole. This allows for the proper amount of paste to reside between the pins of the part, and the surrounding plated hole. With a properly designed board, proper preparation is the next consideration.

As mentioned above, THR components require the use of a solder paste. The solder paste is applied over a stencil, by way of a dispenser. Care must be taken to ensure the holes are completely filled with paste. This will ensure adequate paste is left between the pin and the surrounding holes after the pin is inserted into the board. The pin length should not exceed 1.5 mm past the opposing side, so as to not displace too much solder paste. The last phase is the reflow soldering process itself.

The circuit board is placed on a conveyor and passed through a special oven which gradually raises the temperature of the entire assembly to a point at which the solder paste melts. Heating profiles vary, but in general most peak temperatures will reach 260[degrees]C within two to three minutes. At this temperature, the solder paste becomes fluid and fills the holes by capillary action, soldering the THR components to the circuit board. It is then gradually cooled to room temperature. A good solder joint is evidenced by a solder meniscus on both sides of the PCB. If the pins do not pass completely through the board, a meniscus will only exist on the component side. For that reason, some THR designs have standoffs which not only aid in visual inspection, but keep the solder past from contacting the component. Periodic inspection of the board underside should be done to ensure adequate solder wetting is taking place.

By Michael Balekdjian, Application Engineer, RIA Connect, Inc., www.riaconnect.com
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Title Annotation:DESIGN TALK
Author:Balekdjian, Michael
Publication:ECN-Electronic Component News
Date:Jun 1, 2010
Words:835
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