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A lot of boards with predominantly SMT components have TH connectors on them for things like headers and power connectors. Take a standard barrel power jack for example:

TH:

Through-hole power jack

SMT:

Surface-mount power jack

When designing a board, how do you decide whether a connector can be SMT or not?

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I think for connectors that take stress, through hole should be preferred. So then SMT would be a compromise. maybe you need to get the board smaller, and by going to an SMT connector, you're able to freely route traces under the part, or route more traces. Or maybe you can get a discount if your board requires no holes to be drilled at all. –  Kaz May 29 at 19:07
    
Much as they all say: Solder is not a good means of mechanical retention. If an object is subject to mechanical stresses apart from its weight, then they need to be dealt with by other than the solder connections. Even heavier components or those of substantial 'height', so impact forces through the c of g cause significant stresses, need some means of reacting mechanical forces. –  Russell McMahon May 30 at 8:58
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3 Answers 3

up vote 8 down vote accepted

The choice of through-hole versus surface-mount comes down to you, the designer of the PCB. To make that choice, you have to consider things like:

  • Assembly and tooling. Does the company or person assembling the PCB have the necessary tooling for the desired form factor?

  • Device construction and clearance. Does the device ultimately need to be space-saving? Are through-hole leads going to cause clearance problems in a restricted form factor? Is there another PCB or something nearby which through-hole leads and the resulting solder could be too close to?

  • Strain/durability. Is the DC jack going to be regularly exposed to physical wear and tear? Are users going to be plugging and unplugging the device often? Through-hole may afford some additional strain resistance and durability.

  • Cost. If you're building 100k units, a small difference in cost between the TH and SMT version of this component might matter.

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The shear strength of SMT pads on a frequently jarred power cord gives rise to metallurgic fatigue.

The reliability of this interface is poor unless there is significant pressure on the connector to resist strain from stress on the pad joint from mechanical torque.

Unless you can provide this extra durability protection or prove that the plug is infrequently used, you must choose THT.

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Would there be any disadvantages to a connector which used through-hole mechanical support but surface-mount electrical connections, if the electrical connections were made by legs that had a little elastic stretchiness? I would think that would be the ideal mounting approach. –  supercat May 29 at 17:23
    
Yes, if mechanical strain relief is good, but this photo shows an epoxy sealed contact for SMT leads. –  user43594 May 29 at 17:50
    
The SMT connector pictured didn't seem to have any through-hole mechanical aspect. I guess a disadvantage of having through-hole pegs would be that it would preclude use of the flip-side of that part of the board for other purposes, but mechanically I think it would seem superior. Actually, even with a pure SMT part, I would think having a solid "mechanical" pad and slightly-springy legs would be better than using the same pads for both support and connections. The legs wouldn't need to be very springy--just more springy than the mechanical pad. –  supercat May 29 at 18:38
    
There must be mechanical stops to prevent stress , even more, with compliant leads to pads. This is why THT power receptacles are much stronger. If you choose SMT, I might consider epoxy around base to board. –  user43594 May 29 at 18:51
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Agreed, the same is true for iPad plugs, Macbook power jacks, and many other mobile plugs and jacks where strain relief on the plug wires as well as the receptacle, are so marginal that it boggles the mind or perhaps it is planned obsolescence. –  user43594 May 29 at 22:54
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If you're going to production, the cost of the selective soldering process will add considerably to assembly cost per board, particularly if its the only TH part. If not, try to localize THs to one part of the board to try to control cost.

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