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Coming to electronics as a hobbyist I'm not sure I understand why I have to solder header pins? On more than one occasion when dealing with Arduino / breadboard projects the circuit will not work until the header pins are soldered. Holding them in place will not solve this problem. Why? Low voltage? Need for persistent connection? Holding them in place doesn't work nearly as well as it looks like it should? Why?

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While a loose "hold" might not work that well, on thousands of occasions I've jammed a 6, 8, or even 10+ pin header crooked into holes on a PCB to program the device. Generally it takes a good bit of force to one side to prevent something like what Andy drew. – Nick T Mar 24 '14 at 4:10
up vote 21 down vote accepted

Think about it mechanically - you have a straight row of pins and you insert them in slightly loose fitting holes (all in a line). Even if you hold them in place - can you be sure that one of the pins isn't fractionally bent in one direction different to the others. Think about this for a 3 pin header: -

enter image description here

Clearly the pin in the middle isn't touching the inside of the yellow hole until you put unreasonable amount of pressure on the connector. Please, no complaints about the colours.

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Plus the fact that contact area (at the corners) of the pins with no gap is still tiny. – Ben Voigt Mar 24 '14 at 15:38
The drawing was exactly what I needed. Thanks very much Andy! – Dave Kaye Mar 24 '14 at 17:01

The holes in the PC board are somewhat larger than the pins on the header so that you can easily insert the header. The normal headers are intended to be soldered into the board. Because of the loose fit of the pins in the holes, simply placing the header in the board without soldering will not ensure a good connection.

There are "press fit" headers available that will make a secure connection when inserted into holes of the correct size - but there is no guarantee that the common boards (Arduino shields, etc.) have the correct size holes to accept the press-fit headers.

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Electrons are really small. What looks like a solid connection to the naked eye at a macro scale could have tiny gaps which look massive to an electron at the tiny scale that they care about.

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It'll work, but you have to give them a shove to make the gap – W5VO Mar 23 '14 at 23:41
I assumed arcing was outside the scope of this question – NickHalden Mar 24 '14 at 0:03
@W5VO That sounds potentially difficult. – Adam Davis Mar 24 '14 at 15:29

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