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I am going to apologize ahead of time if this is a really simple question. I started learning to solder during my internship but I had no EE knowledge prior. When I was working with surface mount soldering my internship adviser mentioned this property or effect, the name of which I'm blanking on, that would cause the pieces when heated to move into place on the pads of the board. Does anyone know what the name of that property or effect is?

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    \$\begingroup\$ Surface tension. \$\endgroup\$ – The Photon Nov 28 '18 at 17:30
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It's just the surface tension of the solder in liquid state.

A physical system will settle in its lowest achievable energy configuration. Given the surface tension of the solder and the adhesion forces to the different surfaces it is in contact with, the lowest energy configuration would be all the component pins in the center of their respective pads. Of course, this assumes the presence of a solder mask and the resultant force overcoming any friction present.

It is the same principle that generates the meniscus of a liquid in a container.

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  • \$\begingroup\$ I didn't know about the solder mask. It promotes adhesion of liquid solder to copper? \$\endgroup\$ – tjt263 Nov 28 '18 at 19:42
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    \$\begingroup\$ No, it avoids adhesion of the solder to the places you don't want it to go. \$\endgroup\$ – Edgar Brown Nov 28 '18 at 19:44
  • \$\begingroup\$ So it's a substance applied to the board then, not the copper contacts? \$\endgroup\$ – tjt263 Nov 28 '18 at 19:47
  • \$\begingroup\$ The contacts are not commonly copper, these are tin, or even gold. Copper develops a patina rather quickly which makes soldering to it more difficult. For normal commercial boards, solder mask is an isolator that covers everything, including copper traces, except where you want solder to go or to provide contact surfaces. \$\endgroup\$ – Edgar Brown Nov 28 '18 at 19:55
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I think you are talking of surface tension. Here is a video of a BGA that is not centred on the pads being reflowed. https://www.youtube.com/watch?v=rmb3uLqueNU

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