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I was curious as to why the bottom of these SMT pogo pins are slightly rounded. Wouldn't it cause them to fall over before they are reflowed ? It seems that a completly flat bottom would make sense like most SMD pads. enter image description here

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The curved bottom will allow for betting wetting when using solder paste, as it gives more surface area on the bottom where the solder paste actually hits.

This is less important when hand soldering as you generally use far more solder and it will wet up the sides of the pin, but in large scale manufacturing they use much less solder paste and you want to maximize the surface area exposed to the solder paste. If it was flat the bottom would lay flat against the pad when it is reflowed and give far less surface area.

In addition the solder paste is very viscous and and will hold the pin in place, so the pin doesn't actually need to balance on a flat bottom.

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  • \$\begingroup\$ Just as a follow up, if a slightly rounded surface gives better wetting, why don't they make all SMD pads slightly rounded at the bottom ? \$\endgroup\$
    – VanGo
    Commented Aug 30, 2020 at 5:51
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    \$\begingroup\$ @VanGo for the most part they are actually rounded, for example unique-leds.com/images/thumbnails/545/360/detailed/2/… \$\endgroup\$
    – Hatman
    Commented Aug 31, 2020 at 13:47
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Interesting that this one is the only one of the 8 similar items in Harwin's line-up that has that well-rounded bottom (the others have a slight chamfer but no radius). For example, this one, which is the same diameter.

I might speculate that this one is perhaps not intended to be soldered at all, but to be held in a molding between two PCB pads.

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I would guess that since this is a pogo and will be pushed against, the rounded surface will allow more solder on the sides of it thereby allowing better solderability.

T

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