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I found these triangular solder joints in a 24v power supply. Why are they made as triangles? This is the first time I have seen it and I was unable to google anything about it.

enter image description here

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  • \$\begingroup\$ looks to me like it allows for increase in error with the ammount of solder used \$\endgroup\$
    – tuskiomi
    Nov 17 '16 at 19:40
  • \$\begingroup\$ May have something to do with wave soldering. (As larger pads on IC edges to gather excess solder). \$\endgroup\$
    – Wesley Lee
    Nov 17 '16 at 19:54
  • \$\begingroup\$ The triangle portion is in the direction of the trace so it could have to do with reliability of the signals. There is also an awful lot of solder on this board which could be why you have bigger bonding pads for solder. \$\endgroup\$
    – 12Lappie
    Nov 17 '16 at 19:55
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    \$\begingroup\$ I don't know the reason it is designed like that, but it's super funky. I don't know why, but when I look at that, I see John Travolta dancing "Stayin' alive". From now on, I shall design all my PCBs in that fashion... \$\endgroup\$
    – dim
    Nov 17 '16 at 19:56
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    \$\begingroup\$ Fascinating! Could be for thermal dissipation and / or current capacity. Those long pointy solder shapes are only on copper pours or heavy traces. \$\endgroup\$
    – user98663
    Nov 17 '16 at 20:23
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Please note a little notch (adjacent to "R7"), a narrow slot in the thick trace. These special-shape traces are used to minimize (separate) effects of one high-current path from another high-current path, or maybe from a sensitive signal.

The triangles are deliberate solder mask openings to allow a solder build-up, to beef-up the trace near the joint, to compensate for reduced current carrier capability of the PCB traces at the junction point.

Since many of the through-hole pins are of a large diameter, there is often little annular copper remaining to form a current path on it's own, so the solder itself is used to provide increased current capacity at those locations.

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  • \$\begingroup\$ The resistance of solder is a lot greater than that of pure copper. The result of applying a lot of solder is only a small increase of current capacity. \$\endgroup\$
    – Uwe
    Nov 18 '16 at 9:57
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    \$\begingroup\$ @Uwe, yes, solder has smaller conductivity. But the build-up is much thicker than the skinny copper-clad trace. Here are some considerations presented, although too optimistic for this particular triangle case: electronics.stackexchange.com/questions/265185/… \$\endgroup\$ Nov 18 '16 at 19:23
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    \$\begingroup\$ I concur with Ali's point on beefed up traces. This occurs often for heavy thruhole magnetic parts on topside. I can recount a dozen experiences where big name companies (like Apple's $4k CRT's, LG stove relays etc, )had defective designs due to vibration induced invisible micro-fractures on solder joints with insulating annular rings for magnetic , relays and it helps to put rigid polyurethane adhesive on all large THT passive parts to board. \$\endgroup\$ Nov 18 '16 at 20:07
  • \$\begingroup\$ Do you know this as a fact, or are you guessing? \$\endgroup\$ Nov 18 '16 at 20:10
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    \$\begingroup\$ I know this as a standard design practice having been Eng mgr in a large Contract Mfg company and from Failure Analysis on those which did not do this, although I have not seen the J.T. patterns before,, the triangular patterns offer greater strength withstanding to vibration in HALT/HASS screening, \$\endgroup\$ Nov 18 '16 at 20:13
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Note the glue holding the components to the board, which is most obvious underneath U3. Considering this along with the prevalence of through-hole components, I believe the triangular openings in the solder mask are to assist in heat transfer during wave soldering.

With wave soldering, you run one side of the pcb over a wave of molten solder, which is attracted to the exposed metal areas of the board (and its components). The glue keeps the components from washing off into the solder.

Wave soldering is used when there are through-hole components on the top side of the board, because the solder will wick up around the leads and fill the through-holes (if they are plated). It will also, incidentally, make nice solder joints on surface-mount components on the bottom side :)

One problem with wave soldering is that large copper areas covered with solder mask are slow to heat up. Similarly, it can take a lot of time to heat up through-hole component leads enough for the solder to flow. This is especially true in a power supply, where the components might be large inductors or connectors. Having extra exposed metal (without solder mask) helps quite a bit.


Another reason (perhaps the primary reason?) is explained in Ali Chen's answer.

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These are drainage shapes or solder thieves.

This board is wave soldered, and wave is a messy process. Just when the solder cools off and solidifies, it can form ugly shapes. Flux will often prevent this from happening, but sometimes, special keepout shapes in the soldermask or even extra copper shapes are used to 'convince' the extra, unwanted solder to go where it won't do any damage. This is what you can see here; note how the edges of the shapes point towards inward an to traces they are connected to anyway.

These shapes are created in an effort to avoid solder bridges where you don't want to have them - across isolation spacings or towards comparatively small SMD components right next to the large through-hole pins where a lot of solder might accumulate.

Here is another example of seemingly strange patterns in the soldermask's keepout.

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