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See the annotated image below.

The board takes AC power in, and feeds it to two relays (plus an empty spot where a 3rd relay could go). On the other side, it has two connections labeled AC1 and AC2. AC1 feeds to the relays, and AC2 is connected directly to several other headers all called COM.

I am curious about the unusual solder patterns. AC1 starts with heavy solder and gets progressively thinner with each relay. As you can see, the connection to Relay A is stripy. What does the stripy pattern do? Does it have a name?

The AC2 <=> COM connections have their own stripy pattern. It's a little harder to see in the picture, but it has a bunch of long, 45º stripes running between all the headers. They're more continuous than the ones to Relay A, looking more like a "grill".

What is up with these traces?

A bit more detail: This board is one of 3 found in a 240V space heater, so it's 240V between AC1 and AC2. Most of this board is simply converting down to 5V for a microcontroller (found on another PCB). The relays actually control other relays found on the 3rd PCB, where the heating coils are connected.

Annotated image of PCB with funky traces

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Some engineers like to add extra solder to high-current traces, to increase the trace's cross-sectional area; people argue about whether this has any significant effect, given solder's substantially worse conductivity than copper. (I'm on the side of "just go for 2-ounce copper instead", personally.)

That's what's been done here. By leaving areas of soldermask open, large amounts of solder from the HASL finish and/or wave-soldering processes are left on these high-current traces. The reason for the stepped design going to the relays is presumably because some of the current will divert through each relay, meaning the traces after that point will never need to carry as much current as those before it.

It appears that additional solder was placed on the right side of the relay trace, beyond what HASL or wave soldering would leave. Presumably, this was done by printing extra solder paste on that part of the board when the surface-mount components were reflowed.

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    \$\begingroup\$ Glue is visible on some components; this was certainly a single-pass wave soldered assembly. No amount of paste would melt that tall, or stay in place; that blob must've been added by hand. \$\endgroup\$ Dec 19, 2023 at 5:19

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