This is from a power supply for an old PC monitor. I opened it up to see if I could fix it (likely bad caps) and saw this rectangle matrix. What's the purpose?
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Probably an attempt to reduce the resistance of the trace (and better dissipate heat) by putting solder in parallel.
The pattern in the solder mask is likely so that a solid section of solder does not warp the board excessively as it cools (the CTE of solder is much higher than that of FR4).
You can see shorter traces to the right that are covered almost entirely by solder.
answered Mar 25, 2017 at 22:40
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\$\begingroup\$ More likely heat exchange than electrical resistance, though why they would do that under a cap I have no idea. \$\endgroup\$– Trevor_GCommented Mar 26, 2017 at 12:55
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4\$\begingroup\$ Dave Jones of EEVBlog tested this and found that the solder reduces trace resistance by about 50%. youtube.com/watch?v=L9q5vwCESEQ \$\endgroup\$ Commented Apr 13, 2017 at 8:13