PCB Bring-Up & Debug — question 10 of 10
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You have current probes, an oscilloscope, a hot air rework station, and a couple of programmable electronic loads available on the bench.
Two independent daughter modules connect to a main board through a shared 5V rail and connector. Neither module alone causes any problem, running for hours without issue. But whenever both are attached together, the 5V rail intermittently sags within minutes and resets both modules. Swapping in different physical units of either module doesn't change the behavior, which suggests the fault isn't in either module individually but in how the main board's shared rail responds to both loads at once. Design a rework-based test to localize the actual cause without permanently damaging any of the three boards.