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I am looking at a schematic for an RF sensing switch, and I notice that there are two resistors that are marked in the schematic as 18 Ω resistors. They are 2512 SMT types and are installed in parallel to each other in the circuit as R11 and R12.

(see schematic

In troubleshooting this device, I found that the developed resistance of the parallel pair, rather than being about 9 Ω, was a whopping 1.07 MΩ! Closer inspection showed that both of these resistors had cracked laterally, right through the "8" in the resistor marking. There is absolutely no indication of heat or other physical damage to the board or its components. Any ideas what might have caused these two resistors to have broken as they did?

As requested, here is a photo of the board...

MFJ-1708B PCB

The reason that R11 looks disturbed is because I heated the one side of it to verify the crack in the resistor. Two relays, four connectors, and a binocular transformer have been removed. Connectors were removed to get the board out of its enclosure, and the transformer was removed for modification.

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  • \$\begingroup\$ @Andrew -- BTW - the resistors are above and between the two relays, marked R11 and R12. R11 is the one that I heated the left side on to verify the break in the resistor body. The board appears to have been machine-soldered - oven, etc, as opposed to hand working it. I am halfway thinking that the heat of an oven caused the two resistors to crack, but that should not happen... \$\endgroup\$
    – ChrisP
    Commented May 23 at 11:22
  • \$\begingroup\$ Was it faulty from the factory, or went wrong in service? \$\endgroup\$
    – Andrew Morton
    Commented May 23 at 11:38
  • \$\begingroup\$ @Andrew - My belief is that it failed in service (which does not support the oven idea), though I get different reports from the two users who had the device connected to transceivers at the radio club that owns it. It is something that only gets used once a year -- at Field Day -- and when they were prepping materials for this year's event, they found it non-operational. I cannot say for sure when or if it ever worked correctly, as it was before my time there. \$\endgroup\$
    – ChrisP
    Commented May 23 at 11:53
  • \$\begingroup\$ If they are both cracked the same way, perhaps the equipment underwent a rapid deceleration event and the board flexed most just where they are. If it's only used once a year, that's plenty of time for it to get moved around in a cupboard without anyone remembering if anything untoward happened to it. \$\endgroup\$
    – Andrew Morton
    Commented May 23 at 12:23
  • \$\begingroup\$ What's the voltage applied to those resistors? If 24V or higher then that would be the reason. \$\endgroup\$
    – Lundin
    Commented May 23 at 12:39

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Assuming the resistors were broken cleanly and didn't look burned before you worked on the board, my first guess would be that they cracked due to board flex.

A very common cause of excessive flex is screwing the board down onto imperfect mounting bosses (non planar, wrong separation, etc).

Large SMT components are quite vulnerable to this. I've seen it several times with large caps. I haven't seen it before with resistors, but I don't see why they wouldn't be vulnerable if the substrate is ceramic or similar.

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