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I am using this Power Metal Film Resistor - which as per the datasheet, is coated with a "red, non-flammable lacquer which provides electrical, mechanical and climatic protection". Now, on my PCB, I have two vias to GND placed underneath the power resistor placed for thermal relief in case the resistor gets heated up. Maybe that was my mistake, but the interesting thing is, part goes in to the field and is returned because smoke starts emitting from the resistor. On debug, I realize that the lacquer coating of the resistor had been chipped a little exposing the metal body of the resistor. There was very little clearance b/w the resistor and the PCB and in this particular case, the resistor happened to touch the via - boom! short to GND and resistor began heating up.

Questions:

  1. In this failing part, there might have been friction with another object or some mechanical stress that caused the coating to chip off, thereby exposing the potential problem (promixity to GND via). However, in other parts, assuming there is no friction - should I still expect the coating to come off, either due to normal ageing or due to moisture or thermal stress, etc?

  2. It seems dangerous in general, if the coating of metal film resistors can come off due to mechanical stress - thereby exposing the metal body of the resistor to contact with say, the enclosure for example. Is this a known problem - how is this dealt with or prepared for?

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  • \$\begingroup\$ These are both questions you should be bringing up with Vishay engineering support. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 11 '15 at 19:16
  • \$\begingroup\$ Vishay is incidental - the question is general. \$\endgroup\$ – NK2020 Jun 11 '15 at 19:18
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  1. You should not depend on the lacquer for insulation. It could become damaged.

  2. Allowing a lacquered resistor to be in contact with (or even close to) a metal object is not good design. Any potential danger is avoided by preventing contact entirely.

Occasionally you would see toy-type or other low-end consumer products that had haphazard assembly of lacquered through-hole resistors such that they could rub together and short. Not so much a safety issue (plastic enclosure proper clearances and creepages for safety) or it wouldn't pass safety agency standards, but the safety guys don't care if the thing stops working.

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    \$\begingroup\$ @SpheroPefhany +1. PR0x resistors aren't UL-recognized and as such the lacquer isn't considered insulation at all (from a safety standpoint). \$\endgroup\$ – Adam Lawrence Jun 11 '15 at 19:34
  • \$\begingroup\$ If the fab house allows it, you could try "tenting" the vias (covering them with solder mask.) But it would be better to move them. \$\endgroup\$ – rdtsc Jul 28 '17 at 2:11
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  1. leaded resistors will be derating on 70C, if temperature continue increasing, the resistors power will be not as your orginal design. if the circuit is parallel a capacitor, your have to consider capacitance and resistor power. If the power metal film resistors could be continue under 70C, the resistors will be not failed.
  2. the min insulation voltage of metal film resistors is 300 voltage. Please check resistor datasheet.
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