Best practices when soldering/placing bleeder resistors on a PCB

[pitch-pitch]

Hello, this is my first post here!
I'm using a X2 cap to filter 230VAC voltage input and got a few other capacitors at similar voltages on a PCB I'm designing. I'm a bit afraid of having the PCB - or even worse the mains prongs - energized if a bleeder resistor were to detach itself (although I realize this is quite unlikely).

I've read the best practice was to solder the bleeder resistor on the same pads as the ones from the capacitor or even directly on the cap's terminals. Is that true? I've never seen it on existing PCBs of appliances and other devices.

Have you ever seen a bleeder resistor detach itself which left the capacitor at dangerous voltage levels? How would you avoid that?

[Jon_S]

I have not seen that requirement in any European standards. I would love to be corrected if I have missed something.

Can't recall a problem in that area on returned product, but I did see one not populated once. Or rather felt it when I picked the damn thing up! We solved that with 100% AOI.

[paul@yahrprobert.com]

My toyota corolla's clock has a vacuum fluorescent display, and in the high voltage section there are some very long surface mount resistors.  These routinely fail by breaking the solder joints.  So much so that there's many videos on youtube telling you how to re-solder those joints.  Its obvious what's happening: the thermal expansion of the resistors repeatedly stresses those solder joints.  This will be worse for longer resistors obviously.  The way to avoid this would be to make the resistor out of many shorter resistors in series or use a through-hole resistor.  Another easy thing to do is design the part so that the temperature rise is small.  Estimate the thermal resistance to ambient surroundings and aim for a small delta-T.  Also, a good idea would be to use two resistors in parallel, to lower the odds of complete failure.

[DavidAlfa]

If you're really concerned about the resistors dettaching, use TH parts.

[TimFox]

But they are not fashionable in this millennium.

[DavidAlfa]

So will you use racing tyres in snow instead wearing chains because it's more fashionable?

TH still has a lot of uses, this is one of them.
They are safer, won't fall or break easily, they tolerate pcb flexing / expanding / contracting much better than SMD.

You see this everywhere if designed by a decent engineer thinking on the safety.
You could use a decent sized resistor like 1206 or 2512, they won't dettach easily, but they large size make them brittle, might break if subjected to vibrations or flexing.

[TimFox]

No:  I don't follow fashion slavishly.  I should follow fashion and see which of these emojis means "sarcasm".

[pitch-pitch]

Thanks for the very interesting replies! I was indeed thinking of using a good ol' axial TH 1/4W resistor (or even two in parallel to have a fail safe as mentionned by Paul), but was even then concerned about potential hazards. Maybe I'm thinking too much and a TH resistor soldered through two pads close to the capacitor's is safe enough. I guess TH components don't desolder themselves that much and it would be unlikely for the PCB to snap in half between the resistor and the cap.

[nctnico]

Another option is to use several smaller SMT resistors in series. Takes more parts but is more resilient against flexing of the board. Through hole seems overkill to me. Thruhole can fail just as well BTW. If reliability / safety is a concern, then put 2 strings on the board and a neon bulb that is lit while the capacitors are still charged to dangerous voltage levels.

Be sure to include a MOV or TVS diode in your circuit to protect against overvoltage surge events.

[TimFox]

IF you worry about board flex, a TH resistor with extra lead length can reduce stress on the resistor body.
1/4 Watt TH resistors typically are rated for 250 V (assuming resistance high enough to keep below rated power), and 1/2 Watt are rated for 350 V.