Metal migration in 1206 resistors in Flyback SMPS?

[cupoftea]

Hi,
Will R20 and R21, (RCD and diode snubber) over the years of constant use, suffer metal migration due to the pulse currents they take?
(LTspice and jpeg attached)
They only have dissipation of <50mW....but their power is pulsed , in 140ns pulses. They are general purpose 1206 resistors.
Maybe over 5-6 years of 12hr/day use, they will start to fail due to metal migration, and thus MMA0204 style resistors must be used instead?
Its a Flyback of 24v, from 90-265, and 20W. 65kHz.

 

[dick_freebird]

If the metal film is a refractory metal or compound then time averaged
current limit probably covers everything. Only soft metals like aluminum,
gold and to lesser extent copper move much with current, and those are
not generally used as the resistor body / film material. NiCr is much more
electromigration-resistant (but thin film has a power-density-to-blow
which again has a relation to pulse width, and is another corner of the box).

If you were really concerned with such details you would talk to the
specific manufacturer about the specific family, and ask about DC and
pulsed current ratings and deratings. Because specific construction
details matter.

 

[Easy peasy]

1206 thick film / metal film is a safe choice considering the pulse power - I have never seen metal migration in or around a 1206

--- Updated Aug 5, 2023 ---

obviously melf's are even better - from a voltage and pulse handling point of view

 

[KlausST]

and there are dedicated "pulse prove" resistors.

Klaus