A couple of years ago, I spent time making a 'resistor wrecker" system
to answer a question similar to this. I had noticed that if my scope
probe shorted out a capacitor that was charged to 3.3V, the 1 Ohm series
resistor would often fail (open circuit). I figured that the resistors
had a threshold similar to the I^2t (current squared times time) curves
for fuses. My system connected my DUT (resistor) to a capacitor (with a
programmable voltage charge) using a FET switch. The system ramped the
voltage in small steps, discharged the cap into the resistor, then
recorded the resistance of the DUT. The test ended when the resistor
had changed value by 50%, but usually the resistor blew (open circuit)
the next step after a 5% to 10% change in value. After recording the
average, max, min, and std-dev of my resistors (ranging from 1 Ohm to 20
Ohm), I plotted the results. The data clusters were "all over the map",
giving one bit of useful information to the question of "how much
current?".
Answer: "It depends."
And it's worse than that. Chips from different vendors (and probably
from different manufacturing lots from the same vendor) are different.
I think it is because the parts are made from a film (thin or thick)
that is "some thickness" and is LASER trimmed to value, typically with
an "L cut" or "S cut" . The amount of trimming needed depends on the
thickness of the film and the desired resistance. A thicker film
requires deeper notch in the conductor to make the correct resistance
value. A range of values can be made from the same thickness of film,
limited by how deep they can trim the notch for the highest value in
that range. The conductor cross sectional area at the notch is where
the part fails when the pulse current density exceeds the short term
thermal capabilities of the notch.
Conclusions: Short duration high current surges are limited by the
narrowest point in the resistor, with the part failing by vaporizing the
conductive film from the substrate, creating an open circuit. Lower
levels of over-power faults cause the whole part to heat up, with the
failure caused by over-temperature. The heat causes a temperature rise
which causes a permanent shift in the resistor value. The power level
required for this effect depends on the ability of the part to conduct
heat to the environment.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair Hogg
Sent: Thursday, December 02, 2010 12:39 PM
To: [log in to unmask]
Subject: [TN] [Somewhat NTC] When does a resistor become a fuse?
This is a little off the normal Technet topic genre, but I was wondering
if anyone out there may have some data on how long a resistor can
withstand overcurrent before it changes resistance / fuses open? I'm
taling about a standard 0805 carbon film resistor, for example 100 ohms:
at 1 mA it drops 100 mV and dissipates 100 uW, no big deal for a 125 mW
device
at 10 mA it drops 1 V and dissipates 10 mW, still no big deal
at 100 mA it drops 10 V and dissipates 1 W, until the resistance
increases dramatically
How much of a time duration can the device withstand at 100 mA? Is there
a correlation between overload and time?
The graph in
http://www.koaspeer.com/pdfs/Thick_Film_Pulse_Limitation.pdf
seems to indicate that an 0805 (2A on the graph) can dissipate 1.5 W for
1 second. Does this make sense?
Thanks,
Blair
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