At 02:46 PM 2/10/2004 -0500, you wrote:
>Hi list,
>
>         does anyone know how to calculate how much of
>a current pulse resistors can tolerate? I'm looking
>at tech note TN-95 from KOA SPEER as a starting point.
>
>         The lengthy description is: I'm helping diagnose
>a problem on a product that we buy from another vendor.
>The product is sold in the US and Europe. It is powered
>by a 14.4V battery. It is a 3R9 0805 that is failing.
>Currently we have around 30 out of 1500 that failed.
>All failures occured in January, and all in the US. No
>failures of this nature in Europe. The vendor thinks it
>may be related to the inrush current from plugging in the
>battery but I have my doubts. It seems strange that they
>only failed in the US, and only in January. My first
>suspicion would be condensation or ESD.
>
>         circuit is a 14.4V battery powering an LM2594M
>through a FDN5618P p-channel MOSFET and a 3R9 0805
>resistor. The input to the regulator has a 33uF electrolytic
>in parallel with a 100nF ceramic. The battery remains
>disconnected from the circuit until the user presses a
>button. It is controlled by a 16F73 that keeps the circuit
>powered up after the button is pressed. Current draw from
>the circuit in normal operation is between 40mA and 90mA.
>
>         If I ignore the fet resistance and assume a perfect
>capacitor for worst-case approximation, then I basically
>have a 14.4V battery charging a 33uF cap through a 3R9
>resistor. Following the tech note shows an equivalent 50W
>pulse for 200usec. So where do I go from here? Is that
>correct, and is that too much for an 0805?
>
>         Thanks for any and all ideas!

ONLY go to the manufacturer's data and don't expect it
to be the same from one manufacturer to the next. That
IS a fairly large power pulse for a tiny bit of film.
How much power would you guess it would take to vaporize
that little bit of stuff off the top of the ceramic?

As far as the failure distribution goes, it might be a
temperature-related issue. Try testing at
<< 0=B0C, as if the tool was left in an unheated garage or
back of a truce. The northern US states (and Canada) are
generally colder than most of Europe in the winter.

Ok, pulling down my Philips resistor manual, I see that
their design is 10:1 higher than the maximum SINGLE-PULSE
rating for Philips' 0805 5% resistors. There's your problem.

OTOH, that's easily within the range of cheap pulse-rated
leaded parts, but the cost and size will be a bit more.
You can get special 1206 pulse rated parts that are just
okay for single 50W/200usec pulses, and maybe could be
fitted to the same board pattern as a stopgap.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the=
 reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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