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From: "Ken Finney"
Subject: Re: Drift/Worst Case calculations of SMD Chip Resistors
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Date: Wed, 2 Oct 2002 15:43:07 GMT
"Klaus Vestergaard Kragelund" wrote in message
> When browsing datasheets for resistors I'm allways a litte confused as to
> the validity of the specification stated. I wonder if you could help me
> with these simple and somewhat embarrasing questions:
> A standard 0805 SMD 1% resistor is chosen (datasheet:
> OK - so the maximum deviation at the production line is 1%, thats fine -
> what I'm curious off is the spread of production of the PCB and drift
> I would normally just use the 1% spread and calculate the drift coming
> the temperature coefficient. But is this enough - several other factors
> described in the datasheet:
> Shelf life, short term overload, climatic, rapid temperature change,
> soldering and vibration.
> My question is which of these factors will make permanent changes to the
> resistor value?
> Also the "Temperature rapid change" isn't that a bit dubious - rapid is
> pretty relative? (from person to person)
> And finally what would you guys from experience use for your worst
> case/drift calculations? - load, soldering and vibration - or all of them?
The military spec for normal chip resistors (MIL-PRF-55342) has allowable
drifts for each of these items (more or less). Some commercial chip
have characteristics that are much better than the military spec, some are
much worse. The "standard' way in high reliability design is to "root sum
square" all the factors (i.e. square all the factors, add them together, and
take the square root of the result). Doing this for 100 ppm drift parts
yields an allowable drift (in addition to the original tolerance) of 1.18%
after 2000 hours at 70 C and 1.7% after 10,000 hours at 70 C. So, a
0.1% resistor can become a 1.8% resistor at the end of its life. Not
bad, considering a carbon comp resistor can drift 42% over its life.
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