Preaching to the choir here for sure. I've had fewer bad experiences with
substitution of resistors than capacitors. I recently learned that;
higher voltage rating Tantalum caps, of the same series can have higher ESR.
So, if you are looking for a specific ESR from a specific capacitor series,
higher voltage rating might be bad.
In addition, apparently, X5R caps have generally lower ESR than X7R so
improved temperature coefficients might not give better function.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Friday, January 05, 2007 3:02 PM
To: [log in to unmask]
Subject: Re: [TN] Resistor Substitutions
You have to be careful about substitutions.
For both caps and resistors, a higher tolerance applies only to the
capacitance or resistance value range within a given temperature,
humidity, or operating voltage range. For example, you can get certain
capacitor and resistor values with a tighter tolerance between temps of
20C to 80C (1% versus 5%), but the 1% parts may be derated at a higher
percentage at hotter or colder temperatures or humidity levels or
voltages, etc. So if you substitute a resistor with a tighter tolerance,
be sure it has the same temperature, humidity, and operating voltage
derating or better. You have to pay attention to the entire component
specification and understand the application of the device it is going
into. I have seen substitutions made for a capacitor with a tighter
tolerance at room temp, but its actual application was in a device that
typically operated at 150C. The substituted capacitors failed because of
their lesser ability to hold their capacitance value at those
temperatures (50% derating above 80C for the 1% parts as opposed to 10%
derating of the 5% parts).
If it is an unauthorized substitution, guess who pays for the rework?
Another example, a design engineer (working with input from a
reliability engineer) specifies a certain part because they know it will
remain within a 5% tolerance slightly above its rated 25V power level,
and he uses it in a 30V circuit. He knows that another part with better
(1%) tolerance will fail at the 30 volt level due to a greater voltage
derating factor. The buyer substitutes the part with a 1% tolerance
assuming that 1% is greater than 5%, and does not document this change
in the form of a deviation, waiver, or ECN request.
How is the design engineer going to know the substitution has been made?
Who's gonna get fired when the devices begin failing in the field? How
much longer will it take them to discover the root cause of the problem
without the deviation, waiver, or ECN documenting the change?
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Friday, January 05, 2007 12:53 PM
To: [log in to unmask]
Subject: Re: [TN] Resistor Substitutions
Hi Eric!
In the past I've had it both ways, one large military customer we had
says that a "better than" part was okay to use, and gave us a blanket
okay for that. But then another of our military customers wanted to know
of ANY suggested substitutions to the BOM and then give the
authorization whether or not to use, which is fine.
The important thing is make sure that your buyers know that. A lot of
them are used to buying "better than" parts routinely. At a past company
that I worked at we got spanked pretty badly by one of our customers for
buying better parts without asking first...
-Steve Gregory-
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of ekalgren
Sent: Friday, January 05, 2007 11:41 AM
To: [log in to unmask]
Subject: [TN] Resistor Substitutions
Good afternoon everyone. I hope your new year is going well so far. I
just got off the phone with our folks in purchasing who were telling me
that there was a 13 week lead time for a resistor. I checked a few
places on-line and was able to find a better (in terms of failure rate)
resistor in stock.
My question is does anybody working with mil spec components have a
blanket substitution policy along the lines of "S or T failure rates can
be used in place of R failure rates without engineering approval" or do
you require approval for every sub?
Eric Kalgren
Electrical Engineer
Cobham Defense Electronic Systems
Continental Microwave Division
11 Continental Drive
Exeter, NH 03833 USA
Tel: 603-775-5200
Fax: 603-775-5201
E-mail:[log in to unmask]
Website: <http://www.cobhamdes.com/> http://www.cobhamdes.com
<mailto:[log in to unmask]>
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