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'[EE] Ionic contamination on PCBs'
2005\08\16@154436 by James Newton, Host

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---
James.



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> -

2005\08\16@161533 by Bob Axtell

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I've done several references using SMT precision resistors. What I
learned- the
hard way- is that I got into problems whenever I undersized the
precision resistors.
If calculations indicate that a 1206 will be fine, use a 2512 instead.
Ensure that
the full contact surface is actually soldered down, so that self-heating
is pulled
away from the precision resistor. Keep the precision resistor away from
other
heat sources, too.

While contamination is possible, I am inclined to think overheating is
the problem.

--Bob



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2005\08\16@182056 by David Van Horn

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I've done several references using SMT precision resistors. What I
learned- the hard way- is that I got into problems whenever I undersized
the
precision resistors.
If calculations indicate that a 1206 will be fine, use a 2512 instead.
Ensure that the full contact surface is actually soldered down, so that
self-heating is pulled away from the precision resistor. Keep the
precision resistor away from other heat sources, too.



It's as far away as practical, with a ground plane layer below to spread
the heat.


While contamination is possible, I am inclined to think overheating is
the problem.

Worst case dissipation would be 9.5 V into the series combination of
332k and 100k, so the 332k gets a grand total of 160uW.  Seems within
tolerance for an 0603?




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