Another *very* important point to remember concerning chip
resistors is power handling ability. I do a lot of industrial
designs where the resistor carries some current and dissipates
some power, often continuously under overload or error
conditions. They are also mostly high-reliability designs. Factor=

in elevated ambient temperatures and things get a bit more
complicated as the power dissipation of the chip is reduced.

Remember that a 1206 chip is basically a 1/8W->1/10W resistor. I=

don't like to run more than about 60% maximum power on any
resistor to reduce the maximum chip temperature and maximize long=

term reliability.

Once you get to 0805, 0603 and 0402 chips, you begin to have to
worry about even LOGIC signals possibly damaging a resistor in an=

overload state -- for instance a 220 Ohm, 0402 in series with a
PIC output to say a Mosfet gate. If the gate gets shorted to
ground and the PIC output is high (near 5V), the power in the
resistor becomes ~110mW and that will cook an 0402 resistor (or
even an 0603 and will be borderline on an 0805, depending on
ambient temperature). This is also something to remember when
sizing resistors for input overload clamping circuits. It's easy=

to forget when designing with SMD chip resistors [not that I've
ever made that mistake ;-)]

I can see where the smaller parts will continue to be cheaper and=

preferred over time, but I also see a need for the larger chips
for a long time also. I don't think they'll go away any time
soon, but I have started using 0805's wherever possible.

Matt Pobursky
Maximum Performance Systems

On Fri, 15 Feb 2002 19:21:14 -0500, Spehro Pefhany wrote:
>At 09:31 AM 2/16/02 +1000, you wrote:
>>Hi Everyone,
>>
>>Just to add my $0.02.. I recently went to talk with an=
 assembly
>>contractor and they "suggested" that I stick with 0805s or
>>0603s as the 1206s where being phased out by the manufactures
>>as the 0805 is much more popular.
>
>The prices on 0603 resistors are less than 0805s- in quantity
>(100K+)
>
>0402 here we come... and those pesky 0201s are probably not far
>behind, at least for resistors. Capacitors won't shrink as=
 fast,
>for obvious reasons.
>

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