Alt-BEAM Archive

Message #01217



To: beam@corp.sgi.com
From: Justin jaf60@student.canterbury.ac.nz
Date: Wed, 03 Mar 1999 19:47:38 +1300
Subject: [alt-beam] Re: More Questions (SMT hunters guide)


> 5) I know that this has already come up, but how can I identify SMT
> componants. I have a board with a whole bungh of SMT componants on it.
> Can I just heat the other side of the board to unsolder them?

Yeah, I brought it up. Haven't done too much since then, but I'll share
what (little) I've found so far :-)




Justin's Guide to SMT. (version 0.89 :-)
========================================

Gosh, that's an impressive and knowledgable-looking start. Now, let's
see...

Regarding removing them, I use a scalpel to cut the solder on things
like resistors and caps. I've also done this on an IC, but you generally
cut the leads instead, which shortens them by about half a mm. I don't
mind, but if you needed the extra length, I guess you try the
butane-torch-to-the-back-of-the-board approach :-)

Regarding identifying them:

Resistors are easy - they have a 3 digit number on them. The last digit
is the multiplier (eg 473 = 47K) Note - this number is on the component
itself (you may need a magnifying glass), it's _not_ the number on the
board beside the component.

Chips are easy - read the label. If the label is obscured by flux (or
something - sometimes seems to happen on one side of double-sided
boards), lightly sanding it off (with fine grain, say, 300-400?) seems
to work. Don't sand too much off though, else you might lose the
markings you're trying to read :-) You should, of course, narrow your
search considerably by only looking at those IC's with 14 or 20 (or
whatever you're after) pins.

Diodes are difficult - they use the same case as transistors, helpfully
having the distinctive 3 leads that have uniquely identified diodes as
diodes since time began...
I haven't messed with identifying unknown diodes since I can just buy
them. The code on the cases read: A6s30 (where 30 is written at 90
degrees to the rest) and looking down on them from above, with the two
leads at the bottom and the one lead at the top, the one at the top is
positive, the one at bottom left is negative, and the bottom right is
unknown and probably unneeded as far as we're concerned.
I think I've also found a diode in a larger case with a lead at each end
(a diode with only two leads? how curious!) and a band indicating
polarity. The label on it was "2". Useful... Who comes up with these?
[Note - as I haven't used the transistor-cased diodes yet, I haven't
ruled out the posibility that they may in fact be transistors - the guy
at the shop selected them because of the drawer they were in, and seemed
a little uncertain regarding the 3 leads... At first fiddle, they seem
to work as diodes though]

Transistors - haven't messed with these as I can just buy them. They use
the same cases as diodes. Known values:
1Fs64 (64 at 90 degrees) BC548 near-equivalent (might be 547)
6Dp73 (73 at 90 degrees) BC327 or near-equivalent.
Note, I have not yet used these, so their "equivalency" is not yet known
:)

Capacitors are Bastards.
On the board, these have no markings, and come in a variety of cases and
packages. Get a handle on which components are capacitors by looking at
the component number next to them (if it's something like C162, it's a
cap, R means resistor, Q means transistor (I think), IC means, well,
guess :-)
The larger caps will have their values and polarity written on them, but
if you're like me, what you're after are those very useful .22uF caps.
While I was stumped as to how to identify caps, Steven Bolt suggested

>Improvise an RC-generator, calibrate using a known .22uF, then try
>the SMT caps in the bitbox. Needs a scope or a frequency meter; you
>could improvise the latter, if your DMM doesn't oblige.using a normal cap

And I thought "Why didn't I think of that?!?" as you do when confronted
by astoundingly good ideas :-)
Unfortunately, I lacked most of the necessary equipment, so set up the
following;

Power supply - 2 AA batteries (3.V)
Two momentary switches. The first connects the power supply to a known
.22uF cap, charging it. The second connects the side of the cap just
charged positive to an 8M2 resistor, which in turn is connected to the
base of a BC548 transistor.
The positive from the batteries is connected to the collector, while the
emitter is connected to an LED, the negative lead of the LED is
connected to the negative of the batteries.
The multimeter, set to measure milliamps is connected to each lead of
the LED.

So I push the first button (charges the cap), release it, push the
second button (discharges the cap through resistor into transistor) and
watch the multimeter. With the .22uF cap, it takes about four seconds
for the value to fall to X (about 0.02mA with my setup). Then I start
timing SMT caps in place of the .22uF cap.
I quickly found that .22uf SMT caps are in a slightly larger case than
normal SMT caps, which narrows the search to almost exclusively .22uF
caps.
Of the caps I tested, they were either bang-on four seconds, or a long
way off - only two seconds, or fully six seconds. Presumably an
indication that 0.1uF and 0.33uF caps are the next step up or down.
(Can't remember if I checked against a known 0.1uF cap to make sure, I
think I did).

Now, I havn't actually got so far as to freeform a circuit with my new
caps (I'm still need one more, so need to find another circuit board to
plunder first), but I'm pretty certain they're the correct value.


I hope his helps.
If anyone has _anything_ to add to this, please post it.


Seeya
Justin

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