On Mon, 28 Mar 2016 22:15:42 -0500, Jon Elson
wrote:
What does this suppose to mean, do, or prove?
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Jeff Liebermann wrote:
The original failure mode was shorted MLCC caps caused by either
thermal shock or board flex. The bad ones were easy to find with an
ESR meter. However, when I tried to install replacements (and
guessing the part value because Apple doesn't supply service
information to non-authorized repair shops), I managed to crack and
short several known good MLCC caps with a soldering iron. Having
learned the lesson, I used some solder paste and a hot air SMT reflow
gun to do the soldering. I also pre-heated the PCB and let the caps
cool down slowly. I don't know if that was necessary, but it worked
every time. I'm told that two solding irons used as a tweezer also
works, but I haven't tried that yet.
I've heard these stories a number of times. And, yes, maybe some people use
insanely hot irons or in some other way cause this problem. And, some
really large caps are prone to this damage.
Yep. As James Arthur mentioned, it's not a problem with the
physically small caps, which come to a uniform thermal equilibrium
rather rapidly. It's only the big caps, with lots of plates, thin low
voltage ceramic dielectric, and larger thermal mass, that were a
problem for me.
But, as I say, I have hand-
soldered over 25,000 0805 MLCC caps of modest value without seeing this
problem.
One exception will break any such rule. Dig out a larger and higher
capacitance MLCC cap in a 1210 or larger package. Try soldering it
with a single soldering iron. Unless you're very good, it won't take
much to trash the cap.
http://www.avx.com/products/ceramic-capacitors/high-voltage/high-voltage-mlc-chips/
"Chip sizes 1210 and larger should be reflow soldered only."
I use a very good Weller temperature-controlled iron, and run it
at a modest temperature.
I use several ancient Weller WTCP TC201 and TC202 (because they were
free) soldering stations. Most of my tips are 750F with a few at
850F.
Much better to use an iron with really good
thermal conductivity at a lower temperature than one with poor conductivity
at a very high temperature.
I don't understand. All the tips are made of plated iron. There's
little difference in thermal conductivity between tips, unless you
want to throw in copper tips heated with a gas burner. What part of
the soldering iron varies in thermal conductivity?
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558