On Thu, 31 Mar 2016 04:20:59 -0400, "Michael A. Terrell"
wrote:


Jeff Liebermann wrote:

On Sat, 26 Mar 2016 19:56:12 +0000 (UTC), Andre Majorel
wrote:

Sorry for the delay but I missed the followups to my comment.

On 2016-03-22, Jeff Liebermann wrote:

If it uses an MLCC capacitor, use a hot air gun to reflow, not
a soldering iron tip.

News to me. In what way are multi-layer ceramic caps and
soldering irons incompatible ?

Thermal shock easily cracks MLCC caps. I learned that the hard way
while fixing several Apple Mac Mini computahs, which feature a
collection of MLCC on the bottom of the main board.
https://web.archive.org/web/20130606062903/http://blog.helpmymac.ru/?p=3585
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.

How were the new caps stored? If it is where they can adsorb
moisture, you can damage them with an iron that is too hot.

Well, I must admit that I didn't take any special precautions.
Methinks that the relative humidity in my office runs between 40% and
60% but is not monitored or recorded. Occasionally, it gets low
enough to where static electricity becomes a problem, or high enough
to where I'm rather uncomfortable, but those are rare. The caps a mix
of cut tape and loose bags stored in Ziploc bags (mostly pink
anti-static) and in paper coin envelopes. Nothing in hard plastic or
metal drawers that might chip or crack them. Although I know that
these caps make tolerable hydrometers (and microphones), I don't think
they can absorb enough moisture from the air to where a steam
explosion would be a problem.

I did some digging to see if humidity might be a problem in storage
conditions. There were plenty of notes on how a cracked capacitor
might allow water to enter the dielectric. Soft (solder) termination
is the recommended fix. Some suggests pre-heating the capacitors
before soldering to drive off any moisture. One demands that the caps
be used within 12 months. I didn't see humidity as being a problem
until AFTER the capacitors had cracked. It would take some time for
the moisture to alter the capacitor characteristics. With my hand
soldering technique, I managed to instantly produce shorted
capacitors, which methinks was more likely due to uneven thermal
expansion, than to moisture incursion.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558