Saving my Mac SE/30. My first foray into SMD work
tothwolf at concentric.net
Mon Apr 27 22:45:01 CDT 2015
On Mon, 27 Apr 2015, Ian Finder wrote:
> FWIW the NeXTcube has the same shoddy SMT can caps, but last time I gave
> it a peek to see if they were actually leaking, they were not- whereas
> all the cans in its contemporaries had spilled electrolyte.
> Perhaps a different MFG or SKU of cap?
> Pete, were yours actually leaking? Or just the same shoddy type?
Modern made top brand SMD aluminum electrolytics (such as Panasonic,
Nichicon, Rubycon) also don't tend to exhibit this failure mode. There
were two primary causes of the end-seals in the late 1980s to mid 1990s
era SMD aluminum electrolytics failing. One was that the "rubber" compound
used for the capacitors end-seal didn't hold up well to the higher
temperatures used in reflow soldering and would deform. Another was the
use of chlorinated solvents to clean boards after soldering.
Manufacturers solved the end-seal issue by switching to rubber compounds
which could better handle the higher temperatures encountered in reflow
soldering. This is why modern parts soldered at even higher temperatures
using lead-free solder aren't failing like these early SMD aluminum
electrolytics did. Chlorinated cleaning solvents OTOH, are still a problem
for aluminum electrolytic capacitors, even for through-hole parts, and
most capacitor manufacturers explicitly state in their datasheets not to
use them. Not only can chlorinated solvents damage the rubber compound
used for the capacitors end-seal, but they can wick into the capacitor and
cause the aluminum itself to corrode.
It wasn't that those early SMD aluminum electrolytics were shoddy or
defective, it was that the electronics industry didn't understand how to
handle them during assembly. This is similar to what happened in more
recent times when manufacturers first began switching from tin/lead solder
to lead-free solder. Instead of replacing process equipment which had been
designed for tin/lead process with equipment designed explicitly for
lead-free process, many board houses at first tried just running their
reflow ovens and wave soldering machines at higher temperatures. In
hindsight, the results of doing this sure look to have been predictable.
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