robm said:
i have a dead mobo that has domed caps, not worth sending off but worth an
attempt at repair
i am amateur solderer i have couple different weller irons 15w 25w 60w
I read somewhere about cutting the caps in half and exposing the two cap
posts then soldering new caps to the posts instead of trying to completely
remove and replace the caps
Is this a bad idea ?
is there a good reason not to repair this way ?
I haven't done it as you describe, but I can understand why someone
may have come up with that method after destroying an otherwise good
board trying to get the wires out with just an iron. The problem is
that these boards are at least 4 layers, with the caps soldered to
vias that connect to big pours of copper in the inner layers. By the
time you get enough heat into one end of the lead to melt solder all
the way through the via, you may well remove the outer pads.
I recently replaced all the large electrolytics on a mother board just
as you describe. I ended up preheating the board with a hot air gun
till it was at about the boiling point of water (put a drop of water
on the board near the joint being worked on and wait for it to start
to bubble), before applying heat to both leads with an iron heating a
big blob of solder. Harder than removing the caps was the process of
cleaning the holes well enough to insert the new caps. This also
needed the preheat and a length of stranded wire I used as a solder
wick. Soldering the new caps in required preheat, also. Using low
temperature solder (63%tin, 37 % lead or 62% tin, 36% lead, 2% silver)
is very helpful.
By the way, to make this process worth while, be sure you have a good
grade of replacement caps on hand, or the new ones may not last long.
I used the 105 degree rated, high ripple current, low ESR FM series
by Panasonic, from Digikey. I was also able to put slightly larger
value caps into the same space.
The ripple voltage on the board was way down after the replacements
were installed. The board was very unstable before the replacements,
but has been running perfectly since.