replace caps on motherboard issue

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 ?

TIA
robm

 

With a solder sucker, you need not do that. The spring
loaded solder sucker will pull solder out of the hole so that
a new cap can be properly mounted. It is far better - more
reliable - to install new caps properly.

 

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.

 

I've never had any problems just heating one lead at a time while applying
force on the cap on the other side of the board. When the solder gets loose,
the other pin bends, and most of the heated pin comes out of the hole.
Reverse the process on the other pin, bending the other direction. Usually
a couple of iterations gets the cap off (probably not a recommended
technique for a board you are worried about destroying, but it worked
for me on a couple of old boards).

Once the cap is out, you can use a solder sucker fairly easily on the
holes one at a time to get enough room to put in the new cap.
--email: icbm: Delray Beach, FL |
<URL:http://home.att.net/~Tom.Horsley> Free Software and Politics <<==+

 

John has demonstrated another technique that professional
use - and that most anyone can do. Another factor in removing
those leads is the quality of the soldering iron. Best irons
will increase the heat as necessary when the load - the size
of the solder joint - becomes larger. Just another trick that
makes removing solder in a hole easier with less damage to the
board.

Also pay attention to the numbers and specs that John has
provided for capacitors.

 

Least of which is the diameter of the replacement cap! Some boards
do not have clearance.

 

It's quite a good idea.

If the component is known to be defective then it matters most that the pcb
isn't damaged.

I often chop components in half so as to make removal easier and hence avoid
collatteral pcb damage. I haven't done it with electrolytics though. Be careful
about the electrolyte they contain. It may be slighly toxic.

Was it an Abit mobo btw ? I need to recap my own Abit KT7-RAID soon.

See this link http://badcaps.com/

Graham

 

I wish it was Abit kt7

it is a ECS k7s5a, i have a couple and they run fine for what they are doing
i tried recaping one once by removing the CAP but i think i damaged the
trace trying to get solder out of hole so now a second board has problem and
i'd like to not damage the board

it is using the famed luxon bad caps

i have nichicons from mouser to replace