[time-nuts] I love the smell of tantalum in the morning

[Magnus Danielson]

Dear Tom,

On 11/05/2016 08:12 PM, Tom Van Baak wrote:
> See C13 in the attached photo. I need to replace some blown caps on a few boards [1]. In one instance the cap got so hot it melted itself off the board. Quiet convenient, actually -- it acts like its own fuse -- but I don't think the 5071 designers had that clever feature in mind.

You're clearly lucky, one failure mode is short-circuit, and they can 
bring other stuff with them as they fail, and hence hated by some.

> Having not done SMT before, how should I do it with minimal risk to the very precious PCB. Or, what equipment should I use this as a good excuse to buy?

Now, for these caps, you can use a normal soldering-iron without too 
much trouble, but I strongly recommend pre-heating the board with a 
hot-air gun.

The trick is to pre-heat the board widely so it becomes hot, but not 
enough to melt any solder. As you now apply your soldering iron, the 
heat-transfer won't be as large as if you had a room-temperature board, 
simply because the lower temperature gradient.
The effect is that your heating up goes quicker and that part of the 
board won't experience excess heat for too long.

Another trick I use is to solder new solder onto the joints. This breaks 
through the oxide layer, which is a poor heat conductor. I solder onto 
the joints and let them cool. Then I come back again and now the 
soldering iron melts it all up nicely.

I still do SMD with my Weller WECP-20, but it's not optimal.
At work we moved the otherwise so high valued (over-valued) Metcal to 
the side as the new JBC stations is much better. Metcal's doesn't keep 
the temperature good enough and the new JBCs is beeter. Metcal's also 
have a failure-mode in their tips which makes them break way to early. 
There exist replacement tips which is in fact better than the original.

So, if you need an excuse to buy a new toy, look at the JBC-stuff:
http://www.jbctools.com/

However, I would probably be able to replace that cap before your get 
your new and shiny toy on the table.

Cheers,
Magnus