[time-nuts] I love the smell of tantalum in the morning
erm1eaae7 at ermione.com
Sat Nov 5 16:49:03 EDT 2016
On Sat, Nov 05, 2016 at 12:12:18PM -0700, Tom Van Baak wrote:
> 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?
I do this kind of repairs quite often and as you have already read from others,
almost every method like hot air, hot tweezers and two irons is suitable.
However, if you have not previous experience of smd reworking and, being
lucky that the component has only two leads, if you can have someone who
will help you and there is enough clearance around the components, I suggest
you use two irons of normal power (> 50W) with flat tip.
Hot air is a much better way to do this work, but it's also more risky:
the temperature and air flux should be right (too low and you will heat too
much everything before desoldering the component, too high and you do the same).
You can end up with melted plastic parts in other components around the area.
In some cases when sensitive components are very near, I build little fences
out of sheet metal to lay on board around the component, letting just the space
to pull up it with tweezers.
To solder back, clean the pads with solder wick, then with some form of
liquid cleaner to remove traces of rosin (I use alcohol, or thinner in some
stubborn cases). Hold the component in position pushing down to the board and
solder it manually at the sides. Forget to use SMD paste and hot air for
soldering. This has not any sense with parts so big and with only two pins
and it's trickier than desoldering about temperature and airflow...
There is another thing to look at. In some process, mainly time ago, the SMD
components were glued before soldering, so the component, even if solder is
already melt, need some force to be pulled up.
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