[Wolves] [OT] SMT soldering

Chris Ellis chris at intrbiz.com
Sat Mar 19 19:33:28 UTC 2022

Hi James

On Sat, Mar 19, 2022 at 2:52 PM James Dutton via Wolves
<wolves at mailman.lug.org.uk> wrote:
> Hi,
> I know this is not Linux, but I know some people on this list play with electronics.
> I have a component I wish to replace on a PCB.
> NCP1250
> It is a TSOP−6 package about 1.5 x 3 mm. It is a chip with 6 pins.
> I have not done surface mount stuff before.
> As it is so small, what do people suggest as options for going about doing it?

That should be pretty manageable with just a standard soldering iron.

The general trick with SMT hand iron work is rather counter-intuitive,
basically be messy and then clean up.  Desoldering braid and some flux
paste is very essential.

Essentially, tack one lead into place to align it.  Then drag a solder
ball over the pins, don't worry about bridges.  Then clean up with the
desoldering braid.  Volia a clean looking SMT joint.

Removing the part with just an iron might be more difficult and
soldering a new one.  I'd probably carefully cut the leads of the
broken chip and then remove each lead.

The other option is to buy a hot air SMD rework station, you should be
able to get one on Ebay/Amazon for ~£30ish (tend to go by the name
858D).  You can use this to reflow just the specific parts.  Easy to
remove the part.
Then add solder to each pad with an iron, cover in flux and use the
hot air to reflow.

> I am trying to repair a PSU because I don't like waste.
> I understand electronics, and know how to solder normal through hole components well enough.
> Other complications, when it broke, it has fused resistors (causing them to be open circuit) and at the same time missing a label, so it is quite difficult for me to determine what value resistor they were. Any ideas?

This is pretty tricky, you'd have to try and reverse the PCB to a
schematic to work out roughly what the part does.  Hopefully it's not
important like a feedback resistor where the value really matters.

> I also understand that it would be cheaper and easier to just go out and purchase a new PSU, but that is not the point here. It's more of the challenge and environmental aspect that I am doing it for.

SMT work is pretty fun and quite satisfying once you've learnt some of
the tricks.  It can be surprising how easy SOICs and stuff can be with
the drag soldering trick.  Hot air is very handy but also a bit of a
pain in the arse at times, especially when the air moves the part around.

> Kind Regards
> James
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