SMT soldering

[MartinJones]

Just wondering if any of you has experience of SMT (surface mount) soldering.

I fancy messing around with some op-amps, but will need to to solder them to a 'soic to dil adaptor' (current one(s) in situ are dil fittings).

I'm competent (I think!) doing the bigger stuff (built WAD kits), but this seems very small & obviously I worry about too much heat getting into the component as I faff around!

Martin

[MartinJones]

I should add: I'm not looking for someone to do my soldering, just some advice.

Martin

[Phil Y]

Hello Martin, first of all what sort of iron do you have? A proper temp. controlled one will make things a lot more easy. The problem with an old 15 watt mains type is that with a suitably small bit it will have very little oomph if you are soldering anything but the finest tracks. Also a pair of fine pointed tweezers are pretty much a must for holding surface mount parts. Thats a first couple of things that come to mind.

Regards, Phil.

[soulminer]

The correct way to solder SMT is to use solder paste and a hot air gun. Could be pricey for the amateur though. Have fun !!

[Audiognome]

Hi Martin,

although solder paste + reflow is the way it's done in volume manufacture, you can solder SMT just fine with a soldering iron. Tweezers & a fine-tipped iron are essential, though. Ideally the iron should be temperature controlled, set at around 300- 350degC or so. Depending on how good your eyes are, you may also need to work under a magnifier.

To hand solder an SOIC, flow a bit of solder onto one of the pads and then place the device with your tweezers. Line it up carefully with the pads and then touch the pad that you have already wetted again with the iron. This should stick the device down, so you can now let go with the tweezers. Then go round and solder the other joints, applying the iron tip to the pad (not the device leg) and then gently touching the solder wire onto the pad also. The solder should melt, flow across the pad and "wick" up the device leg. When this happens, remove the iron. Don't overdo it on the solder - only a very small amount is needed. The thing you musn't do is put any force on the device leg when everything is hot - the IC package will have softened slightly and the leg may fall off!
Also, don't forget to touch-up the first joint you did, if it doesn't look as nice as the others.
Most SMT devices will withstand up to about 5 or even 10 seconds of heat from an iron.

I'm making it sound much more difficult than it actually is - once you get the hang of it, it's very quick & easy. You could buy a couple of cheap op-amps to practice on, if you're nervous.

I must have soldered literally thousands of SMT parts by hand, over the years. 0402 resistors are easy enough (that's a 40thou x 20thou package!), so long as you don't sneeze and lose them!
"quad flat packs" with >100 legs at 25thou or less spacing, are a bit more hairy. Soldering iron isn't recommended for these !!

Nigel.

[John T]

Yes they can be soldered by hand.

Many moons ago I built a digital watch from parts imported from the States (the type that used red LED's for the display). This entailed soldering to a PCB a chip approx. 1cm square with 10 contacts per side plus a number of SM capacitors. The trick was to initially hold the component in place on the board with a piece of double sided tape. Using a soldering iron with the tip turned down to almost needle-like proportions and very fine gauge solder - plus a very steady hand - it was possible - and very buttock clenching!!