>> It'd be easier to leave such tiny work to machines. Why do you need
>> to solder them manually?
>
>I'm building a one-off design for a psychology PhD experiment. I do have
>access to University College London's Electrical Engineering department but
>I doubt they have robots for building surface mount systems.
They will have technicians, lecturers and students who should know what to do though. Ask nicely and someone might give you a demonstration and show you the tools they have available.
Folks are starting to experiment with reflow soldering in toaster ovens. I've read good reports on the web, but have absolutely no personal experience nor have I even talked to someone who's done it, so I can't vouch for it. One of these days I'll give it a try, in the meantime you can search on toaster oven reflow and get a slew of pages.
There's at least one company
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who'll even make prototype stencils for your solder paste, although for stuff that's not too fine you can dispense paste from a syringe.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Get a decent temperature controlled soldering iron, run it at 600F/315C and use low melting point solder. No need for preheating or any other messing about.
On some boards chips can be tricky to remove because they are glued in place.
OK, so this is basically emulating the reflow process. The only issue with this is that there is no real temperature control and you can damage both the board and the components if you are not careful (as is the case with all soldering).
The technique I use is lots of flux and a soldering iron. It is key to use flux so that the solder flows properly. Sometimes a microscope is required and then occasional washing to remove the solidified gunky flux [wash in ethyl alcohol]. However if you want to solder an IC you can run the soldering iron along the side of the IC. Slope the board and then run the soldering iron down the board from the top most pin to the bottom most one and the solder meniscus will stick/flow with the soldering iron. You will end up with the last 2 or 3 pins with a bridge which can easily be removed with solder braid. You do however need to tack the IC down to start off with so that it doesn't move whilst soldering. I learned this from a very skilled tech and it works incredibly well. Especially for 25-50mil pitch QFPs. For Passives, it's a lot of work and the technique given in the link below may be better if you have lots of them.
**FLUX** is always key though! Water soluble if possible as it's easier to clean off. Just wash it and wait for it to dry. Don't try and heat it dry since this can cause steam to form under the solder joints (and the ICs) and cause adverse pressure. The pressure can cause joints to fail and parts to crack.
I would also like to point out something else about the technique given in the link below. Bake out! One real issue in SMT soldering is that IC's have to be baked out if they've been sitting in a normal environmnet (humidity) since the epoxy absorbs water. The water then gets heated by the IR reflow process/oven and can cause the IC to crack. The way around this is to heat the IC/components gently to above 100 deg C (125 deg C is OK). This bkes out the water. USually in an industrial process this is done for about an hour or 2. The IC's are then sealed in a waffle tray and bag with anhydrous silicone crystals to make sure no water gets reabsorbed. You shoudl only be wary of this if your IC's have been lying around in a damp warm atmosphere for a few days. For normal prototyping this is not so much of a problem.
You can also get the reflow "hairdryer" kinds of devices. These work well also and are a little bit more controllable than an oven. If you paste as described by the link below, just heat around the IC/components until the solder goes shiny. It'll reflow at that point. Let it cool off and check it.
It is interesting to note that if you look at one of the winners (Robert Lacoste) of the Circuit Cellar design contest (Steve Ciarcia's embedded microcontroller design magazine) ... he addressed many of the issues that you raised.
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In fact enough, with a $150 toaster oven and a low-cost controller, he achieved reflows with good results. A few mfg. were very interested in his design and approach (Robert has his own consulting company in France).
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