SMD soldering- hot air

[Oznog]

Just wanted to report on this "hot tip":
I'd asked awhile back about SMD soldering for ICs. After getting some of the Zephyrtronics paste, I took advice and preheated the board in the oven to flow the flux, then used a dry, blunt soldering iron point to drag across the leads to flow the solder.

It does work, but it's hard to avoid pushing the device around, and can still leave bridges and a bit of mess if there's too much solder. It's repairable, but there's something better.

The butane Weller soldering iron (Portasol P-1k) has a catalytic hot air blower with a tiny jet, very similar to the electric ones Zephyrtronics wants $900 for. Only I bought the full set brand new with soldering tip, hot knife, air blower, and torch tips, and a case, for $20 on eBay. The results are quick, beautiful, and there's no spots the heat won't touch. Flows perfectly! Not that the soldering iron tip didn't work, but this works much much better.

 

[zevon8]

congrats.. you just got yourself a hot-air rework station for 20 bucks... very nice!

only thing, watch the time you apply the air stream, since its not really controlled, and can do mega damage really fast! I have lifted a few tracks doing this when I used one of these for reflowing on PCB's but never tried it on new parts. I found if you practised on a piece of wood, you could visualize where the "hot-spot" was in the air stream, to get an idea of the distance to use.

 

[Oznog]

Well, the rule is that usually a hotter tool has less potential for damage. You spend less time applying heat to the leads which means less heat soaks down into the sensitive component or board. I left the tool on max, let it warm up fully, and it does a row of leads in a good sweep. The temp seems just right, although it can go lower if I wanted to.

Haven't needed to remove an SMD yet. Zephyrtronics and several other places use an alloy which you wick into the component's leads, it alloys with the solder to form a new low melting point alloy, and then the whole lead set can melt at a moderate temp that you can safely bring the whole board up to with an oven or big hot air stream.

 

[zevon8]

Well, the rule is that usually a hotter tool has less potential for damage. You spend less time applying heat to the leads which means less heat soaks down into the sensitive component or board.

True, I did a poor job explaining what I meant, being trying to find the hottest part of the flow, and if it had a "convergence" point of sorts. They one I used had a "swirl" to it that sorta focused at a point, then dispersed fairly quickly.

BTW, I always wondered if that special solder would be good for rework or if it needed to be purely applied. It and the air torch might be just the thing for rework on high density SMT passive device PCB's where operating heat has damaged a board with cold joints. I would guess mixing it would alter the eutectic point to make it not worth while.