Hot air soldering beginner - any tips and tricks?

[shadewind]

I just got me a hot air reworking station which I'm going to use to solder exposed pad devices and QFN devices. Since I don't have all the parts for my current project yet, I decided to test it out by removing some parts off an old useless graphics card and it seemes to work pretty well. I have a few questions though:

• I used a temperature of somewhere around 300 to 350 degrees C. Is this an appropriate temperature or should I use something lower? I guess it depends on what you're doing but some pointers would be nice. I noticed that for parts containing plastic (in this case a PLCC socket) I had to use a lower temperature to prevent the plastic from melting (about 260 C).
• I wasn't able to remove the GPU itself which was a rather large QFP device since I didn't really come up with any way to heat all the sides at the same time. Aoyue has special nozzles for this that are square shaped. Is this required or is there some trick I haven't figured out?

Any additional tips and hints would be very much appreciated.

[IanB]

I have no practical experience in this regard, so take what I say as purely theoretical.

For installing SMT devices it might work better to use a soldering iron rather than hot air. Hot air might best be reserved for unsoldering and reworking previously assembled parts. See these videos on selection and use of tip shapes by Hakko for examples of SMT soldering:

http://www.hakko.com/english/tip_selection/flv/tip_movie.html?id=d-1

For removing larger parts, preheating of the board is advised. A specialized board preheater can be used, or it can be improvised for example with a coffee cup heater.

Also for removing large parts, a bigger source of hot air might help, for example a hot air gun for heat shrink tubing or embossing. You can protect the surrounding parts with aluminium foil with a hole cut in it to expose just the part you want to remove.

[shadewind]

I do know how to solder SMT parts with a soldering iron, that's not problem. But you can't (at least not most of the time) solder QFN and exposed pad devices properly without hot air or other reflow technique since you can reach all of the pads with a soldering iron.

[IanB]

Right, I didn't pay proper attention to QFN in your post.

But I do believe a preheater is a big help. It avoids undue stress on the board and it leaves the hot air gun with less temperature rise to achieve.

[sacherjj]

Biggest thing is to use plenty of flux.  Solder paste.  Go at it.  I've really found it easier than normal soldering and parts self align very well.

[Fraser]

HI,

I recently started using a hot air station and found it very beneficial to obtain some scrap modern PC motherboards for practice work. Try to get both 'non ROHS' and 'ROHS' compliant PCB's as the hot air temperature requirements differ due to the differing solder types. I have become proficient in removing the high lead density IC's, cleaning the PCB and reinstalling the IC's. All done with the hot air station and at zero risk to a valuable PCB or IC's 

As has been stated, lots of decent quality flux on all solder pads and quality solder paste are important for a good result. You can practice with differing temperatures and air flows to gain experience of the effect that these have on the desoldering & resoldering of parts. I also invested in a PACE 'Heat Wave' pre-heater and that makes life a little simpler and less stressful for the delicate IC's. My unit was a used bargain so it is worth keeping an eye out for one if you intend to do a lot of SMT work.

[shadewind]

One thing I'm wondering a bit about is how to solder and desolder plastic parts. I tried removing a PLCC socket from a lead free motherboard and this ended up melting the plastic even at temperatures that are so low that they won't even make the solder reflow.

[IanB]

One thing I'm wondering a bit about is how to solder and desolder plastic parts. I tried removing a PLCC socket from a lead free motherboard and this ended up melting the plastic even at temperatures that are so low that they won't even make the solder reflow.

Puzzling. Parts designed for SMT soldering are usually robust enough to withstand the typical reflow temperature profile in a convection oven. How did the PLCC socket get soldered down originally, one wonders? However it was first soldered is how it should be unsoldered.