Soldering with Hot Air Question

[rfdes]

Hi -
I have a commercial product with a handful of PCB locations which are void of components.  I wish to solder these missing parts in an effort to introduce a functional upgrade.  The board is 100% SMD and the entire board appears to have been reflow (vapor phased) soldered.  The locations with the missing parts have solder reflowed onto each of the pads.  There a appears to be plenty of solder on each pad so it seems that if I clean the area with alcohol and use flux that I can just drop the ICs into place and reflow solder them with a hot air pencil.  Will this work or do I need to solder wick off the solder and start fresh? The existing solder certainly looks clean and fresh am hoping that this would work.  I plan on trying it this coming week but wanted to get some opinions whether this is a successful strategy.
Thanks

[moffy]

The use of flux should freshen up the solder, as long as there are not excessive amounts of solder on the pads, then you would need to use solder wick, but I am just talking from what I have seen on the videos, I will solder my first TSSOP28 and VSSOP10 packages in a few days. Best of luck.

[sahko123]

As above having a healthy amount of flux should be enough. As long as you keep the solder from oxidising you should get a good result.

[rfdes]

Appreciate your response.  I'll give it a go this week.
take care

[Shock]

Proper rework would be remove the solder with wick, solder sucker or vacuum tool (as appropriate) and apply fresh solder or paste depending on how you chose to solder. This eliminates contaminants and the possibility the old solder having reflow issues. You would use IPA to clean both before after each step for optimal results.

Gear for you own use - if there aren't excessive contaminants and the solder looks good after inspection, generally it's "whatever gets it done".  But you must use additional flux, not using it can be problematic (especially with old solder) and besides it will look crap.

Some components won't balance on soldered pads well. So preheat the board a bit with flux on the pads then use tweezers to place and hold lightly in position. You know when it's done as the solder will wick onto the legs uniformly. Some fluxes (such as tacky flux varieties) give extended work time and prevent the component from moving around, this can be advantageous as well. Another trick is to apply a little flux on the component legs.

It's not unusual to require touch ups with a soldering iron. It's debatable if it's any more efficient than removing the solder in the first place, but I'll let you discover that on your own.

[EPAIII]

The above advise is good but there is another consideration. A hot air gun not only dispenses hot air, but MOVING air. That moving, hot air can easily reflow the solder on adjacent components and then blow them around. So while your new component is being nicely attached to the PCB, one or MORE other components may wind up being, at worst, removed or perhaps just moved about enough to create shorts or open circuits. This is one really good reason for not doing it this way.

Any time I use a hot air gun on a PCB I try to first install shields with something like heavy duty aluminum foil to keep that blast of hot air from hitting the surrounding the parts. The weight of aluminum foil that is used for disposable pans and pie and cake pans is about the right weight so it too is not blown away. Use big pieces or weigh them down with nuts or washers inside the folds. And a lower speed on the air gun is best.