OK, maybe I don't quite understand what solder paste really is? But after looking into some info and found that:

Is solder paste have solder and flux in a liquid state?

That was I think right now, so my question is what a good solder paste to use in electronics?

 

Solder paste is applied to a bare board with a mask: sorta like silkscreening. The parts are place with a pick and place machine and the solder pastes helps hold the parts in place during the trip through the reflow oven.

What good is it? It is absolutely essential to surface mount manufacturing. It would be impossible without it.

 

Would a Hot Air SMD rework station work with that kind of paste? If so, I have one of those Which paste would you use?

 

Yeah it would. What do you mean by what kind of paste? You don't realy need hot air though or solder paste for that matter...you can solder most components (except for thing slike BGAs) with solder and an iron.

 

Yea, I did just bought a brand new solder station as well for like $60.00 that has a temp controlled tips. Since I did also have a hot air station, though it would be easier to solder those smds that I need to solder using the hot air instead of hand soldering them. Don't you still need some kind of flux to solder then hand anyway?

 

Yeah, for SMD work the rosin core in the solder isn't enough. YOu need to add your own to the joint. But I just think handling a hard solder wire and a bottle of flux is easier and cleaner than handling a bunch of solder paste which is fluidic, and can't be stuck into a bottle.

I've never used hot-air, but from what I've read it's easier to use an iron to attach it and use hot-air to remove it. SO with hot-air you don't have the overwhelming fear of having to remove it if you botch it (as I have).

 

You can use regular solder with excellent results. The technique is (for TQFP-80):

1. tin your pads. Put some rosin over the pads then run a drop of molten solder using your regular soldering iron. You'll get nice little bumps on your pads covered with thin film of rosin.
2. Put a little rosin on IC pins. Kester RMA-186 pencil is handy for dispensing small amounts of rosin. You can also wet a piece of paper towel and then put an IC over wet spot for a second.
3. Position IC over pads. Don't sweat it - the part will self-center thanks to surface tension of molten tin. So as long as your pins are positioned over right pads you are fine.
4. Heat up your hot-air to 300 degC. Set the air flow to low 20%. This is important - molten rosin is slicky and the strong stream of air will be moving your part out of alignment. Now start heating your part from above - if it's big, you can move your air stream around the part. When you heat it up enough, you will see your part settle down and level on the pads. Put your air gun away, let the thing cool down and inspect your work under strong magnifier.

On the attached picture you can see the result - it's TQFP package with 0.5mm pitch. Pins are perfectly aligned and the amount of tin is right.

The hardest part is to put the correct (small and uniform) amount of tin on the pads before soldering. It's relatively easy with small pads (see above); bigger pads tend to take more tin than necessary. For 0603 and bigger sizes I found it easier to tin the part itself instead of the pads it is going to - you overdo it just a little, then, when the tin melts the excess spreads on the pad.

Also, there are air soldering tutorials on sparkfun site - check them out.

Attached Thumbnails

 

 

I also used solder paste with hot air. For most cases results are the same, for packages like QFN regular tin applied on pins solders more reliably. Also it needs to be kept in refrigerator between uses.

 

I can't believe there are serious hobbyists who don't use hot air tools. They are the absolute best for any SMD work. Sure, I use a pair of irons too, but I've got two hot air pencils, one that blasts a lot of air and one that is quite fine for delicate stuff. I can't imagine working without them, at least for SMD which is mostly what I work with.

If you look at solder paste up really really close, like through a microscope, what you see is tiny solder balls immersed in a paste flux. When working with it pretty much behaves like a thick paste, you can smear it around, wipe it off, dap it onto spots and even press it through small holes in a paste mask. I usually don't use paste for putting down new ICs unless they are BGA. If the leads are accessible, I like to just tin the pads first with an iron, and then hot-air the part down. The main reason isn't the way the paste behaves on the board, its that there is annoying clean-up work to keep the paste dispenser in good order, and if your just doing a little bit, its too much trouble.

__________________
RadioRon