I build/rework/repair a lot of fine-pitch SMT boards by hand on a daily basis. I don't ever bother with solder paste. It takes more time futzing around with paste than just using wire solder.
Flux is your friend. It will help
tremendously with solder bridges on fine pitch parts. Use it liberally. It helps to get the solder to wick to pins and pads and form nice fillets. Don't be afraid to reapply more flux if you're working on a tough area. Flux will burn up and become ineffective, and you can get chunks of solder mask, silk screen and other impurities that cause problems. Use it liberally. As you get better and work faster you will probably require less.
Flux pen here...
https://www.sparkfun.com/products/8967
Soldering two-pin SMT parts:
For two pin SMT parts (e.g. resistors & capacitors), I will typically tin one pad on the board, pick up the part with my tweezer and place the part while heating the tinned pad with my iron. Then solder the second pin. This way you don't have to try to hold the part with one hand, hold the iron in the other, and apply solder with yet a third hand.
Soldering SMT packages with leads:
For your SOT and TSSOP packages start with clean pads on your board. If you have solder on the pads, clean them with solder wick. You want the part to be able to sit flat on the board.
On a leaded part like this, I will typically tin one of the corner pads on the board footprint for the part. Get a good mound of solder (reverse meniscus) on the pad. Apply flux to the board. Lay the part onto the footprint using tweezers. It won't lay completely flat because of the one tinned pad. When you get it into place, you can press down on the top of the part with your tweezers to hold it in place while you touch that tinned corner pad with your soldering iron. When the solder wets, the part will lay flat. Now that the part is tacked in one corner, you can get a solder ball on you iron tip and tack the opposite corner of the part while holding it in place with your tweezers.
Now that you have opposite corners of the part tacked you can easily adjust the alignment of the part if needed by heating up one corner or the other and shifting the part with your tweezers until your pins are reasonably well centered on the pads. Having two opposing corners tacked when you start soldering the rest of the pins will keep the part in place when you start soldering the rest of the pins. You can also easily remove the part at this point without destroying it if you realize you've done something wrong.
Once the part is centered, you can use the drag technique described by phalanx. Again,
flux is your friend.
Soldering leadless SMT packages:
For leadless packages, such as QFN and DFN packages, you'll need hot air, or a hotplate. With your soldering iron, tin the pads with wire solder and get a nice mound of solder on all of the pads. If the part has a center slug, use only a small amount of solder on the center slug, otherwise, the part will tend to float on the solder on the center mound and may not contact the outter pads. Also, if there is too much solder on the center pad and you push the part down, the solder can squish out and short to other pins.
Now that the pads are tinned, apply flux and place the part on the footprint, centered as best you can. Apply heat (hotplate or hot air). Be careful if you're using hot air not to blow the part off the board. When the solder wets, the surface tension will tend to pull the part down and it will typically center itself nicely. If it is not centered you can nudge it with your tweezers. Remove heat and let the solder solidify. If you have a very fine tip on your soldering iron you can use it to touch up pins. Again, flux helps a lot with preventing bridges when touching up. Use it liberally.
A side note on hot air vs hotplate; note that you can only really use a hotplate if there are no components on the back side of the board. Hot air can be used on two sided boards. If you have parts on the other side of the board from where you're working, typically the surface tension of the solder will hold them in place and keep them from falling off the other side. You may have a problem with larger parts, or if the board gets jarred when the solder is melted.
When you're done:
Make sure you clean the board of flux when you've finished. I use a tooth brush and distilled or deionized water to scrub my work area, and canned compressed air (e.g. Dust-off) to blow water off the board and from under parts. You can use regular water, but distilled or deionized works better to remove contaminants since it has no minerals and is 'hungry' for ions. You can get distilled water at the grocery store.
SMT work is intimidating at first, but in truth it is very easy with a little bit of practice.
Keeping your soldering iron tip clean and in good shape is critical for doing good fine pitch work.
Recommended tools for fine pitch SMT work. You don't need all of this, but this stuff is handy for an SMT work bench:
- Fine point tweezers (https://www.sparkfun.com/products/10603?)
- Fine tip soldering iron
- a damp sponge to keep your tip clean - a regular sponge, not a 'brass sponge'
- tip tinner/cleaner (https://www.sparkfun.com/products/8966)
- water clean flux pen (https://www.sparkfun.com/products/8967)
- fine wire solder (https://www.sparkfun.com/products/10240) - I use 15 mil SN63PB37 solder (No it isn't RoHS, but we're not talking production quantities here)
- solder wick (https://www.sparkfun.com/products/9327)
- clamp or vise to hold your work
- deionized or distilled water for cleaning boards
- hot air gun for removing larger SMT parts
- magnifying glass or microscope to better see your work
- squeeze water bottle - keep your iron cleaning sponge damp, clean your boards
- tooth brush or acid brush with trimmed bristles
- cotton swabs - for spot cleaning
- canned compressed air - to blow water off boards
- work lamp - good light is important to see your solder joints
- assorted small hand tools - needle nose pliers, side cutters, strippers, dentist picks, screwdrivers, etc.
- wire wrap wire - for rework
