Surface mount devices.

Well I've pretty much finished off my power-amp and pre-amp combo (just have to finish off the chasis for the power-suply), thanks to everyone here, and I'm moving onto my next project, a USB audio device. Only thing is that one of the devices I'm using only comes in surface mount form (SSOP-28), and these usually come in moisture free bags and so on. So I was wondering if there's anything special I have to do with them before using them. Baking them is something I've heard of and so on...

Also, if anyone knows of any good tips on soldering those suckers I'd appreciate it. I'm confident I'll be able to do it even though it's small, but any tricks that'll help me out and make it that much easier would greatly be apreciated.
 
Here is what I would recommend:

Equipment

Hot air gun - 300-600 degrees
Small diameter round attachment 1/8 -1/4" for heat gun
No-Clean liquid solder flux
Soldering iron
Toothbrush

I will assume you are mounting on a PWB. Take a soldering iron and tin all the pads where you want to mount the component. Leave just enough solder on the pads but no so much that you leave a bump.

Next, fire up the heat gun with the small diameter tip and gradually heat the PCB around the pads. Hopefully there are no sensetive components nearby like polypropolyene caps or plastic connectors (these should be mounted later anyway) which may melt. If there are other SMD components nearby you may wish to keep an eye on them. They may become hot and blow off their pads.

Remember to gradually heat the area and the component to prevent thermal shock which may crack components.

Once the area is pre-heated, grab your component with a pair of tweezers or small needle nose. The ones which are bent at the end work great. Align the component on the heated pads and gradually apply heat to the area, alternating from one side of the component to the other.

After some time (depending on the component size, ground planes, and other considerations which absorb heat) the tinned pads will start to flow and you can remove the heat. Wait 15 seconds or so and carefully release the component from the tweezers if you release too soon the component will not be aligned with the pads. The majority of pins and pads should now be tacked on.

Gently brush a small ammount of no-clean liquid solder flux to the pins and drag a loaded soldering iron tip across the pins in order to "wave" solder them. At this point you may wish to manually solder the pins using a magnifying glass.

The reason for the flux is that the loaded soldering iron tip will have burnt off any flux in the solder and also it will help to prevent the solder from bridging between the closely spaced pins.

Even though this is "No-clean" solder flux you should wash the area with a toothbrush under clean water to remove any residual.

This is the process I have used for many years reworking surface mount boards at an electronic contract manufacturer. Hot air is much faster and cleaner than using standard solder/desolder tools.

Then there is the old fashioned way.

Simply tack down two corners with a soldering iron and carefully solder the pins with the use of a magnifying glass.

All the standard precautions apply with regard to safe handling of semiconductors.

A bit much but I hope it helps

Stephen
 
I've tried hot air but here is what works well for me.

Equip:
Liquid flux (no-clean)
Solder wick (no clean)
LMP solder (silver 2% or more)
Overcoat pen
Magnifier (the kind that you wear on your head)

Tin the pads. Wait for the board to cool and apply the overcoat pen around where the chip is to be mounted. (but NOT on the pads!) I can't recommend this step enough. The coat will probably require a couple of hours to cure.

Apply flux to the pads and to the IC pins. Load up a small amount of solder on the iron. Center the IC on the pads, holding it so that it cannot move. Run the iron tip quickly down the pads, pressing hard enough to ensure a solid join. Don't try to solder pads individually and don't worry about bridges.

Once both sides are soldered, check that there is contact between all traces and pins. If not, correct. If everything is connected, use the wick to remove any bridges. If you do this properly, it is rather difficult to pull the solder off the pads, but if you do a hot iron (no solder) should be enough to correct the mistake. Use lots of flux; the wick will almost magically remove only the solder that shouldn't be there.

The key is to work quickly, to avoid overheating the case or lifting any traces. The overcoat helps enormously with the latter difficulty.

Good luck with your project. I think you'll find everyone has their own favorite way to do SMD...I recommend practice more than anything else. You'll find out what works best for you.

(A great way to practice, or to recycle some components, is to thread some enameled wire under the leads of a SMD. Fix one end, and pull on the other while heating the pad to lift it from the board. You can remove 28-pin ICs in about 10s this way, usually without damage.)