Soldering SMT components

[cypher2001]

I'm curious how people like us (ie: average joes of the world) could prototype using SMT technology. How can you solder pins you can bearly see? How about when there is no pins at all (QFN packages) Is it possible?

I'm specifically looking at some small QFN packages I have laying around here. I'd like to "play" with assembling prototype circuits, but can't for the life of me figure out how I would connect them.

Ideas? Comments?

Thanks

 

[Doug S]

The best advice I have ever received on this topic came from Wayne Y [dat2zip]. Bite the bullet and buy a good stereomicroscope.
Get a good quality SMT solder paste [About $40 for 35g]. Apply with a 22 GA or smaller needle on a 3cc or smaller medical syringe with needle cut down to 1/8". Any longer and there is too much resistance for the SMT paste to flow.
Buy some good tweezers.

 

[LED_ASAP]

NewBie demonstrated a simple way---hold the QFN package up-side-down with a wig wire, then solder wires to the pads. But I couldn't find his original post now /ubbthreads/images/graemlins/frown.gif

 

[PEU]

I learned in this forum way ago that the best way to do one offs is to use the dead bug technique:

put the components upside down, glue them to a copper board on do the wiring.

good pliers are a must, and if you can get a self locking one is like having an extra arm /ubbthreads/images/graemlins/smile.gif

Also advised in this thread, a good magnifier helps A LOT.

Or if you have enough dough, you can buy a pick-and-place machine... /ubbthreads/images/graemlins/grin.gif


I burnt my eyes trying to do a simple zetex converter...

Pablo

 

[NewBie]

Yucky looking quick prototype:
http://www.molalla.net/~leeper/sminver.jpg

Same thing built for use:
http://www.molalla.net/~leeper/sol_inv.jpg

Further along:

Another 98% eff for 1A out at 3.9V:
http://www.molalla.net/~leeper/th1000.jpg



A tri-level output made for 1AA, 2AA, 123, r123, has three output levels depending on input voltage range:

 

[cypher2001]

Thanks for the input. Atleast its good to know it "CAN" be done..


Now to keep telling myself that when I fail.

 

[Mednanu]

It's really not as bad as you think, but does require some skill and specialized tools, nonetheless. Keep in mind, I have done soldering of this type with those cheap radio shack bargain bin $5 soldering irons with the huge, "clubbo" style tips.....but I'm surprised the circuit even worked after I got done. As an amateur/hobbyist, I would suggest the following hardware.

1) High magnification reading glasses ( couple bucks at most drug stores ).
2) A circuit board holder - these usually have two large alligator clips and fully adjustable arms which allow it to hold the circuit board in any orientation ( $5 - $ 15 bucks at an electronics store or on eBay ).
3) A low-end Weller soldering station ( $40 - $60 on eBay or at your local electronics store ). I believe the model number is WP-60.
4) This one is important ! The very smallest soldering tip you can find ! I managed to find one that got down to 4 thousandths of an inch and ground it down to 1 thousandths.....it's still a little big, but gets the job done.
5) Alternately, if you can't afford the weller, you could make your own solder-tweezers ( which actually worked BETTER than the weller, but weren't temperature regulated ). I've posted instructions for making them here .

Then, I recommend building up a small solder bead on the circuit board pads where you'll be soldering the SMT's ( even the smallest solder wire is usually too big to try pushing it against the SMT and circuit board when you solder them together ). Then just glue down the smt's ( with a small dot of rtv or something that allows for removal if you blow the device during soldering ) onto the circuit board ( so they don't stick to your soldering tip when you pull the iron away ), and solder away.........quickly. You can't linger more than a fraction of a second while making the solder bead or you'll fry the SMT. It's sort of a swiping maneuver rather than a hold and flow maneuver. If you use the tweezers, it's even easier, but you have to make sure they don't get too hot. I had to turn mine on, then off again, and let them cool for a minute or two before it was safe to solder with them.

 

[NewBie]

[ QUOTE ]
cypher2001 said:
Thanks for the input. Atleast its good to know it "CAN" be done..


Now to keep telling myself that when I fail.

[/ QUOTE ]


The key is skipping Pepsi and Coffee for the previous 8 hours, imho. They make you shake.

The WP-60 is fine, but it will need something to back off the power. Even a 40W is too much, so you need to either regulate it, back it off, or make a plate that pulls heat out of the tip when it is in the holder.

The Metcal system, which you'll see up on ebay for a substantial discount works very nicely for this sort of thing, it does an awesome job of holding the tip temperature constant, and ramping up the power really fast to hold the tip temperature constant, such as going from a ground plane to a lead on a chip.

If you make the boards, and mount the chips as intended, a heat gun, paint stripper gun, and/or hotplate are useful. It takes a bit of practice, but can be done.

 

[PhotonWrangler]

Lots of good advice here. Also consider picking up some SurfBoard adaptors from Digi-Key. They're SMD-to-solder-pad adaptor boards for experimenting with SMD parts. They have all sorts of different configurations for different package styles and they can be a lifesaver.

Oh, and the thing about avoiding caffeine is true! :::)
/ubbthreads/images/graemlins/tongue.gif