Soldering SMD's

[Tony Clark]

On Fri, 14 Apr 2000, Jaakko Lofstrom wrote:

> What's the minimum configuration needed to solder SMD's?
> How do you do it?
> I mean do you need special soldering tips or stuff or is it possible to
> use a hot air blower in a wave-soldering fashion? As you can see, I'm
> totally lost with the subject ;)

   I swear there must be a conspiracy!  I leave town and a whole 
discussion of SMT/SMD pops up!  ;)
   Since I do SMT stuff, I'll give you all of my secrets.

   First, don't try soldering anything under an 0805 unless you have a 
magnifying glass.  I made the mistake of using 0603s on my stuff and it 
gets really tough with conventional soldering equipment!
   Equipment wise, everyone has made good suggestions.  Get a soldering 
iron tip that is as small as possible.  You don't need any exotic or 
expensive equipment to _solder_ SMT stuff, HOWEVER, it gets fun when you 
try to _unsolder_ SMT parts!
   Use a fine solder, like .020, use whatever you like using, I won't 
start a solder thread here.
   Yes, you will need a good set of tweezers, preferrably NON-magnetic!  
However, even non-magnetic tweezers WILL still exhibit magnetic 
properties with small SMT parts.  Just be aware of that!

   Now, to _solder_ SMT parts is rather easy.  Here's what I do:

   Dot _one_ contact pad with a tiny bit of solder.  Doesn't matter if 
you are doing a two-lead 0805, MELF, 14-pin SOIC, or 44-pin PLCC.  Grab 
up your part with your tweezers (if you are doing LARGE parts, you might 
want to invest in one of those hand held vacuum devices for holding 
parts).  Next, place the SMT part over the pads and with your iron, melt 
the pre-soldered pad.  Push the part down and remove the iron.  Wait a 
second until the solder has solidified and then proceed to solder the 
other pad(s) in a conventional manner.

   The trouble begins if you have to _unsolder_ an SMT part!  Now I have 
successfully unsoldered a 14-pin SOIC with a _normal_ soldering iron and 
without damaging it (I soldered it in backwards *DOH!*).  As much as that 
would make a lot of people cringe, I sincerely don't suggest anyone try 
this unless they are extremely good at controlling heat!
   However, for two-pad parts, its easy to just use the tip to heat one 
side and then (if you are quick) move the tip to the other side and push 
on the part, if the solder is still fluid, the part will just _slide_ off 
of the pads and can be then removed from the board.  I don't have much 
luck with de-soldering braid, so I don't use it.
   You _can_ do an 8-pin SOIC by grabbing the body of the part with a set 
of tweezers and then lay the iron flat across the 4 pins on one side.  
Lift up with the tweezers and it should lift the one side off of the 
pads.  Repeat for the other side. (Yes, this is how I got the 14-pin off, 
but its extremely hard to get the iron flat across all 7 pins!).
   Again, soldering isn't hard.  De-soldering IS!  For proper 
desoldering, I would recommend getting a separate soldering iron with 
tips made to fit specific jobs.  For SOICs, a blade tip is best.  Yes, 
you can get those soldering tweezers, but I doubt many people want to 
shell out $400 or more for one.
   If anyone wants to know more about SMT, I just installed an SMT line 
for my company and we're now starting to run production boards through 
it!  Maybe now I can convince my boss to shell out for one of those 
expensive SMT rework stations!

   Cheers,

   Tony

------------------------------------,----------------------------------
I can't drive (my Moog) 55!         |     The E-Music DIY Archive
------------------------------------|  
Tony Clark -- clark at andrews.edu     | aupe.phys.andrews.edu/diy_archive
http://aupe.phys.andrews.edu/~clark |     Contributions welcomed!
------------------------------------'----------------------------------