Tuesday, March 02, 2010

Hand soldering SMT components

Below is based on an email that describes how I do hand soldering of SMT.  I'll try to add a few photos soon.

I use a temperature controlled iron with a good tip, set to 315C.  A temperature controlled soldering iron with a good tip is KEY!

I bought a Aoyue 937+ digital soldering station for about $50 through Amazon.  You can buy a "fire-starter" soldering iron without temperature control for as little as $10.  But even an "expensive" soldering iron without temperature control makes SMT soldering more difficult, your tips don't last as long, and you are at greater risk of heat damaging parts and your board.  Excessive heat can lift traces and pads.  If you need to save money, you can buy the Aoyue 936 analog controlled soldering station for $40.

For SMT, I use a roll of .015" (very thin) Sn63/Pb37 no-clean solder.  Not lead-free.  Lead-free takes at least twice the skill.  That's for pro's or "pro-sumers", not us hackers. 

I use a magnifying glass on a third-hand base to see my work.  I keep both eyes open so I can use depth of field.  I don't use the alligator clips, I just use the third hand to position the magnifier where I need it.  The board is held in position with scrap PCB's taped around it.  I use a gooseneck lamp or a strong LED flashlight to illuminate my work.  A ring light with a center magnifier would probably work better, but I don't have one (yet).  Decent microscopes are too expensive and don't allow me enough freedom in choosing my angle of view.  If you can afford one, you are a pro-sumer and should be working on your lead-free skills.

No prep on the pad, just drop the part on and position it with your very-fine tipped tweezers.  Note that one side of my supposedly "anti-magnetic" tweezers have already become slightly magnetized.  But titanium tweezers are over $20, "anti-magnetic" stainless are just $5, so I just deal with it.  Solder wire doesn't magnetize, so I use it to hold down 0603 and 0402 parts while I pull away my tweezers.

Make sure your soldering iron tip is clean and well tinned, but no excess solder.  A very tiny bit of excess solder on the tip will help heat conduction, but it also too often grabs the part with the surface tension of the solder to cause it to tomb-stone.  That's where the part pops up to a vertical position, like a tomb-stone sticking out of your PCB.

Touch one side of pad and end of part with the tip.  Count to 2 or 3, touch the pad with solder wire. Exhale (to blow fumes away instead of inhaling them).  Solder should flow across pad and up the lead of the part.  0603 parts will tombstone about 25% of the time, 0402 about 75% (less with practice).  When it happens, use the end of your solder wire to quickly push the part back down.  If part has rotated, maybe 10% of the time, push it straight with solder wire. Pull iron away.  Try not to dwell overlong positioning the part with the iron heating the joint, as you can damage the part and/or the pad.  Of course, you have to remember to remove the solder wire before the iron.

On other side of the part, touch iron to pad and lead, count to 2 or 3, touch pad with solder, pull solder away and pull iron away almost simultaneously.

Admire handy-work.  Soderwick (tm) is your friend at this stage, but very rarely needed, about 5% or less of my joints need rework thanks to using thin, lead-based solder and my temperature controlled iron with good tip.  Be generous with oderwick. Once used, cut the end away and always use solder-free Soderwick. 

I have found I can hald-solder SOT-3 parts.  These have 5 leads in about the size of a 0603 component.  I was surprised that I could align the parts and solder them with basically the above technique.  Even with my magnifier, it is very hard to see the leads to tell if they are aligned.  For hand soldering, using larger than recommended pad lengths helps. 

I do not have a very good technique for QFN (leadless) IC's.  Basically, I align the part carefully and try to hand solder one corner pin without moving the part.  Then I drag solder the sides (starting on opposite corner from first pin) resulting in many shorts.  I clean those up with Soderwick.  Lay it snug along one side of the part, and drag the soldering iron across the Soderwick.  It easily pulls all the excess solder out, leaving just enough to make joints.  I ohm out all adjacent pins to verify no shorts.

I have one QFN part that requires a solder pad under the body to be connected.  No choice there but to use a SMT reflow hotplate or toaster oven.

Always ohm out power to ground when you are finished, before applying power.

LED's are about the most easily damaged SMT part.  My digital meter has a diode check function.  I apply that through the load resistor to check for LED operation (and correct orientation).


Vinod said...
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Vinod said...
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Vinod said...
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