SMT Soldering

Soldering SMT components is not that difficult, the steps are essentially the same for soldering all types of SMT components:

  1. Put flux on the pad with the smallest thermal area:  This makes the next tacking step faster as it takes less heat transfer to do the job.

  2. Position the component on the pads and use the iron to melt the solder plating on the PWB pad to tack the component in place:  This frees your hands for the next step.

  3. Put a drop of flux on the component and pads:  This provides plenty of flux to clean all surfaces when the heat is applied in the next step...  I will often skip this step, relying only on the flux contained in the solder wire itself.

  4. Place the solder wire in contact with component and pad using the overlap length to set the amount of solder.  DO NOT FEED the solder into the joint!

  5. Lightly press the iron against solder to transfer heat to solder, component and pad until the solder has flowed freely coating the exposed surfaces.  DO NOT GRIND the soldering iron into the component or the PWB pad... it is there just to provide the heat!

  6. Remove the iron before the joint over-heats and burns off the flux completely.

  7. Solder the rest of the terminals in the same manner.

The final joint should be concave with 100% coverage of the component to pad interface with no air pockets.  Tin/lead solder joints can be expected to be shiny where lead-free solder can have a slight grainy texture for acceptable joints (K100LD looks very close to Tin/Lead however).

This is a typical 1206 capacitor after soldering and cleaning off the flux residue.  Don't spend a lot of time trying to get the perfect joint shape however... you'll get better with practice.  What you really are trying to achieve is a solid, void free connection between the pad and terminal where the solder has blended with the metals from both surfaces.


My favorite soldering iron setup is the 50W Hakko 936-12 temperature controlled station (set to 450C) with a 900M 60 degree bevel tip.  The mass of the tip determines the speed of temperature rise at the joint:  Too small and the tip will freeze to the joint, too big (and / or hot) and the flux will burn off exposing the joint to the air before you have time to react.  The bevel draws any excess solder back away from the joint.  I use the same setup to solder 0201 parts as well as dead-bug BGAs (That's #43 wire on a 0.5mm pitch BGA).

The solder shown is my standard Kester K100LD 0.015" Dia lead free RMA from Stanley supply (Pricing) or TechniTool (Pricing).

Just to show how forgiving this process can be, the movie at left is the same 1206 size capacitor being soldered with my 100W Weller stained-glass iron using 0.042" dia Tin/Lead solder.  That iron hasn't been cleaned or wiped for 15 years!
Cross section of joint from the first example video.  Notice the clean concave solder fillet and lack of voids. The good whetting quality all the way to the inside edge of the component terminal is an indication of sufficient heating of the joint.  The fact that no clear line between the copper and the solder can be seen indicates good intermetallic alloying has occurred.  Even the solder under the component (Far right) has a smooth concave fillet.

Any questions or comments?
 This page last updated on March 27, 2010