Surface mount device (SMD) soldering involves a different technique and soldering tip than through-hole soldering. This technique is a bit more difficult and can be intimidating for beginners, but with a steady hand and practice, SMD soldering can be mastered.
The technique discussed here is only for surface mount devices with small exposed feet (seen below) or a chip with accessible contacts. Different techniques and equipment are required for SMD’s with inaccessible contacts.
- Flux (syringe works best)
- Screwdriver or chisel tip for soldering iron
STEP 1: CHANGE THE TIP / HEAT UP THE IRON
For SMD soldering, replace the tip of the iron with a screwdriver or chisel tip. The iron for surface mount soldering needs to be even cooler than for through-hole or wires as the heat can quickly damage the small surface mount devices. Ensure the tip is well tinned.
STEP 2: APPLY THE FLUX
Using either a small brush or a syringe, apply flux over the contact points for the SMD on the PCB. The layer should be thick enough to evenly cover the contacts, but no thicker than that will be difficult to place the SMD.
STEP 3: PLACE THE SMD
Using the tweezers, place the SMD on to the flux-covered contacts with the feet/contacts aligned against their contact pads on the PCB.
STEP 4: ADD SOLDER TO THE TIP RESERVOIR
With the flat end of the soldering iron tip pointing up, melt a small amount of solder into the reservoir. The solder should form a bead on the tip of the iron.
STEP 5: APPLY THE SOLDER
This is the tricky part which takes practice. Using the tweezers to secure the SMD chip in its place, turn the soldering tip with the solder facing down, and gently drag the iron across the contacts. Move the iron at a medium speed with a very light touch, pausing between each pass to examine the flow of the solder into the contacts. Capillary action will pull the solder in between the contacts. Once a few of the pins have been soldered, you can pull the tweezers away while the other pins are soldered. Continue making passes until all pins have a good connection. Remember, it is always better to add too little than too much. Reference the photo at the top of the next page.
The pins are connected by too much solder.
This is a common problem when learning how to SMD solder. If too much solder causes a short between legs/contacts, there are a few techniques that can be utilized. First, try using a clean soldering iron tip to melt and then pull the solder away. Next, soldering braid can be dragged against the melted solder to pull it away. Last, a solder sucker can be placed directly against the soldering tip and used to suck up the solder.
The solder is not flowing on to the chip and pads
This technique is difficult the first time but does get easier with practice. Two common reasons the solder isn’t flowing are the angle of the soldering iron tip relative to the board, and too little flux. The iron should be held relatively flat to the pins and pad, moving parallel to the board. Sometimes the flux gets thinned out during placement and more may be needed to get the solder to flow properly.