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 Here I will show you some techniques for soldering very small, fine pitch SMT packages using standard soldering equipment.
These techniques will help you to master the SMT soldering skills needed for assembling for instance my stepper motor driver board. The methods described here are based upon my personal experience working with electronics for several years, but there might of course be other methods as efficient (or even more efficient).
EquipmentThe minimum equipment needed (shown in the image above) is: - Soldering iron with small tip
- Solder (0,8mm or thinner is preferrable)
- Solder wick / Desoldering Braid (braided copper strip for removing excess solder)
In addition you will find it even easier to do a nice job if you have (image below) - Magnifier/microscope (e.g. sn USB microscope) to inspect your work
- A pair of pliers to hold the small components
- Isopropanol (or other rubbing alcohol) to clean off excess flux
- Solder flux in small dispenser, e.g. a mini-fluxer pen as shown in image below
General technique - Apply flux to the PCB pads and the component pins
- Solder one pin/pad to hold component in place
- Check alignment, and adjust if neccessary by re-melting the solder
- Solder pin/pad number two. For large multi pin components (e.g ICs) select a pin on opposite side of the component
- Re-Check alignment. After this point it will be very hard to realign the component
- Solder remaining pins. Either one by one, or if the pins are very small and closely spaced, solder everything without caring about shorts
- Remove any excess solder using the solder wick/braid
- If any solder joint need a touch-up, either because too much solder was removed, or because the solder looks rugged or gray/matte due to overheating, apply some flux and reheat the solder. This will give you back the nice, glossy finish. If neccessary, apply some more solder.
- Clean of any remaining flux using rubbing alcohol.
The following sections details the echnique for different types of components
Resistors and capacitors Soldering SMT resistors and capacitors is fairly easy, but you need a steady hand, and a pair of pliers to hold the small resistors. I have successfully soldered resistors as small as 0402 (thats 0.04" × 0.02" / 1.0 mm × 0.5 mm)using this technique. - Apply flux to the PCB pads
- Add a small amount of solder to one of the two pads
- Re-melt the solder, and place the resistor on the pad. Be quick, overheating the pads or resistor might damage the PCB or the component, and reduce solder finishing (you will see the solder turning matte gray)
- Solder the second pad
- Clean any remaining flux using a cotton swab soaked in rubbing alcohol
Electrolytic capacitor (aluminum can type)
The technique is a little bit different for the electrolytic capacitors of can type than other SMT components, as these have very long leads that need to be soldered to the pad along the complete pad to ensure mechanical strength.
- Apply flux to the PCB pads
- Add solder to both pads
- Remove as much solder as possible on the second pad using the solder wick/braid. Only a thin coating should be left.
- Apply a thin coating of solder to the capacitor pins
- Reapply more flux to the pads and capacitor pins (both now covered in solder)
- Place the capcitor and re-melt the solder on the first pad. This will complete the soldering of the first pin
- Apply solder to the second pin
- Clean any remaining flux using a cotton swab soaked in rubbing alcohol
Fine pitch ICs (TSSOP, SOIC, TQFP etc, also including thermal pad) This technique applies to all kind of multi-pin SMT devices/ICs, both with and without thermal pads. If the IC does not have a thermal pad (centre large pad on the underside of the IC), skip step 2,3,4,5,10,11,12,13 and 14.
- Apply flux to the PCB pads and package pins
- If the IC has a thermal pad you should make the footprint with a relatively large hole through the thermalpad (as done in my stepper driver design)
- Apply a thin coating of solder on the thermal pad on the PCB. Use the solder wick/braid to remove as much of the solder as possible
- Apply a thin coating of solder on the thermal pad on the IC. Use the solder wick/braid to remove as much of the solder as possible
- Reapply flux to the thermal pads on the PCB and the IC
- Add solder to one of the pins on the PCB
- Align the IC, and re-melt the solder on the single pin.
- Check alignment
- Solder a second pin. If the IC has a thermal pad, the second pin MUST be on the same side as the first pin.
- Flip the PCB, and heat the thermal pad trough the hole
- Add a small amount of solder on one edge of the hole to ensure good heat transfer down to the thermal pad
- When thermal pad is heated add more solder until the hole completely fills
- Apply pressure to the pcb, ensuring good and close contact between the IC and the PCB
- Remove the soldering iron while keeping the pressure, until the solder solidifies
- Solder all the pins to the PCB, on all sides if the IC. Dont mind to much bridges and shorts between the pins
- Remove excessive solder using the solder wick/braid
- Retouch if neccessary any bad looking solder/pins by applying more flux, reheating and if needed, applying more solder
- Clean any remaining flux using a cotton swab soaked in rubbing alcohol
The last pictures shows the finished result, as seen trough a microscope. Last two pictures show a 0.5mm pitch TQFP package soldered using this technique.
Lead less ICs (QFN, MLF, LLC etc) Soldering lead less packages can be done using the same technique as leaded ICs, as described above.
- Apply flux to the PCB pads and package pins
- Add solder to one (or a couple) of the pins on the PCB
- Align the IC, and re-melt the solder
- Check alignment
- Apply solder to the pins on the oposite side of the first pin
- Apply solder to the remaining pins on the same side as the first pin
- Remove excessive solder using the solder wick/braid
A little bit simpler method, requiring a bit more practice, is to just apply plenty of solder to the soldering tip, and then slide the tip along the pads. Note that you need to have the exact right temperature and the correct speed while sliding along the pads in order to get this to work correctly and a give neat result. Also use plenty of flux. On the last image below you see the chip is more or less flooded with solder flux.
Finished board This image shows the completely soldered stepper motor driver board, soldered using the methods described in this article.  If you have other devices/packages, and would like me to add a description here of how to solder them, leave me a comment below, and I will see if I can add a description.
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Comments
Thanks
John
easiest way is to solder a big solder blob on each of the two pads to join. Let them cool down, and then very quicly add a new solder blob between them. If you heat for too long, the bridge will be broken (lead collecting at either one of the pads, so be QUICK. Then when cooled down, re-melt the bridge again to get a good joint. Ill see if I can make some pictures and add to the article this weekend.
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