The construction of my DCC locomotive decoder forced me to delve in areas of surface mount technology I had never previously explored.

Soldering Iron

Yes you can use a soldering iron for many SMD components.

My favoured method is to use a “manual wave” soldering tip, especially in a JBC handpiece.
Such a tip has a small concave reservoir at the tip which holds a small pool of molten solder.
To use it, one loads up the pool with solder. I find life is a lot better if the solder also contains silver.

I get the best results using an electronics paste flux (not plumbers!).
NOTE: this is a pure flux, ie it does not hold solder balls.

Flux the PCB pads, then place your device.

With the pool of solder facing downwards on the tip, solder the component down.
For larger ICs it is best to solder the corner pins first.

During soldering, solder is drawn from the pool of molten solder.
The flux and inherent surface tension of the pool helps to avoid excess solder at the actual joint.

You can literally solder one side of a SOIC package in one steady swipe across the pins.

Reflow Soldering

Prior to my locomotive decoder assembly I was aware of using solder paste and heat, but had never actually tried it.

Using a QFN device however forced the issue.
I simply had to use some sort of reflow technique to mount a device which has no exposed pins when mounted and are all hidden from view on the bottom surface.

I did the usual rounds of Youtube seeing what others were using to reflow SMD components.

I found popular methods to be electric fry pans, toaster ovens, infrared lamps and hot air.

After much deliberation, I decided to try using hot air.
I had already used a heat gun at work for removing faulty SMD components, at which they excel.
I figured I could also use it for heat shrink tubing.

I was able to secure an Atten 858D+ hot air station for ~60 AUD on eBay.

Next was to get some solder paste.
I’m not a fan of lead free, so I got a 25g syringe of tin/lead solder paste from Element14 (nee Farnell)

This stuff supposedly has a short shelf life, I suspect more so due to evaporation of alcohol from the flux component.
I figured as I’m not using mine in a production environment I could always try adding a drop of alcohol if required. It is hard to believe the solder balls will magically expire after time X.

Dispensing tips

Next important item is a dispensing tip.
I tried using a toothpick, but it is very difficult to transfer nice deposits of solder.
I bought a selection of tips from Mouser along with parts to build my decoder.

I first tried a pink 20g tip but quickly found far too much paste would be applied.

I then tried an orange 25g tip.
This supplies a nice thin bead which is good for the QFN pads, but is awfully hard to squeeze out.
Nonetheless this is the tip I have used to build my decoders till now.

I also have a blue 22g tip, but it is a bit long at 1.5”.
I should trim this back to 0.5” and give it a go. I think this size will be a nice compromise.

Application

The part I found hard to get over was laying a strip of solder paste across all the pins of an IC and relying upon surface tension to pull solder to the joints during reheat.
I don’t know why I stressed about doing this. It simply works, and works very well.

I have now placed 4 QFN devices using this technique and they have all mounted OK without any shorts and all pins connected.

Normal components like resistors and capacitors just require a small drop on each pad.

Placing components

It goes without saying you have to use tweezers. Thumbs are just too big.

I also find tweezers with a bent tip are far easier to use than straight tweezers.

Reflow

This is where it all takes shape.
I use a narrow nose piece, and set my hot air gun set to 350 degC.
I let the gun heat up then gently wave it about 30mm away from the components to be soldered.

Within a few seconds the paste will start to flux off, and the solder begins to flow.
A couple of more seconds ensures the solder has completely reflowed and then the heat gun is removed.

The nifty part is when all the pads are holding molten solder, the devices do tend to self align (within reasonable limits) due to the surface tension.

Cleanup

Most solder pastes these days are claimed to not require cleanup. Perhaps that is the case, but I still prefer to remove any flux residue.

I find Isopropyl Alcohol is still be best to use.
A few drops on a cotton bud and a bit of gentle wiping soon converts the board into a nice clean affair and a dirty cotton bud.
I have still yet to find a reasonable cost source of Isopropyl Alcohol.
You can buy spray cans of contact cleaner at some hardware stores, but I’d still prefer simple liquid in a bottle.