Introduction: SMD Soldering – Standard, No-lead and Thermal Pad Packages

By Custom Electronics
About: Engineer, retired. Have always had an interest in electronics, often related to radio control. This evolved into a part time business that I still practice. I do some voluntary work in schools to encourage …

Surface mount soldering is straight forward for most components – as will be shown here. They can be soldered with a standard iron, preferably temperature controlled, with a fine (0.5mm) tip. I will also show some more tricky parts that require use of a hot plate and/or hot air iron, plus some other techniques.

I will show three levels of difficulty: standard, No-lead packages, Thermal pad packages.

Equipment needed:

Standard:

  • Soldering iron – ideally 50W temperature controlled with fine(0.5mm) tip
  • Decent desk light – 100W incandescent equivalent
  • Solder paste – best transferred to a 1mm syringe, plus 0.9mm OD needle

No-lead packages:

Thermal pad packages:

  • As above, or using soldering iron in place of hot plate.

Some other tips:

  • I keep my solder paste in the fridge (along with my cyano adhesive), double wrapped for safety. It has a short shelf life but can go a very long way! Hence keep it cool to extend its life. The paste used in the videos is 2 years beyond its ‘end date’.
  • Concentrate on a steady hand holding the components in place – unless you are keen on micro-tiddly-winks!
  • A magnifying lamp can be helpful for the finer components.
  • When laying out a circuit board try to keep the pads that are to be soldered in the same direction. This minimises the need to rotate the board when soldering.

Equipment:

Most of the equipment can be obtained at relatively low cost. I use an Antex TC50 iron (50W) with my own controller. This uses a thermocouple rather than a thermistor for temperature measurement – so no calibration is needed and irons can be swapped without requiring a change in setting. (I swap irons rather than change bits as this is faster). My hot air gun is an 858D that is under £30 on ebay. My hot plate is basically an aluminium block with a soldering iron element inserted. See the linked instructable above. You can also use a cooking hot plate – but temperature control is pretty difficult. Others have said an upside-down clothes iron works (cotton setting is around 200C).

Step 1: Standard SMD Soldering

My first boards used 1206 size resistors and capacitors. After a short while I found that 0805 were just as easy to solder. I have soldered 0603 size and the occasional 0402 but these require a pretty steady hand, plus a magnifying lamp helps.

Leaded ICs with 1.27mm pin spacing are easy – as shown below. SSOP and TQFP packages have spacing of 0.65mm and 0.5mm and require more care as it is easy to get a solder bridge. I use the same technique but start with the finest bead of solder. Also its helps to only touch the iron on one pin at a time. If you get a bridge then use solder wick or solder sucker to clear it. I find the solder wick works better if dipped first in some liquid flux.

My SMD soldering approach is to place and solder one lead on each component first, and then, when all (or most) are in place, put paste on the remaining leads and then solder these. Note if soldering the right hand leads then work from the right hand side of the board to avoid melting solder on the components that are only fixed by the one lead.

I hope the video below shows how easy the standard SMD soldering is:

Key Stages:

Place component

Add solder paste

Solder first side

Solder all second sides

Step 2: No-lead Packages (e.g. QFN)

These are more difficult, especially the ones where the pads do not go around the edge of the component. These need solder paste putting on the board pads before placing the component. This leads to the question of how to get this to flow (without a reflow oven). Some have just used a hot plate and effectively transfer heat from under the board. Applying heat only from above – e.g. with a hot air gun – risks overheating the component. After some experimentation I ended up using a combination of the two. I set the hot plate to around 180C. This warms the board approximately to the reflow preheat/soak temperature. I then apply a hot air gun from above with the air gun set to 240C.

The solder paste can be applied by hand to each pad. The amount/heights should be matched as closely as possible. The ideal approach is to use a solder mask with holes just where the paste is needed. Since making the video below I tried a solder mask cut by hand, using a scalpel, from clear plastic transparency sheet (thickness around 0.1mm). This was very effective. There are also companies offering custom solder masks.

No-lead component and PCB

Key Stages:

Add solder paste

Place component. Then heat with hotplate below and air gun above

Step 3: Thermal Pad Chips

One is more likely to come across these chips in a rework situation because the circuit board will probably have a set vias under the chip to take heat away to a copper area on the opposite side of the board. Commercial boards can also have the complication of components on both sides - as is the case shown in the video. If the components are on one side only then the hot plate method as used for the QFN packages will probably work. It may be necessary to run the hot plate a shade hotter given the good heat transfer through the board. An alternative to a hotplate is a soldering iron with suitable size bit. In the case shown a 2.3mm bit covered a good portion of the vias through the board. The iron was adjusted so that solder just melted on the tip.

The same comments as above apply to putting paste on the board. However this might not be necessary if there is solder still there from the original chip (assuming a rework job). Flux should be added. Then the chip placed carefully into position. Note getting solder contact over just half the thermal pad is usually considered adequate.

Hot air is applied (again 240C) at the same time as turning up the soldering iron under the board. Keep an eye on the solder on the pads and aim to get all melted at the same time. Then turn the lower iron off and withdraw the hot air.

See video below:

Thermal pad component and PCB

Key Stages:

Locate soldering iron below the thermal pad vias

Add solder paste as required

Apply liquid flux to PCB and chip

Place component. Then apply heat with soldering iron below and air gun above

Step 4: Transferring Solder Paste Into a 1mm Syringe.

See video below. I use a short piece of silicon tubing (RC engine fuel tubing). Aim to avoid getting air in with the paste as this makes it more difficult to control the flow of paste. When connected push the donor syringe while pulling the recipient syringe. This works easier with a new syringe. In the example I was ‘topping up’.

I hope these examples encourage you to get some solder paste and try SMD soldering. After a while the basic method – that covers most situations - becomes as second nature as soldering through-hole components. A big plus with SMD is the size reduction that is possible and hence delivers really neat end products.

Mike