SMD chip soldering
Whilst working on the Funky Flash Cart, I thought maybe some people would find a solder bridge removal how-to of use. So I promised Arto (that is, the one who designed FFC, not myself ;) to contribute one for his project. As getting grips with soldering surface mount devices (SMDs) can be rather tricky for the beginner, I figured why not expand on the topic a bit and write something like this post!.
First up, if you’re a complete beginner with SMD soldering, it’s very advisable to take whatever scrap board with SMDs (f.ex. broken VCR/DVD/CD player) and practice with that prior to moving on to DIY projects. With a little practice and a steady hand, the standard 1206 and 0805 sized parts (that’s 0.126″ × 0.063″ / 3.2 mm × 1.6 mm AND 0.08″ × 0.05″ /2.0 mm × 1.25 mm respectively) can be soldered with most generic tools. Smaller sizes are doable as well but a microscope is usually required for inspecting solder surface (quality etc). 1206 and 0805 you can manage with a decent magnifying glass.
As for tools, besides a soldering iron with a few exchangeable tips (small and large at least), you should also have some flux (preferably no-clean) and tweezers. A soldering iron with temperature adjustment is a big plus. Besides helping to avoid flux charring it’ll also aid working around issues like SMD ceramic capacitor breakage. If excess heat (+300°C) is applied to only one end of such parts, the sudden thermal stress can sometimes fracture the ceramics. So, either buy a second soldering iron (one on each end of the component) or adjust the temperature below 300°C.
With regard to soldering, one thing to keep in mind nowadays is the RoHS (Restriction of Hazardous Materials) directive. The most apparent effect for electronics of this directive is that all consumer devices should be manufactured (at least for the EU market) using lead-free solder and parts. Cutting quite a few corners short on this topic, for the average DIYist it’s most imperative to know that leaded solder should not be mixed with lead-free solder on any board that is to be used for anything else than soldering practices and that lead-free solder has a higher melting temperature. I’m not trying to discourage hobbyist from using lead-free solder, but being aware of the differences is important.
Why these then? Well, mixing leaded / lead-free solders will contribute to a mechanically weak and inferior soldering joint. Thus, any such joints will have a much lower life-cycle (read ‘your fancy DIY project goes bonkers much faster’). The melting temperatures are usually between 215°C to 230°C for lead-free solder whereas leaded is around 180°C. The exact temperatures depend a bit on the alloy mixture. If working with lead-free solder, make sure the flux can withstand the higher melting temperature required by the solder. No need to wreck that fancy new soldering bit you just bought with flux char ;)
Ok then, on to some actual working tips.. I’m going to build on the FFC solder bridge removal instructions, so no need to wonder if parts of this text sound a bit familiar.
- Using tweezers, place the chip on the PCB and line it up with the solder pads.
- Solder the first pin from either end of any edge to “anchor” the chip down. Use the smallest soldering tip you have. Start warming up the pad/pin combo by placing the tip to the pad without touching the point where it meets the chip pin. If there’s room for a small gap between the soldering tip and the chip pin, even better. Then, I wait a short moment for the pad to heat up and carefully apply solder to the point where the pad and iron tip meet. The solder should flow to the chip pin if the pad is hot enough. Lift the soldering iron away and wait a few seconds for the solder to cool down before moving on to the next pin.
- Verify the chip alignment. If you managed to misalign the chip during soldering the first pin, it’s still easy to warm up (or remove) the solder and re-align the part.
- Solder the second pin from opposite edge and end of the chip. Verify alignment.
- Solder remaining pins as carefully as possible, but don’t bother yourself excessively about making shorts between pins. Personally, when manually soldering SMD chips smaller than SO-8, I usually end up with at least one short per edge of chip ;)
- If you managed to solder some short-circuits, wait for the chip to cool down a bit and then apply flux over the pins/areas that are shorted.
- Change to a bigger soldering tip and wait for it to warm up.
- Using the large tip, swipe the solder bridge away. You should be sweeping gently along the length of the pin/pad, starting from the top of the chip and moving towards the end of the pad (see picture below). Remove solder from the tip between passes.
Fancy mspaint arrows ftw! \:D/
The trick here being, when the solder bridge is cold and you gently sweep over it with the hot soldering tip, the latter will draw excess solder to it. Depending on the size of tip and the amount of solder in the shorting point you might need to do multiple passes. A bigger tip will of course draw more solder than a small one.
Some flux might also need to be applied in-between passes to make the remaining solder more fluid. The more often (and longer) you apply heat to the joint, the more flux burns away in turn making the solder more rigid and harder to remove. Most importantly, the remaining solder should be allowed to cool down between passes as this method will not work as well if both surfaces are equally hot. F.ex. working on multiple short-circuits at a time will allow the others to cool down while one is being worked on.
And that’s about it, I think. Comments / opinions / your own methods welcome, as usual.