Hand-soldering SMD for fun and profit
If you want to inspire fear/admiration/incredulity(* - delete as applicable) in people, simply tell them that you solder surface-mount components by hand. Go on, try it ... I'll wait.
What if I told you that it's easier, and a hell of a sight quicker, than solding through-hole components?
When you mention surface mount technology, most people will immediately think of the insanely small components that you'd typically find on a PC motherboard or in your phone, and they'd be right. However, not all surface-mount devices are created equal ... the insanely small stuff in phones and on motherboards is picked, placed and soldered by machines, however 'bigger' surface mount components are easily soldered up by mere mortals given a few basic tools.
Over time, it has been quite gratifying to watch my module designs evolve from rather nasty looking through-hole prototypes on perfboard to professionally fabbed boards with hand-soldered surface-mount components. I haven't abandoned through-hole construction altogether - it still has it's uses, and in some cases a through-hole component may cost considerably less than the SMD equivalent (case in point: the LF398 sample and hold amplifier - the SOIC version is about twice the cost of the PTH chip)
At this point I have absolutely no desire to get into a dick-swinging contest with those people who claim that they can hand-solder 0402-sized components blindfolded with one arm tied behind their back. If you can do that, great. Me? I'm in my mid-40s, my eyesight isn't the greatest and I know my limits. Those limits? 0805 and SOIC.
You only need four things to solder SMD:
- A good quality, temperature controlled soldering iron (I got something suitable from Farnell for just over £50). Iron temperature isn't critical - I generally run mine at 370C for 'normal' use or 400C if I'm planning to remove components or am working on a board with a large copper pour.
- Good quality solder - lead or lead-free, it's entirely up to you. 0.5mm or 0.7mm diameter. Do yourself a favour and get yourself a 250g or 500g reel - the initial outlay may be large but it'll last you a long time.
- Flux. Trust me - you can never have enough flux. Flux pens are useful in a pinch, but I prefer the sort of stuff that comes in 50ml bottles and has a little applicator brush.
- Desoldering braid and/or a solder sucker - accidents will happen. Count on it. Splash the cash on a 10m reel of good quality desoldering braid - the cheap stuff off eBay is crap and not worth it.
Technically, you need five things but 'a steady hand' is kind of implied (I'm serious too - do not attempt SMD soldering if you've hit the coffee a bit too hard. Don't ask how I know this)
Hit your favourite video sharing site and you'll find a whole host of tutorial videos, but the techique boils down to this:
- Apply solder to one of the component pads.
- Take hold of the component in a pair of tweezers, melt the solder and manoeuvre the component into position (make sure it is aligned correctly etc.)
- If necessary, gently press the component down and melt the solder so that the component is sitting flush against the circuit board.
- Add flux to the other pad and solder it.
Easy! If you've done it right, you should have nice, shiny and rounded solder joints - if any of them look dull, pop a bit of flux on the offending joint and re-melt the solder.
The technique for ICs really isn't much different:
- Apply solder to one of the corner pads.
- Using the same technique as above, maneouvre the IC into position - the main difference here is that you need to make sure that all of the IC's legs align with their appropriate pads. For 8-pin devices or MMBT transistors this is fairly straightforward - for 14 and 16-pin devices you may need a couple of goes to get things right.
- Tack the chip down by soldering the leg at the opposite corner.
- Apply flux and solder each of the remaining legs in turn - in the event that you end up with a solder bridge between pins use either desoldering braid or a solder sucker to remove the excess. With enough flux, surface tension should do it's thing and the solder will flow to where it is needed and nowhere else.
Again, if you've done it right you should see nice, bright joints. Using this method it's easy to solder 'standard' SOIC or TSSOP packages (150mil pitch) providing you have a steady hand - larger TQFP packages (think in terms of the Atmega 328 TQFP) are also relatively straightforward. BGA? Forget it.
Why use SMD though? Here's a few reasons:
- SMD components are cheap. And I mean really, /really/ cheap. The sweet spot for price/size seems to be 0603, but even in larger sizes like 0805 you can get more components than you'll ever need for a relatively small outlay (eg. a reel of 5000 0805 1k resistors cost me about a fiver). SMD resistors are ridiculously cheap, capacitors less so. Surface-mount versions of CMOS chips tend to be a /lot/ cheaper than their through-hole counterparts.
- It's quick - once you've got the technique down, assembling a board takes far less time than it's through-hole equivalent would take. For a start, you don't have to muck about bending components and trimming leads, just put some solder on a pad, drag the component into place and solder the other pad(s) down.
- Better use of board space - well, duh! Obviously SMD components are smaller than their through-hole counterparts so you can make far better use of available board space. In addition, it makes using both sides of a board a no-brainer providing you're happy with a few vias here and there.
- Removal of components is a cinch - let's face it, having to remove a through-hole resistor from a board is a monumental pain. With SMD, it's just a matter of heating both component pads and lifting the offending device off with tweezers or, if you're lucky, the tip of your iron. Obviously you want to be careful not to lift any of the pads, but with enough flux and a bit of care this is pretty difficult.
But what about the downsides?
Well, prototyping with SMD is a bit hit and miss - if you're using 0805 or similar components then you can use regular perfboard if you're enthusiastic enough. A few companies used to do SMD prototyping systems but these seem to have been discontinued. Fortunately, the Internet rode to the rescue - I stumbled across this article on surface mount soldering (worth a read) and it would appear that the author had similar issues - I had a bunch of these fabbed up and now I have everything I need for building prototype SMD circuits prior to committing them to a 'real' board.