[sdiy] Moving to Surface Mount - lots of questions

[Gordon JC Pearce]

On 06/12/11 07:45, Mike Beauchamp wrote:
> Hey all,
> after assembling a few PCB's with a few hundred resistors, I've been
> thinking about moving to Surface Mount for my next project. Please
> convince me, or talk me out of it :)

I haven't got the patience, dexterity or enthusiasm to sit and fiddle 
with awkward through-hole components any more.  They're just plain 
horrible to work with.

You will need a proper soldering iron.  I've done it with 
non-thermostatic irons in the past, but it's far easier with a 
temperature-controlled iron with a decent tip.

There is a lot of moaning in amateur radio circles about kits with SMT 
parts, which are mostly related to people using unsuitable tools and 
solder.  You only need to dig back a couple of decades to see people 
moaning about how awkward and fiddly these newfangled printed circuit 
boards are, and how the iron they use for their tagstrip constructions 
just burns the board material!  So, put that 50W Antex to one side for 
the moment, and drop some cash on a cheap Chinese temperature-controlled 
soldering station like an Aoyue 936 and a few spare tips and elements. 
It'll save you a lot of stress and aggravation.

> I've watched a few videos of people using stencils to apply solder
> paste, placing the components, and then oven-heating the boards. That
> looks like it would be pretty fast to assemble, but I do have some
> questions for the experts:
>
> Speed: Having never done it before, how much faster is it to
> stencil-place-oven as opposed to insert-solder-clip?

Stencil and solder paste isn't really worth it unless you're doing a lot 
of leadless ICs, or mass assembly.  For small numbers of parts, just do 
it by hand.  It's much faster to stick down SMT parts once you get used 
to it.  Do you remember how hard you found soldering through-hole bits 
neatly the first time you built a PCB?  Go and look at the first PCB you 
ever built.  Yeah, not so tidy, is it?

Do not glue the components down.  That's a silly idea.

The one thing that people find daunting is sticking down ICs.  You do 
these in exactly the same way you do through-hole, except you don't have 
to fiddle about for ten minutes getting the legs through the holes (that 
don't quite line up properly because they're not quite drilled 
straight).  Plop the chip on the board in the right place, line it up, 
then tack down the power pins.  Once you've got 0V stuck down, you can 
adjust the chip until it's square on the pads and then do the +V pin, 
followed by all the rest.  Here's a tip - run a bead of liquid flux 
along the rows of pins, then run your iron along the pins dragging a 
bead of solder along them.  The solder will only stick to the pins 
thanks to the flux.  This is far, far quicker than soldering a pin at a 
time, even on through-hole chips.  If you get too much on, wick it off 
with some braid.  Do the quad flat pack chips in the same way.

> Reliability: I know that repairing a through-hole board is easier for
> most people, but is the reliability of SMT stuff on par with TH?

Typically reliability is about the same.  Many of my real genuine 
repairs (ie. a hardware fault, not "finger trouble" or misprogrammed 
equipment) come from replacing ceramic caps that have gone leaky - SMD 
and through-hole fail at about the same rate but SMD is far easier to 
replace.

Getting SMT chips off isn't as easy but if you're certain the chip has 
failed you just cut its legs away and wick them off with braid leaving 
the pads nice and clean.  You can get special hot air rework tools with 
nozzles that fit over the chip shielding it from the heat and directing 
hot air onto the legs - this allows you to lift a chip off and solder it 
back down in seconds.

> Storage: Those SMD resistors are tiny! What are some good ways to
> store 30-40 different values so they can easily be grabbed for
> populating?

They come in strips or reels of tape with little pockets containing 
individual parts.  Write what the part is on the end of the reel in 
marker pen.

> Toxicity: Soldering irritates my asthma, although my fume extractor
> helps heaps. Does sticking solder paste into an oven cause similar
> fumes?

Again, I don't use solder paste and don't see the point outside 
production work.  As with all soldering you should do it in a 
well-ventilated area.  If your fan is a bit brutal it might blow some of 
the very tiny components around, so be careful ;-)

It's not synthesizer-related, but the Kanga Finningley software defined 
radio kit is all SMT except the sockets.  It's an excellent first 
project, with everything laid out really clearly in the instructions. 
It's worth even just grabbing the PDF of the assembly instructions off 
the site for a read.

--
Gordon JC Pearce MM0YEQ