[sdiy] pros and cons of SMT in DIY

[charlie wallace]

a few of us are putting together 400 boards this week/end, it is
mostly smd 0805 with lqfp (which i personally dislike but i was
outvoted, i prefer qfn since it solders better)

we have a stencil printer
http://www.ebay.com/itm/332130477725?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
it is not great but once you get it dialed in it works, in the past we
just put hinges on a piece of MDF and the stencil, then cut out an
outline of the pcb to be made , and use those as guides

the stencil comes from the pcb people or we laser cut or cnc one out,
these are generated from the tcream layer in eagle, with cheap ebay
40w laser i use the engrave method rather than cut , it gives better
edges; if i cnc then i use brass shims, usually 3mil and i schrink the
 tcream a little first.

the paste is applied using a thin spring steel metal scraper that i
used for clay sculpting, it works the best of anything i've found so
far, i use aim leaded water soluble solder the no clean stuff is no
good for these boards, and its a nightmare to clean if you have too

then to a small pnp the neoden tm220a, though wish i'd gone for the
240a at the time (bought by generating litecoins with old gpus a few
years ago) there is a nice ulp dp-tm220a.ulp that handles it, once its
up and running it does pretty good but not a lot of components can fit
it, we have a modified juki as well but its in storage.

once as many parts as the pnp can place, the board goes into the
kitchen and the others place the few smd components, and then it goes
on to the madell hot plate, for a 170o cycle, after the thru hole
parts are added with the metal under either the amscope microscopes or
the mantis (all ebayed or bought from amscope directly during sales)

then the board is de-briged if needed, usually the lqfp arm chips
since this time i let the board house do the stencil gerber and they
didn't shrink it.

there are about 60 parts placed, the good thing about smd is that part
placement only has to be pretty close, the surface tension of the
solder will pull the parts in (usually)

with two of us we can do about 10 boards an hour at a relaxed pace,

qfn's i like since you can practically throw them onto the board and
they'll self right, and they tend not to bridge, we did a test where
we could see how lax the part could be placed and it'll still solder
in properly.

smd size doesn't really matter if you're hot plating, once you get
down to like 0201 and such its all about magnification and a steady
hand, and if you don't have that then use a manual xy pick and place
tool, or an automatic one. manually soldering them is also straight
forward you can glue them in place then solder if you're not steady, a
small tipped iron with the right temperature it goes well , takes a
little bit of practice on how best to hold them and avoid tombstones
if not glued, again you can also use the xy tool to place and hold a
part while you hand solder it.

we've taught 100s of people to smd aver the years, some have never
even picked up an iron before and they're placing chips, 0602s with no
bother , though occasionally people have soldered chips upside down
before ,another good argument for qfn

basically you just need a good iron, good microscope (amscope rp type
is fine) i'd avoid most usb microscopes since too much lag so hard to
solder and stick to glass, but you can $5 a playstation eye , mod the
lense and and get an ok picture at 180fps, flux and a 10/5ml tube of
solder paste.



On Sat, May 20, 2017 at 8:15 AM, Richie Burnett
<rburnett at richieburnett.co.uk> wrote:
> Paula, when you say "working with 0402", do you mean replacing the odd
> component here and there, or soldering 100s of SMT parts on?  And how are
> you actually doing the soldering?  With a fine tipped soldering iron and a
> steady hand?  ...heater tweezers?  ...or stencilling solder paste onto the
> board then placing components into the paste and ovening the board?  Just
> curious what methods you're using, particularly to get consistent results
> with those tiny QFNs.
>
> I can also vouch for the structural integrity of super-gluing down
> wire-wrapping wire or polyurethane enamelled copper (magnet wire) for mods
> to PCBs.  Never seen one fail, and they're almost impossible to rip up if
> you need to.  I've never felt entirely happy with the fumes given off when
> someone has to solder in an area of the board polluted with dried
> Cyanoacrylate glue though.  I remember doing this once and leaping backwards
> with my eyes and nose stinging!  Can any of the chemists on this list
> comment on what vapours are liberated when a hot soldering iron touches
> cured Cyanoacrylate glue?  (I appreciate that I should already have been
> soldering in an area with positive airflow away from me, extraction to
> outdoors, etc...)
>
> -Richie,
>
> -----Original Message----- From: paula at synth.net
> Sent: Saturday, May 20, 2017 3:45 PM
> To: cheater00 cheater00
> Cc: Synth DIY
> Subject: Re: [sdiy] pros and cons of SMT in DIY
>
> On 2017-05-20 03:42, cheater00 cheater00 wrote:
>>
>> Yes, a TH pcb will be able to hold a daughter pcb just by the
>> through-holes, whereas SMD pads will not hold one. So you can easily
>> change topology using an extra pcb on a TH circuit but not on an SMT
>> circuit.
>
>
> You know you can use both SMT and through hole if you need?
> My current project for work has around 100 SMT parts, and 7 through
> hole, some of which are for PCB interconnect.
>
> At the end of the day, the choice is up to the designer.
> I'm really comfortable working with 0402 and QFN (3mm x 3mm) packages,
> and with modding them with mod wires if needed (for the record super
> gluing a mod wire makes it rock solid).
> I'm considering trying 0201 for some parts, just as a personal challenge
> :)
>
> So it's up to each person.
> There's no "right or wrong" or "this way is better", it's just
> different.
>
> Paula
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