[sdiy] SMT rationale? prototyping?

[Jay Schwichtenberg]

I agree with what Jim said about the cost of getting stuff manufactured is
the main reason. But that has influenced other aspects of electronics also.
For example once SMT based electronics is tested, packaged and leaves the
production floor most of it is not repairable. File it and go buy another
unit when it breaks, the manuFATures love that. Another thing is that a
number of components now days (especially in digital land like FPGAs,
processors, DSPs, memory) which you can only get in SMT packages too.

All those years my mom keep me busy as a kid doing arts and crafts paid off
here. My dad also taught me that having the right and good tools for the job
makes all the difference in the world. That applies here too. I'm more or
less a SW/HW engineer (vs HW/SW) and was dragged into SMT for work. After
having a PCI card designed with ~600 components on it and no support from
the hardware guys I ended up being the engineer and technician that got it
to work. Fortunately I have about 37 years of soldering experience and have
gone from tubes to VLSI. It surprises me at how mainly EEs or even techs
that can't solder now days. They just take stuff they need done to the
rework people.

Like Jim said prototyping is a personal taste. I'll go down to Rat Shack and
get a $4 pad per hole proto board or use my solder less breadboard and do
stuff on that first. Not necessarily the whole circuit but enough of it to
make sure that most of it will work. Then I'll go and put the stuff on a
PCB. I've used expressPCB proto service for a couple of boards and like
their service for small stuff. $62 for 3 - 2.5" x 3.8" boards which you can
get a lot on when doing SMT. Only disadvantage is that you use their
software so if you want to go to another board house you'll have to redo
your layout with some real PCB software. Unfortunately I've been out of work
for quite a while and I'm waiting for a job before I tackle any big home
projects. expressPCB boards do not have a solder mask and have a generous
coat of solder on them. I can usually tack a part down without having to
tint a pad because of the amount of solder on them. With no older mask you
have to be careful when soldering parts because the solder will flow along
the traces quite easily. This can create bridges or unsolder parts that have
already been placed.

One key thing when doing any PCB is not necessarily getting the right value
of part on the board. Just getting the part on the board with the right
spacing/package. You can always go back and change the 1K resistor to a 1.5K
later but if you don't have the pads for the resistor you're going to have
to kludge something in.

If I'm doing something that is going to be assembled by machine I lay it out
different than I would if I'm going to hand solder it. With machine assembly
you can go for maximum density and smaller parts. When I have to solder them
myself I don't go smaller than 0805 parts unless I have to and I'm more
generous with spacing between parts. Around taller parts like electrolytics,
smaller parts like 0603s or temp sensitive parts I leave more space so I can
get the iron closer. A part like an 1/4" high electrolytic can block a lot
of area on a surface mount board when you try to get an iron to a part.

For people just starting out maybe use 1206 sized parts, thru hole
electrolytics/tantalum caps, trimmers, connectors and generous spacing would
be a good idea. I personally stay away from j leaded parts also since I have
a hard time soldering those.

Also when doing your circuit layout think of other issues. Make sure the
current going thru a part or on a trace isn't to much for it. We normally
use 1/4 watt resistors when doing thru hole which can handle most of the
current we use in EM circuits. But 0603 and 0805 resistors have a rating of
1/16 watt and 1/8 watt respectively. That may become a factor. There are
some areas where IMHO is not practical. For example you can get 2% & 5%
metal film caps in SMT packages but so far I haven't found any polystyrene
caps in SMT. The metal film caps I've used are very fragile and can melt
quite easily. So maybe thru hole would be a better choice here. Another
thing I wouldn't use SMT for is expo converters. I would use a thru hole
package for the transistor pair and the tempco. Reason here is thermal mass.
I'm assuming that the larger packages would have more thermal mass and thus
be less acceptable (less dynamic, change slower) to temperature changes.

Well maybe someday soon I'll get a job and be able to get that toaster oven,
some good boards and those DSPs I've been lusting after.

Happy soldering.
Jay

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of James Patchell
> Sent: Saturday, July 19, 2003 7:42 AM
> To: Paul Higgins; synth-DIY
> Subject: Re: [sdiy] SMT rationale? prototyping?
>
>
> At 11:49 PM 7/18/2003 -0500, Paul Higgins wrote:
>          Actually, stray inductance would be more close to it, but, the
> real reason for SMT is more related to manufacturing.  SMT parts
> are a lot
> easier to put onto a PC Board using automated machines than
> thruhole.  Getting a surface mount board built is a lot less
> expensive than
> a through hole board.
>
>          Prototyping is a matter of personal taste and means.  I
> personally
> would not do any less than laying out a PC board that would be at least
> professionally made at a PC board shop.  These days you can prototype PC
> boards for $33 each, or less (well, really $100 per order..).  For that
> price, in my opinion, other options are just not cost effective....unless
> you just don't plain have the means.
>
> >Forgive me for asking a (possibly) stupid question--I can't believe I've
> >never thought of this before!  Is the main rationale for the rise of SMT
> >parts--other than the increasing miniaturization of electronics in
> >general--the fact that stray capacitance that plagues through-hole parts
> >limits the frequency at which they can operate?  I just was
> poking around
> >inside my Mac, and it suddenly dawned on me (duhhhh...) that
> when you get
> >into clock speeds exceeding 2 GHz, you're gonna have some problems when
> >every pin looks like a few pF to ground.
> >Furthermore, I remember reading something recently about how the PCB fab
> >people aren't terribly interested in letting engineers know what those
> >stray PCB capacitances really are.  (The author was discussing
> this in the
> >context of long PCB trace runs and their effect on audio).
> >
> >What do people recommend for prototyping with SMT?  I imagine that most
> >DIYers probably don't have robotics stuffing their boards.  :-)
> I've seen
> >those SurfBoards which let you use SMT stuff with breadboards, not to
> >mention making the soldering of SMT parts to a board a bit easier.
> >The conventional wisdom on this list seems to be that they're a rip-off.
> >
> >There is a TI/Burr-Brown part I use that is significantly cheaper in
> >SOIC-8 form than as a DIP-8.  We're talking several $$$ cheaper.  The
> >SOIC-8 package doesn't look too horribly intimidating; I think I can
> >handle this.  However, when I got my Atmel AVR starter kits recently, I
> >freaked out when I saw those TQFP-64 packages!
> >
> >What do people recommend for soldering equipment, etc?  I have a
> >temp-controlled solder station that I've been very happy with;
> I'm pretty
> >sure that I can get SMT tips for it.  Do I just need some liquid
> flux (and
> >a magnifier lamp and a truckload of patience)?  I've been soldering for
> >about 20 years, but I've never touched SMT until now.
> >
> >Also, what brand(s) of solder and flux do people prefer for SMT?
> >
> >Thanks to all.
> >
> >-PRH
> >
> >Paul R. Higgins
> >email: higg0008 at tc.umn.edu
>
>