[sdiy] SMT rationale? prototyping?

[James Patchell]

At 11:00 AM 7/19/2003 -0700, Jay Schwichtenberg wrote:
>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.

         And I have to agree with this...stuff you puchase in the stores no 
longer has any sort of "value" in it.  Not only is it not posible to repair 
it, you cannot repair it.  I found out that even if something sort of major 
happens to the V8 in my Ford Pickup (like say a rod bearing needs 
replacing), it cannot be done...although, at least the engine can be 
recycled....nothing is a lot of the electronic crap we buy these days even 
has any recycling value to it....sortof sad.

         Even the stuff that can be repaired...like the old Westinghouse 
Electric Fan I have (50+ years old)...it cost me about $400 to get the 
motor rewound....a new fan at KMart would have been about $30 (for the same 
size)....something is wrong here...


>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
> >
> >