[sdiy] moving to SMD/SMT

[Tony Clark]

   Hi John,

> Do you recommend a certain brand of resistor? Browsing through the
> Mouser catalog, three brands look pretty comparable: Koa Speer, Vishay
> (CRCW), and Xicon. These are all thick film, 1%.

   I usually buy Panasonics from Digi-Key.  As far as I know, the quality 
will be good from any vendor, so buy whatever is cheapest.  The 
difference may come in packaging (and using those parts in an assembly 
machine).  I know I do tend to see differences in how the tape/reels 
behave on my assembler, but that happens when you run thousands of parts 
in an hour.

> And what about caps? Using ceramic chips in place of ceramic discs,
> and tantalum chips for leaded tantalum seems obvious enough. Are
> aluminum caps the choice for electrolytics?

   Right.  A warning though, be sure to triple check their ratings!  
SMT electrolytics are NOT as robust as their thru-hole counterparts.  I 
highly recommend going one voltage rating HIGHER than your max expected 
voltage!
   In my first SMT modules, I used 16V electrolytics thinking that 
they'd be safe with 15V supplies (after all, the thru-hole ones worked 
fine).  But not so!  Evidently startup spikes lead to massive failures.  
So I now use 35V caps for safety.  No problems since.

> This all started when I found a neat DAC that comes only in a TSSOP
> package. I won't try to handle that until I've got experience with the
> "large, bulky" SMDs. ;-)

   Well if you only have one such small chip to deal with, I'd go for it, 
but definately practice up on other things first!

> By the way, do you make your own PCBs?

   You mean etch/plate them in-house?  Nope.  I let the professionals do 
that.  For the complexity of my PCB designs, it's much better to pay a 
board house to do them.  Even still, I spend a _lot_ of time on the 
design process to make sure that I chase out as many errors as possible 
before sending the files off!  It is a bit easier to screw things up in 
ways you wouldn't normally expect.
   One such way is to get the footprint of a part wrong.  With thru-hole, 
this isn't such a big deal, you can usually reform leads on parts to deal 
with a hole spacing problem.  With SMT, you don't get that luxury!
   Another issue is placement of parts.  If a machine is assembling the 
PCB, you can get parts really tight.  But if you plan to hand-assemble, 
be aware that the soldering iron has to get into every nook and cranny!  
I know I've had difficulty with my own boards, trying to solder in 
J-leaded trim-pots right next to electrolytics.  Doesn't matter which 
part you solder in first, the other one is a bitch!

   Cheers,

   Tony

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