[sdiy] components, smd and pcb

[Tony Clark]

   Hi Juergen!

   Glad to see that others are joining the SMT bandwagon!  As some of you 
might know, I've been working with SMT for over 3 years now, from both a 
business standpoint and from personal DIY projects.  So I'll add some of 
my own insights and experiences to the discussion:

> The background is that I start to make pcb layouts again after
> many years of veroboard-only. So he told me if you want
> to make your boards as unexpensive as possible, then use
> a mix of SMD components and ordinary components.

   It is nearly impossible to use 100% SMT on any project without 
resorting to expensive components or parts that are in general too 
difficult to solder (or rework) by hand.
   As someone pointed out, there aren't many good SMT connectors.  There 
are some nice ones if you are dealing with very small boards and tight 
spaces (ie cell phone manufacturers), but those parts have such fine lead 
pitches that it's nearly impossible to solder by hand.
   There's a very good reason that robots are used to assemble SMT boards!

   Beyond connectors, there are many transistors and discrete parts that 
do not have SMT equivilants.  Many high quality capacitors cannot be 
found in SMT form, although there certainly is a better selection now than 
even 1 year ago (can we say SMT Micas?!).
   So the best bet is to always plan on using mixed-technology when 
involving SMT components. 

> ICs should be SMD, resistors should be ordinary.

   I don't agree with this.  Even when considering the fact that you may 
be limiting yourself to a single-sided board, there are many 
times where a SMT resistor makes sense.  You can put a SMT resistor right 
next to the input pins of op-amp chips.  For high-gain/low-noise 
situations, this is ideal.
   The only disadvantage to using SMT resistors is that they do need to 
be sized accordingly to the current they must carry.  0805s (1/10W) are 
probably  the smallest anyone should attempt, and I'd recommend staying 
with 1206 (1/8W) or 1210's (1/4W) for anyone who doesn't have a good 
steady hand and a very good soldering technique.

> The PCB can be single sided. SMD components go on solder
> side. Normally you *must* make double sided pcbs (if not
> multilayer) when you use SMD, but with a little patience you
> can replace the 2nd copper side with resistors.

   I don't agree with the multi-layer PCB being necessary.  Those only 
really become a requirement when certain high pin count packaging is 
used (BGA, PGA, PLCC, or Quads).  But in most cases where SOIC chips are 
used (or at worst, SSOP), then two-layers is quite sufficient and best to 
use, especially if a board house is doing the manufacturing!

> * Resistors ar standard (can easily read the value)

   Any SMT resistor 0805 or larger has the value printed on it.  I find 
it fairly easy to deal with.  Non-polarized capacitors, as well as most 
(if not all) SMT transistors, do not have any sort of markings on them 
and must be carefully kept separated from other like components!

> Cons:
> * As the SMD parts are on "the other side" now, I have
>    tothink in mirror image when I route the traces.

   Why not keep the SMT parts on the "right side" and put your resistors 
on "the other side"?  It might make it easier to keep track of things?

   Personally I enjoy what SMT allows me to do and would love to see more 
people working with it.  It's true, the technology isn't going away, and 
it does give you access to parts that aren't available in any other 
form.
   One question, Juergen, will your boards be compatible with the 
originals?!  I would be _very_ interested in that!

   Cheers,

   Tony

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