[time-nuts] PCB design questions thread II

[Bruce Griffiths]

christopher hoover wrote:
> John Miles wrote:
>   
>> For one-off PCBs, I've had good luck with www.batchpcb.com .
>>     
>
> I agree.  I've used them once and have been happy with the
> results, you just can't be in any hurry.  Unfortunately, 
> there's no indication of how long it will take a priori.
>
>   
>> SMD is not hard to work with by hand, down to 0603 or thereabouts
>> depending on eyesight and/or equipment.  I find it easier to deal
>> with than through-hole, frankly.  
>>     
>
> I agree with the first part, but I actually find SMD to be easier
> than T/H, if you stick with 0603 and larger for the passives.  I
> find SMD not only easier but quite bit a faster.
>
> SMD IC packages are easy once you get the hang of them, but a
> microscope is needed for inspecting and fixing fine pitched parts.
> (I think this is a bit of surprise to some folks.)
>
> Personally I *hate* turning boards over and clipping leads.
>
> And I'd much rather layout SMD than T/H.
>
> -ch
>   
However for low frequency work the low thermal mass of resistors and 
opamps in smt packages can be problematic.
Also SMT packages are more sensitive to board deflections and vibration.

Using guard rings with some SMT parts is difficult to impossible.
You no longer have the option of directly connecting a leakage sensitive 
lead to a virgin teflon standoff.

Mixers and phase detectors with dc and low frequency isolated grounds 
for the IF and RF ports dont appear to be available in SMT packages.

How do you cope with SMT parts (eg high frequency ADCs) with metal 
thermal transfer /ground connections under the package itself?

Bruce