Homebrew PCBs

Richard wrote:
>  
>
> Hi Derk!
>
> ....mmmmmmmmmmmmm , well, first let me say that your thought process 
> is so powerful, I will have to assume that I fatally missed something 
> in your rational.
>

I guess so. Maybe because my unfortunate use of the term 'arbitrary'. 
Although with my suggestion you could position components arbitrarily 
because software could be written to do the trace printing (component 
locations are taken from the scan), it, of course, does most of the  
time makes no sense electrically/physically, as you pointed out (This is 
self explanatory and I implied it). So you should still do a well 
thought out component layout, but manual placement does not need to be 
as precise as you suggested (making a hardcopy of the layout, put the 
transparent adhesive foil on it, using a vacuum tool etc etc..). That 
should make it easier for quick prototyping in the home lab - wasn't 
that you initial idea?? It also would avoid mistakes in the component 
layout because the trace printing function gets its information from the 
actual physical scan of the components in place.
Hope that makes my point a little clearer

Now, talking about high speed components and capacitors - Besides trace 
resistance, the dielectric properties of the insulating ink and it's 
thickness, which I assume is rather thin (significantly thinner compared 
to multilayer pcb's), will sure have effects like coupling and unwanted 
filtering. Granted, this problem is known for PCBs as well, but it is 
understood to a degree.

What's about heat build up in the epoxy pot?

Another disadvantage, although it probably only plays a role for more 
complex circuits, is, considering you suggested this method for quick 
prototyping,  the fact that it's going to be very difficult to access 
the circuit for probing (even many component pins might not be 
accessible anymore) and  replacing or adding components is absolute 
impossible. For me, these are both things that are extremely important 
in prototyping. As a matter of fact, that's the whole idea of a 
prototype. Spice and breadboarding just don't give you the whole picture 
with increasing frequencies. Wouldn't matter so much for a final 
product, but with that I would go to some pcb manufacturer anyway.

The last point actually would pretty much rule this method out for me, 
even if it would work. I just wouldn't see a place in my development cycle
(design on paper, component selection, simulating w/ SPICE, 
breadboarding if necessary and/or possible, prototyping, probing and 
refining, finalizing).

I still like the idea though, maybe just as a thought experiment - or 
maybe for some special cases I could see an application.

Just some thoughts, Derk








>
> I'll respond briefly on this in hopes I'll get to better understand 
> the merits of your suggestions.
>
> In prima fasia, this would be a complete disaster. Placing the SMDs in 
> a "random" fashion, would seriously violate essential physical and 
> electronic associations that the components function provides.
>
> Example: With the very high speed microchips we use today, very heavy 
> use of decoupling and bypass capacitors must be used and must present 
> themselves IMMEDIATELY (asigned placement) on the IC pins they afford 
> their function to.
>
> "Random placement" of these components would folly at best.
>
> --- In Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>, Derk Steggewentz <derks@...> 
> wrote:
> >
> > Richard,
> > I am following the thread. because I like the creative
> > approach.Regarding accurate placement: Why don't you just place the
> > components arbitrarily and after potting and peeling of the tape you
> > scan the side with the exposed pins. With a piece of software (to be
> > written) the user then can interactively identify the components on the
> > image. With this information and the schematic the software has all the
> > information to do the rest, including printing.
> > Although I can see a few problems with the general approach I think 
> it's
> > worth following.
> > Derk
> >
> >
> > Richard wrote:
> > >
>