Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

Kieth,
There are a lot of variants to the process.  All the way from writing a speck of what the board does and its requred form factor and letting the contract house do everything necessary to deliver you tested and working boards all the way down to just getting a bare board fabricated from your Gerber file.   I have always worked for a company that had an inhouse PCB department.  In the early days before CAD you could even hand them tablet paper sketches of your schematic and they would draw it up on D size vellum for your approval.  Usually the company had an in-hous parts list to choose parts from.  If you had a new part the components group would have to research it and add it to the approved parts list.   Most engineers would do their own schematic drawings because redrawing from sketches was error prone.  You would submit a schematic, BOM and some mechanical and get back a board weeks later.   With the introduction of CAD things got much easier with less errors and shorter correction cycles.

Choosing the parts for your design was always an issue and as important as your schematic.  A board that can't be assembled because the parts are unavailable, don't fit, are too expensive, or of incorrect parameters is worthless.  I always find this is the most time consuming part of board design.  In the early through hold days before there were global foot print specks all the parts were different, you designed in Mfg A's part but Mfg B's wouldn't fit.

Now with SMT a good percent of the foot print issues have been mitigated with good global specifications.   But it still takes a lot of time to pick the correct part.  Lots of things to consider, is it correct electrically, is the size what you need, is it available, what is the production lead time for quantity, how is it packaged --tape and real, trays, loose -- the assembly folks need this info.

There is no easy answer, it all depends on your process flow. 

For my home boards that I get professionally fabricated.  I draw the schematic using my own standard library of parts I have built up over time.  This way I can buy a real of 1000 caps at the same price as buying 50 pieces and keep using them on multiple designs.   For new parts I spend hours and hours on Digikey and others finding just what I want with the specks I need, with the package I want at a cost I can accept and that is available.  Nothing worse than finishing up a board layout and finding out DigiKey is out of stock and the lead time is 20 weeks.  Then I make sure I have all the correct footprints for the parts I am using.  Only then do I layout the board.   The footprint and schematic symbol management is another reason to  build your own set of standard parts to use.  That way you get to check these items once and don't have to be constantly reinventing the wheel making new parts.   Once the board is finished I submit the Gerbers to the  PCB fabricator or  make it my self (doing much less of this).

I have not had any home boards comercially assembled yet.  Most of the PCB fabricators have assembly capability or have partners that can provide this service.  If thinking this route the first thing would be to contact them and get their requirements.   I would guess most will let you supply the parts or will buy them for you based on your BOM documentation.  Keep in mind its going to cost you a lot more if you give them ziplock bag of parts vs  providing the parts on reels for the pick and place machine.  The volume of boards to be built is also a huge item to consider -- normally doesn't make sense to program the pick and place for one board unless you have parts that must be machine placed.

So my flow goes like this:
 High level your design -- pick your parts --check parts availability -- build your schematic symbols and footprints -- draw up the schematic -- layout the pcb  -- check the net list for sanity (catches symbol or footprint screw ups) -- review the gerber -- RFQ the board and get it on order -- place and solder the parts -- test the board. 

Craig