Designing Your Own Printed Circuit Boards
take advantage of our piggy-back
offer
See also:
PCB Layout
Useful PCB Data
PCB Manufacture
PCB Assembly
The following notes will allow will allow
the hobbyist to generate a pcb file that can be included with our next
run on a piggy back basis, this is cost effective for you, and minimises
our cost per board by sharing the setup costs.
Your design
You will probably start with a handrawn
design on paper, that has presumably been tested and optimised with a breadboard
or a circuit simulator (such as Simetrix).
Collect all of the product datasheets and pinouts, and verify that the
components you intend using are available in the package you want at the
quantities you want (note also that there can be as much as a 4:1 cost
difference between a DIP IC and a SOIC IC). Now assign numbers to each
component (from left to right is usual)
Your BOM You
next need a bill of materials listing all of the components used and the
correct packages. Tag each component in the list with the package name
exactly as used in your PCB layout package (eg 1206,SO8,TO-92)
Your schematic
While it is possible to draw a scematic
on paper only, a more professional result is achieved by using a computer
program. For beginners, the easytrax software
is free and easy to use.
First you need an approximate idea of
where to put everything on the schematic, sketched on paper. Normally with
the inputs on the left and outputs on the right.
Start by placing the input and output
connectors, then the IC's, then the passives, finally join the traces.
Note that 0.1" connectors would normally
have pin1 at the bottom when on the left side of a pcb, and pin1 at the
top when on the right side. So if you want your schematic to match your
pcb, it will need to have some of the connector symbols flipped (e.g those
for the pcb left side).
It also helps to use a consistent pin
assigment, e.g. I always use pin 1 as ground.
Next add in the components, starting with
the IC's.
Add some text notes to the schematic,
so that in two weeks time you will know why you did certain things in the
schematic.
Common problems:
Wires go into the component but don't
actually connect to it, best way to check is to drag the component and
see if the wires move with it!
Wires that are meant to join just crossover,
verify by highlighting the net
Wires that crossover get accidently shorted,
verify as above
Mispelt netnames eg "ground" is not "gnd"
, likewise "+15v" is not "+15" ,verify by highlighting the net
Power supplies not connected everywhere,
verify by highlighting the net, and check each IC has power connected.
Wrong package label e.g TO-92a instead
of TO-92c
.
Your BOM and netlist
Get the schematic editor to print a BOM
and check it against your handwritten one, also note the name and directory
that the netlist was saved to. If you make corrections to a schematic ,
ensure that the corrected netlist is in the right subdirectory.
Your PCB layout
Again it helps to have a pencil sketch
drawn of approximately where things belong, and even better if you can
plug all of your components into a piece of veroboard to see how they fit.
Note that connectors, inductors and electrolytic capacitors seem to take
up more room than expected. Import the netlist, and turn on the ratsnest
. Define a board area about 50% bigger than you think you need, place the
connectors first, then the IC's, and the passive components last (some
PCB editors will even "autoplace" these components). Compact the layout
at this stage before routing tracks. Next route the tracks, Try to keep
the top layer horizontal and the bottom layer vertical. Double check that
the traces on the IC's, connectors and transistors go to the pins you expect.
Some tips:
-
use a 0.100" visible and 0.025" snap grid
-
Try to get all the IC's inline with all
the index marks facing the same way
-
Try to place passive components in neat
groups of about 4 components
-
Place the power supply tracks first before
using "autoroute"
-
Connectors on the left edge will have
pin 1 at the bottom generally
-
Place the "fills" last.
Common problems: (the design
rule checker will pick up a lot of errors)
Hole size is smaller than component lead,
especially on connectors, relays, fuses , TO-220
Forgot the mounting holes!
A track almost reaches the pad, but the
gap is hidden under the overlay
Connectors facing the wrong way
Ribbon or IDC connectors may be numbered
two ways: zig zag (ribbon order) or down one side then the other.
Having a via or pad short through to a
ground plane fill.
Not allowing a thermal relief around pads
on ground plane.
--under
construction --
Piggy back boards :
For reasons of cost effectiveness,
PCB's are usually manufactured in large panels (10" x 16"). In order to
provide a rapid turnaround to our customers we often need to have PCB panels
manufactured without filling the entire panel. The remaining space is sometimes
filled with prototype boards. We are happy to offer this spare space to
electronics developers and hobbyists on a standby / at cost basis. The
standard process we use is double-sided, plated through holes on fibreglass
material, either 1/32" or 1/16" thick, green soldermask both sides, white/yellow
top overlay.
-
Conditions.
-
pcb files must be created
with Protel Autotrax or Easytrax.
-
minimum qty normally 4
boards
-
specify required thickness
1/32" or 1/16" or "don't care"
-
normally less than 30holes
/ sq in
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minimum track size 10thou
-
minimum track spacing
10thou
-
cost $1 per sq inch +
postage (typically $3)
-
board boundaries to laid
in the "keep out" layer with 5 thou track, preferably on a 0.1" grid
-
you must own copyright
or be authorised to use the pcb
-
drill size 0.6mm min to
3.2mm max and converted to nearest 0.05mm size
-
Use the following layers
"top", "top overlay" , "bottom" and "keep out"
-
Your boards will normally
be included with our next run, this could be upto 4 weeks away, we cannot
make any delivery guarantee.
email pcbs@cinereus.com
(preferably a week before your pcb file is ready) with details of your
requirements
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