PRINTED CIRCUIT BOARD FABRICATION
BASIC LAYOUT INSTRUCTIONS USING OUR ROUTING MACHINE
I.
Electrical considerations:
·
Voltage break-over:
·
15 mils (.015 inches) = 50 Volts (safe distance for 50V different potential
between traces).
·
25 mils = 150 volts
·
50 mils = 300 volts
·
Current through trace:
·
25 amps/inch of trace width for one ounce copper (.0014 inches thick) and
directly proportional to trace thickness. (50A/in for 2 oz, 12.5 for 1/2oz).
·
Example: A trace of 1oz copper, 15 mils wide, can carry .375 amps.
(1 inch/25amp = .015 inch/x amp) (solve for x).
·
Resistance of trace:
·
R = RsL/W where: R = Trace resistance in ohms.
Rs = Sheet resistivity for copper and equal to 5x10-4
ohms for one ounce copper an inversely
proportional to trace thickness. (2.5x10-4 for two
ounce, etc.).
L = Trace length in inches W = Trace width (inches)
·
High frequency circuits:
·
External grounding (put circuit in an electrically shielded box).
·
Internal grounding (put ground planes around all circuitry).
·
Get some outside help to figure impedance’s, dielectric constant, etc. if
you haven’t had the necessary e-mag background. (Note: dielectric
constant for epoxy fiberglass is 4.9).
·
High impedance circuits or amplifier circuits:
·
Use point to point grounding.
I Mechanical considerations:
·
Board can be single or double sided. However, we can’t make plated through
holes. Thus, it will be necessary to solder a wire from top to bottom of hole if
making a double-sided board. It takes a lot less time to make a single sided board,
so if possible layout as a single sided board. Board size can be anything up to 15
X 17 inches). We will supply the board and charge your budget. Fill out a
purchase request form so I will have all the necessary information.
·
Nothing heavy such as transformers should be mounted on boards (mount in the
end assembly case and jumper wires onto the board).
·
All I/O (input/output) should be pads or connectors placed along any board edge.
The EE391 library doesn’t have any connectors, however, you can find some in
the Eagle libraries (keep in mind they must meet minimum dimensions ).10 mil
spacing is the min dimensions between anything, also min trace width. (A 6mil
spacing could be used in very special cases, but you would have to see me first).
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Use a 15 mil or larger spacing if at all possible since there are milling problems
associated with using the small milling cutters.
·
fasten down large components such as large capacitors with a strap or glue
(silicon rubber or bathtub caulk works well).
·
Use stranded wire for all I/O. There will be a box of wire in the senior design lab
or see the technicians in 727 for some.
III Layout considerations:
·
Know part size and shape (don’t forget heat sinks!).
·
Work out best routing on scrap paper (break the circuit down into blocks if it’s a
complex circuit and route a section at a time. Don’t forget to label all the
components on the schematic).
·
Generally it’s best to route the positive power traces to board center and the
ground traces around the outside of the board.
·
Observe the following guidelines for the component layout (Tplace layer):
·
All components parallel to board edge.
·
Only one lead per hole.
·
Symmetrical lead length.
·
Nothing closer than .1 inch to board edge.
·
Use components to jumper over existing traces.
·
Use a by-pass cap for each IC and place close to the IC (.01 to .1 uf).
·
Label pertinent polarities, I/O, etc. Also, put name, date, and title on the
component layout.
·
Observe the following guidelines for the artwork or bottom layer of board:
·
Avoid acute angles.
·
No overlapping pads.
·
Traces to center of pads.
·
Be consistent with corners. Make all 45 degree or all 90 degree.
·
Good symmetry by keeping things evenly spaced. Use board real estate
evenly across the board. Don’t squash a lot of circuitry into one corner of
the board leaving large empty areas on the other side.
·
Go into oblong end of IC pads (if possible).
·
Label with name, date, and title along with pertinent polarities, I/O, etc.
Label anything you want to come out in copper.
·
*Observe minimum dimensions: (Extremely important!)*
·
Min trace width is 10 mils (Trace width is first determined by current flow
needed - 25A/in for 1oz copper. 12.5 for ½ oz copper). I will use ½ oz
copper unless instructed otherwise. If low currents, the trace width is then
governed by minimum dimensions, so for low currents it should be at or
above 10 mils wide. Minimum distance between anything is 10 mils. I
prefer to keep dimensions at 15 mils or greater if space allows.
·
Pad size. Depends on the size of the hole drilled. Rule of thumb: annular
ring should be as wide as the feeder trace. We have the following drill
sizes: (.0098, .0180, .0236, .0315, .025, .032, .0394, .040, .047, .0472,
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·
IV Eagle:
·
·
.059, .0591, .065, .086, .0866, .1181, .125 mil diameter drill bits) . You
will have to use one of these sizes. If you pick a component from one of
Eagles libraries, verses the EE391 library, that has a different hole size,
you will have to modify that library to comply with the drill sizes we have.
See separate Eagle library modification instructions.
Note that it is always better to design above minimum dimensions if space
will allow. A good overall trace width is 20 to 40 mils. A good overall
pad size is 100 to 120 mil diameters.
After component placement and routing on scrap paper, draw up on Eagle on the
machines in room 723. See separate instructions for Eagle. Try to use
components from the EE391 library if possible. If you use parts from one of the
Eagle libraries make sure they meet minimum dimension criteria and have hole
sizes to match the drill sizes that we have (.0098, .0180, .0236, .0315, .025, .032,
.0394, .040, .047, .0472, .059, .0591, .065, .086, .0866, .1181, .125 mil diameter
drill bits).
After drawing your circuit in Eagle, bring a copy of the Tplace layer and the signal
layers (bottom and /or top layers) to John in room 727 for review. I will also need
a copy of your board file (filename.brd). Make sure you included your name, email, and team number or account. Let me know the minimum dimensions you
used. JM 9-28-04
Note:
For Mike’s Class just indicate “Mike’s Class” under account number.
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