[gta02-core] Notes re: PCB fabrication.
Ron K. Jeffries
rjeffries at gmail.com
Sat Sep 19 20:03:34 CEST 2009
A friend provided these notes re PCB fabrication.
The PCB design should converge with the assembly house capabilities.
If the electrical engineer provides the pcb design house with a full
set of design rules, this would be the normal process if one is in
The netlist and BOM have components that will be on the final board,
but can the assembly folks deal with the component size, bga pitch,
mixed through hole and smt? Get a capabilities list from the assembly
folks, (how many fudicals/board or /bga). PCB finish such as emersion
gold or HASL must be specified.
Now, the pcb design will have gerber files as deliverables. The
assembly house should take these and do the step/repeat to configure
the panel (and panel drawing needed by the pcb fab folks); and order
the stencils for the solder paste. The panel drawing will have a
frame that fits their tooling, and limits the number of individual
boards. Also, it is necessary to specify if the individual boards
will be tabbed or if the individual boards will be cut to allow them
to be snapped when the panel assembly is complete. Consideration for
parts than extend beyond the edge of the individual boards are part of
the framing. Or for low volume, hand soldering and assembly may be
part of the job.
Outline of process:
1. Give the netlist and bom and mechanical .dxf file and design rules
to the pcb designer.
2. If no design rules, get the capabilities from the assembly house
and give this to the pcb designer.
3. Get the assembly house to review the placement and routing during
the check phase.
4. Get someone to do a panel drawing (step/repeat with frames); this
is needed before anyone will fab the pcb. The fab house will take
your panel drawing and step/repeat it on the most economical panel for
the job; they will use part of the panel for their process control,
and you won't get that on delivery. You will also send the
connectivity list (some standard??) to the fab so they know what is
open and shorted.
5. Order your boards from the fab, setup fees, quantity, and delivery
= yield price. Have the finished boards sent to someone for assembly.
Make sure to get them tested, that way they are all equal, and any
design issues will be easy to worked through. You will get panels
that may include boards that fail test and are marked to keep you from
putting parts on them and wasting money.
Your guys can do any part they want to, but you don't get the QC that
comes from an established process.
I got 3.5mil traces on 0.5 oz copper, trace width limits are an aspect
ratio to copper thickness situation. Trace impedance is a ratio of
width to height above the planes and K of the fiberglass; their 40mil
board requirement can have 50 ohm impedance and lots of layers. PCB
material comes with process limitations, and some fabs do not have all
the flavors, (lead free requires higher processing temperature
Ron K. Jeffries
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