Dr. Jolyon A. Browne asked (and I assumed it was no joke):
>I am about to embark on a new project involving PCBs. However, I have to admit
>that my closest association with PCBs has been adding the occassional card or
>two to my computer. I have several question regarding the physical
>characteristics of PCBs that I'm sure someone in this forum can answer. I
>realize that the answers to these questions may depend on the design and the
>specific application for the PCBs. But all the following questions pertain to
>your "standard" multilayer PCB.
Good luck. These answers based on PCB's built in the US for typical
computer and telecom applictions, normal 5V digital products, and the most
common "modern" material and fabrication techniques. This excludes exotic
flex circuits, hybrids, and "motherboards" for mainframe type computers.
>1. What is the range of width of the electronic traces on a multilayer PCB?
0.005 to 0.010" inches for signal connections, with wider traces for power
and ground.
>2. What is the range of spacing distances between electronic traces on a
> multilayer PCB?
typical minimum would be 0.005" with huge variation on the upper limit
depending on layout density. I would guess the industry average for minimum
(as designed) spacing is 0.005" to 0.010".
>3. What is the thickness of the traces on a multilayer PCB?
Internal traces would normally be "one ounce copper" which is 0.0014"
thick. External traces most commonly start with "half ounce" and are
electroplated along with the copper in the plated holes, yielding something
like 0.002".
>4. What material(s) are the electronic traces made of?
Annealed copper foil, with electroplated copper on external layers, and
typically "hot air solder levelling", or a fused coating of tin-lead about
2-40 microns thickness. HASL coating is present wherever the copper is
exposed to permit soldering, otherwise it would be coated with polymeric
solder mask over the "bare" copper.
>5. For a multilayer PCB of about 30 layers, what is the thickness of a single
> layer?
That would depend on the overall thickness of the board. By far the most
common finished board thickness is 0.062", with construction based on
cores, which are pre-laminated sandwiches of glass-epoxy and copper. The
minimum core thickness of a single ply core element is about 0.002", but
this is a somewhat exotic construction. The more conventional constructions
would result in 0.006" to 0.008" separation of the copper cladding or foil.
A 30 layer board represents a very exotic product which would be 0.200" or
more in thickness using conventional drilled and plated via connections,
and the registration of the layers in lamination would be quite difficult.
It is indeed possible to produce such a board, but only with the most
advanced materials and process methods.
>6. What material is the circuit board made of? What type of glue is used
> between the layers?
Woven fiberglass ("E" glass), with multifunctional epoxy resin binder. Tg =
130-140 C. Typical resin systems have large amounts of polymerized bromine
additive for fire retardency. There are other several types of resin in use
for special applictions, but probably 95% of the market is glass-epoxy.
>7. What are the typical dimensions of a multilayered PCB?
Well, Doctor, as a generality, I'd say something like 9 x 6 x 0.062" is the
average. You would probably never see one larger than 18 x 24" in length &
width, and not larger than about 0.125" in thickness unless this were a
very exotic board for a supercomputer. The question is difficult to answer,
you may anticipate huge variations depending on application. Most circuit
boards are fabricated from "panels" which are 18 x 24" in size, and routed
to final dimension based on the design. Normally, there would be 4-6 units
in a panel as fabricated.
If you are located in Sunnyvale, there are many fine board fabricators in
your area. If you ask the right people, you could arrange a visit to speak
with experienced people who could go into more detail.
regards,
Jerry Cupples
Interphase Corporation
Dallas, TX USA
http://www.iphase.com
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