Digital Signal Processing Reference
In-Depth Information
signal integrity rules have been specified and that it in general conforms to good
engineering practice.
4.5
Fabricating and Assembling the Board
The printed circuit fabrication shop (the fab shop) is the manufacture that uses the
circuit board artwork designed by the layout shop to create the actual printed cir-
cuit board. Generally, the layout design shop will directly send the layout artwork
files to the board shop selected by the customer. Often the layout shop can recom-
mend a fab shop with which it has worked that is most suitable for a particular
design.
The boards delivered from the fab shop are drilled and punched to accept com-
ponents such as connectors and some electronic components, but the fab shop does
not install components. That task is the responsibility of an assembly shop or of a
contract manufacturer.
If requested, for controlled impedance boards the fab shop will supply the cus-
tomer with the test coupons they used to verify that they had achieved the target
impedance value. If the customer has access to a time domain reflectometry (TDR),
these coupons can be helpful in determining the actual impedance of a given lot of
boards. If requested, the fab shop can also provide test reports showing the trace
impedance and line widths.
Like layout firms and circuit board manufacturers, contract manufacturers
range in size and in capabilities. Some specialize in specific markets (such as aero-
space, automotive, or telecommunications), but the larger firms tend to work in
all markets. The largest contract manufacturers provide design and consulting ser-
vices, which can include schematic capture, signal integrity analysis, circuit board
artwork generation, circuit board fabrication, circuit board assembly (attaching
components to the circuit board), and electrical testing once all the components
have been attached (“final test”).
4.6
Alternative Methods for Design and Fabrication
Using an experienced layout designer specialist and a circuit board fab shop with
moderate to high-end processing capabilities remains the lowest-risk way to pro-
duce complex or high-performance test and evaluation boards. However, the cre-
ation of small test and evaluation circuit boards is now straightforward with the
proliferation of easy-to-use circuit board layout software and the increasing num-
ber of fab shops willing to manufacture circuit boards in prototype quantities. This
provides an opportunity for the signal integrity engineer to design these boards
without using a specialty layout shop. Small start-up companies and small fabless
semiconductor firms often find this approach particularly attractive.
The cost per board will be reasonably low if the boards have only a few layers
(usually 4 or less) and if the board area is no more than roughly 25 square inches
(160 square cm). The signal integrity engineer can still lay out and have manufac-
tured boards that are larger than this and that have more layers, but the price of
the CAD software necessary to layout these boards will increase, and fewer shops
