The Value of Data Sheets and Application Notes
Understanding the information provided by the manufacturer allows
the designer to be more proficient in the board layout process.
In
the design world today, it is up to the designer to be the master of
the board layout, just as the engineer is the master of the circuitry.
One of the ways a board designer can exhibit that mastery is through
quality placement and routing techniques gleaned from reading and
understanding the manufacturer’s data sheets and application notes.
In
the past, designers used data sheets for gate and footprint information
and little else, rarely reading the application notes. They relied on
the engineers to explain components and/or the circuitry, and to call
attention to any placement or routing criticality. But today, the role
of the designer is more important due to the increasing complexity of
signals, the layer structures, the changing board materials and a
variety of manufacturing issues. Designers must know how the signals
might react on a particular board and how to control them. They must
understand how one seemingly insignificant issue might affect other
design issues on the board – all items that the circuit engineer may
not fully know or understand.
Manufacturer
information is changing just as designers’ needs are changing.
Information is included today that wasn’t necessary, or wasn’t
available just a few years ago. This information includes signal rise
and fall time, source and load information of the pins, impedance
issues, placement insights, RoHS issues, assembly information, and
more.
Knowing the rise and fall time of components
is much more critical now, because signal times are much faster and it
takes a great deal more knowledge and design effort to route signals
correctly. We cannot rely on clock speed to tell us what we need to
know. Attending any high speed design class will explain why control is
necessary, even on a slow clock speed board. Unfortunately, this
information is not always listed in the manufacturer’s data sheets or
application notes, although this would be the first place to look for
it.
How can a designer place components if they don’t
know what component may originate a signal? How can they route a board
if they don’t know if the signal requires critical routing techniques
or stub length control on their particular board? The pin assignment
found on the data sheets should be incorporated into the library parts
and defined as source or load (output or input) while also defining the
function of a signal as a bus, a clock, enable, read/write, etc. This
gives access to the information needed during the layout and ECO
processes. “Slow” boards are more forgiving of poor placement and
routing techniques, but as all manufactured parts are getting faster,
it is a good idea to design boards with these considerations in mind,
even if they are irrelevant today, as any later repairs to the board
will invariably use faster parts.
Manufacturer data
sheets sometimes include impedance specifications needed for a bus or
signals. This information is extremely important to signal integrity
and should be followed and incorporated into board design as closely as
possible. The method used to obtain that impedance is less important
than that the requirement be met. So if the manufacturer’s information
specifies a particular trace width, thickness, spacing, etc, it should
be determined whether the requirements can be implemented on the board.
Often it will not, and the designer will need to calculate how to meet
the impedance required using other methods.
Bus
length matching information may also be included in app notes. The
allowable length skew (or arrival time difference between signals) is
determined by the receiving part on the bus, and is typically 20 to 60
psec, which equates to about 0.100 to 0.300 inch. So again, the
information on the data sheet should be carefully considered for
accuracy if it gives an arbitrarily high tolerance of +/- .050 inch.
Designers should prefer information expressed in time as opposed to
length, because signals travel faster on outer layers of the board than
inner layers.
Placement and routing of parts based
solely on information given by the manufacturer causes some controversy
but is still worthy of consideration. While the information may not
take into account all the specific issues on a particular board, the
designer should read and understand the manufacturer technical data to
understand the requirements of the part. If the information does not
seem valid for a particular design, a discussion with the circuit
designer, manufacturing, or test and repair people may be in order.
The
designer must also incorporate new manufacturing issues, including RoHS
compliance information, into the parts and board layout. These issues
may include, but not limited to, solder mask type or size required,
solder paste type or stencil thickness, squeegee pressure, paste
opening size and shape, step-stencil specifications, cleaning
information, thermal cycles, and wave and reflow soldering information.
Designers
must read and learn to incorporate technical data and accompanying
design requirements to become the board layout masters needed today.
Whether a new design is complex, wide open, or slow or fast, the signal
integrity and EMI (and thus board performance) can be affected by how
components are placed and traces routed. It is vitally important that
the designer understands the information provided by the manufacturer
and incorporates any applicable data into their board layout. PCD&F
