A few guidelines for prototyping PCBs
Aug 1, 2007 12:00 PM, Zulki Khan President and Founder NexLogic Technologies
Inc. San Jose, Calif. www.nexlogic.com
Rapid prototyping in the electronic arena is a bit different than what it is for
mechanical devices. There are no special machines to produce functional boards
overnight. Prototype PCBs are built on production equipment. So it's important to
give manufactures a head start whether or not essential components and detailed
specifications are available.
Design and assembly arrangements might include making sure the right stencils are
on hand, preparing the necessary fixtures, creating the right thermal profile,
conducting BGA (ball-grid array packaged components) studies, and if applicable,
reviewing solder samples.
Design and layout
Component selection, the most critical aspect of rapid prototyping, demands using
standard and readily available components. Ordering components should begin as
soon as the bill of materials is frozen because component lead times can stretch from
8 to 10 weeks and more. Custom components sometimes require a 12 to 14 week lead
time.
An experienced procurement staff knows its options when critical components are on
allocation, unavailable for assembly, or have gone obsolete. It also has a
comprehensive listing of component sources, which reseller carries what particular
product lines, and is familiar with their delivery reputations.
Design-for-test (DFT) and design-for-assembly (DFA) principles play a major role
during the PCB design-layout phase. A careless evaluation of the potential DFT and
DFA issues upfront almost encourages adverse time-delaying consequences. For
example, some engineers avoid allocating sufficient test points for the highest
possible test coverage.
Test coverage is defined as the procedures applied to a given design resulting in
greater than 90% test coverage. All the important modules of the product should get
test coverage through the allocation of test points on the board.
Ideally, test points should be on only one side of the PCB to avoid using dual test
probes at top and bottom. Sufficient test coverage should also be applied to the analog
and digital sections of a mixed-signal PCB so that analog and digital sections can be
tested separately.
Of course, a clean noise-free signal is required for medical devices for proper
diagnostics and detection of diseases. Medical products, such as handheld
communications devices, increasingly use both analog and digital circuitry. In these
applications, analog circuits are high power, high current, and thus, inherently noise
creating. The noise can adversely affect adjoining low power, low-current digital
circuitry when partitioning between the two is incorrectly implemented.
Keeping analog and digital planes properly bifurcated keeps noises to an acceptable
level. The practice, a critical step in rapid prototyping, helps make products free of
noise and cross talks, and cuts debug time.
Also, digital signals are sensitive to noise because they operate at high frequencies.
Correctly placing and routing the digital section is important to completely isolate it
from noisy analog circuitry. Take the clock, for instance. It's the most critical section
in a mixed signal PCB design. So shielding must protect clock traces or critical digital
signals when they unavoidably run through analog circuitry. It's best to run a clock net
and then have a ground trace shield the entire path of the clock trace.
If rapid prototyping involves a high-speed, high-layer count PCB, the signal layer
should be tightly coupled to its adjacent planes. High-speed traces should be routed on
buried layers located between planes. Power and ground planes should be tightly
coupled together. Also, multiple ground layers are functional. The internal ground
plane that makes micro strip and strip-line configurations possible also inherently
shields and isolates signal layers. Thus, ground impedance decreases ground noise
and generates a clean signal, a critical requirement for medical products.
Also, be sure to fulfill the following objectives: Tightly couple a signal layer to
adjacent planes to provide a proper return path. Closely couple power and ground
planes to bury capacitance when needed. Route high-speed signals on buried layers
located between planes. This way, planes act as shields and contain the radiation from
high-speed traces.
When possible, add multiple ground planes because they reduce PCB ground
impedance and common-mode radiation.
Another major layout effort focuses on routing fine-pitch BGA, CSP, and QFN
components. It's easy at design to layout and route two or three mil traces between
two pads of a fine-pitch BGA. However, manufacturing those PCBs can be
challenging. For instance, a board with two and three mil traces can be over etched
which completely wipes out the traces and creates unwanted opens.
Correct BGA routing, for example, calls for a clean signal and signal return paths. If
working on a high-speed PCB design, implement short signal paths and have a precise
return path. That's important so that BGA packages with power and ground voltage
planes carry both signal and return paths. This board feature is extremely important
and effective at high speeds.
A PCB without a return-path connector has a potential problem. If it's not properly
grounded or the return path isn't equal or close to the signal path or the routing is
haphazardly performed, the result is a poorly developed return path and unclean
signal, creating problematic noise.
Some BGA packages use etched lead frames and have no return plane. Without
efficient ground planes, these packages inherently have high self and mutual
inductances. A clean and short return path minimizes or eliminates those inductances.
But the worst characteristic of lead frame packages is mutual inductance between the
leads. If there is a choice, switch a clear frame package with a BGA or CSP to reduce
mutual inductance within those package leads.
Line-to-line coupling is another aspect of BGA routing. If signal lines are too close to
one another and run in parallel at great lengths, then the signal from one line may
interfere with an adjoining one. After BGA partitioning and routing, the PCB designer
must make sure the traces don't run in parallel to each other to create coupling. Lineto-line coupling creates glitches that cause problems during high-to-low logic
transitions, and vice versa.
PCB Fabrication
Special consideration is given here due to the reliability, quality, and repeatability
requirements of medical products during the fabrication of rapid prototyping. Stackup
is of particular importance in high-speed designs because it ensures meeting a board's
impedance requirements. Impedance-controlled traces are required to keep the signal
clean, and suppress noise and crosstalk. Here, a trace is sandwiched between two
ground planes to suppress noise. Without properly defining stackup, finished traces
within the product may not get controlled impedance within the expected 5 to 10%
tolerance.
Stackups can incorporate single-ended, differential pair, multiple single-ended
impedance on the same layer, or multiple differential pairs on the same layer of the
board. A board-fabrication house must incorporate etch-back factors and other
compensations to hit the targeted impedance, which is calculated after defining layer
stackups.
Impedance-control calculations are performed at layout stages and then confirmed at
the fabrication house. Those calculations are needed to assure that the fabricator has
specified the correct differential impedance requirements. These impedance tests
could be single ended, dual strip-line, or micro strip. The purpose of these impedance
checks is to make sure all signals coming out of these components get to their
destination as cleanly as possible with minimal noise or cross-talk.
Differential impedance is calculated on the particular material, its characteristics,
thickness, and level of internal layers in relationship to the ground plane. Different
materials or laminates have different core thicknesses and characteristics, which play
a role in calculating impedance levels on different sections of the board.
Lastly, it's important to avoid unnecessary use of slots, non-standard cutouts, or
unusual shapes, which increases manufacturing time and runs counter to prototyping
quickly. It's best to use standard shapes for slots and cutouts. It's also worth
remembering that laser drilling takes more time than mechanical drilling, especially
for holes of five mils or less. In-house advanced capabilities like this as well as
automated visual inspection to check for opens and shorts in the internal planes are
vital for maintaining a tight rapid-prototyping schedule.
Assembly
For rapid prototyping, it is necessary to order fixtures, tools, and stencils before
assembly starts, probably as soon as the PCB goes to fabrication. The programs for
surface-mount pick and place components and CAD are generated at the same time.
One way to expedite rapid prototyping is to create a thermal profile based on a soldersample PCB from a fabrication house. This profiling sometimes save a half-day to a
day, in some cases, which could be significant for rapid prototyping.
Similar planning saves time when preparing the assembly and inspection
programming. Both should be generated ahead of time and be ready before the project
hits assembly. If not, surface-mount programming can cut into the assembly schedule
and consume a half to two days, depending on PCB complexity and component count.
ECAD programs create visual aids and color coded drawings which help meet a
medical product's demand for repeatability, reliability, and quality. Performing these
steps manually consumes inordinate time and defeats the principle of rapid
prototyping. Plus, performing these steps manually induces the high probability of
human error.
One more factor can save time in the assembly arena: auditing the kit prior to getting
PCBs from a fabrication shop. This ensures that all the correct components are ready
for release to the manufacturing floor and all relevant questions have been answered.
Even if the kit isn't 100% complete, a largely SMT-populated PCB can be released to
assembly as soon as all SMT components are on hand. If the project involves five to
ten percent through-hole components, for example, assembly can get a head start by
first placing SMT components and then adding through-hole components after SMT
components are assembled on the PCB