The Most Important 16 Steps On Microwave PCB Design

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16 Steps On Microwave PCB Design

• Printed circuit boards with high-speed chips and
  microwave PCB structures have numerous parameters
  that differ significantly from those of
  conventional, rigid and flexible printed circuit
  boards. These differences are explained in
  IPC-6018B, Qualification and Performance
  Specifications for Radio Frequency (Microwave)
  Printed Circuit Boards. High frequency is one
  of the three primary classifications of circuit
  boards of the IPC (the other two classifications
  are rigid and flexible circuit boards).

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Microwave PCB Design
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Special requirements Anyone who knows nothing
about the peculiarities of these frequency ranges
will first shake their heads over this chapter.
Because not only because of the losses that
occur other circuit board materials than at low
frequencies have to be used (very often based on
Teflon or ceramics, but also from new, specially
developed organic materials. 
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Microwave PCB Design
The components of microwave PCB look different
(the optimum at the moment is the use of SMD chip
components of sizes 0603 or better 0402, where
0402 means a size of 1mm x 0.5mm) and new ones
are constantly being added. In addition, a
completely different wiring technology must be
used on the printed circuit boards in order to
make the circuits work properly.
The mass fill option provided by Target (and
propagated for this purpose) in such circuits is
only sufficient up to a few hundred megahertz
before new problems start and the method
described below has to be finally changed.
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Simple LC low pass for the wave resistance
On PCB transmission lines, the engendering delay
is given by
Lets take a look at a simple LC low pass for the
wave resistance Z 50 and a cutoff frequency of
100 MHz. The component of microwave PCB values
??themselves are obtained after entering the
filter parameters in one of the modern filter
programs. All components may only be used in the
SMD version (here 1206 for the capacitors, the
coils, on the other hand, as 2220 with an
additional ground connection for the shielding
housing). All of that is still possible and
seems quite normal. It is only with the printed
circuit board that it becomes more
interesting The underside of the circuit board
is provided with a continuous ground surface (
GND) and everything that needs to be earthed gets
its own grounding pad on the top with as many
plated-through holes as possible.
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Simple LC low pass for the wave resistance
In the series, the plated-through holes
themselves are, of course, designed as real
plated-through holes. The use of silver-plated
hollow rivets with a diameter of 0.8 mm ( even
tested up to 10 GHz) works very well with the
first test board.
Input and output connections may only be made via
microstrip lines with the correct wave impedance
Z and the corresponding correct width (which of
course depends on the conductor material, the
board thickness and unfortunately also
somewhat on the operating frequency.
Of course, with filter capacitors with their
often crooked values, you dont try to find such
exotic things anywhere. They are easily realized
by connecting up to three SMD standard values
??from the E12 standard series in parallel. Even
reduces the overall self-inductance and thus
shifts the natural resonance to higher
frequencies. Deviations of up to 1 2 of the
total value are tolerable, which is why we
replace the 33.2 pF with 33 pF and the 57.2 pF
with 56 pF in our example
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The new demands 
The handling of the PCB CAD program and its
properties changes significantly. The new demands
for this process of microwave PCB look like this
b) On the other hand, the solder pads for the SMD
components must be as small as possible, because
they bring additional capacities into the
circuit. These capacities must already be taken
into account in the design and circuit simulation

a) Neither an autorouter nor an autoplacer may be
used. The position of each component on the
circuit board must ensure the shortest connecting
cables to the next component (because every
additional millimeter of cable may mean
additional inductance). This means that
components must be able to be moved with maximum
accuracy without problems or rotated by any
angle. And all by hand.
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The new demands 
The handling of the PCB CAD program and its
properties changes significantly. The new demands
for this process of microwave PCB look like this
g) The lowest level of the circuit board is
completely provided with a copper layer, which is
connected to GND ( ground) via the vias. h)
Consequently, wiring is only carried out on the
top (usually level 1). Of course, you have to be
very careful that the housings of ICs or
transistors may be mirrored correctly if they
were designed for use at the lowest level.
c) Very often you are forced to design new SMD
solder pads or even new housings, because there
is usually nothing in the library for the
required special components. This shouldnt be
secret science and should happen very
quickly. d) The possibility of creating the
vias ( through-plating) must be available. e)
Required ground surfaces must be easy to create
and automatically clear the holes of the vias. f)
At the end, conductor tracks must not be rounded,
their width and length must be adjustable to
within a hundredth of a millimeter.
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Design example ( microwave PCB ) 100 MHz low
pass
Step 3Now the microwave PCB diagram is drawn.
The capacitors come as C 1206 from the
C.BTL3001 library, the coils as L from the
L.BTL3001 library. Entry and exit markers can
be found as references in the pull-down menu
Other components. You can find it by placing
the cursor on the transistor symbol in the scroll
bar and then sliding the mouse pointer a little
to the right. There you also get the mass
symbols. Dont forget each component in the
microwave PCB is now clicked first to mark it.
Then press w until the crosshair flashes. With
ä you get into the change menu and enter the
exact component value there.
We now want to understand the complete design
process for the low pass above. Step 1We start
a new project Circuit board with circuit
diagram and give it a suitable name. Step 2We
switch to the circuit diagram, get a vertical
DIN A4 sheet from the frame library
(RAHMEN.BTL3001) and put it on the screen. Its
best to label your text field right away,
otherwise youll forget it later.
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Design example ( microwave PCB ) 100 MHz low
pass
Step 5Now this board is zoomed to fill the
format. Then you should quickly go behind the
button with the eye to briefly change the
screen grid to 1mm. This makes it easier to
approach the positions of the 4 mounting holes,
as they should sit 3 mm from the edge of the
board.
Step 4Now we need the circuit board and switch
to the circuit board screen by clicking on the
circuit board symbol. There we first delete the
sometimes drawn frame to get an absolutely blank
screen. Then we click on the IC symbol in the
scroll bar and fetch a board with the dimensions
30mm x 50mm via Free housing and the library
PLATINEN.GHS3001.
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Design example ( microwave PCB ) 100 MHz low
pass
e3mm 27mm 47mm 3mm 47mm 27mm Please reset
the screen grid to 0.1mm now! he remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now! remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now!
Step 6 You now place a horizontal auxiliary
line across the microwave PCB board. It must go
clearly left and right over the edge of the board
and have exactly the same width as the 50 ohm
microstrip line. Dont worry after the
following actions this line will be deleted! To
do this, we open the drawing tools menu, click on
the straight line and then on the letter o
(for options). Now it is necessary to set the
line width to 1.83 mm, not to round off the ends
and to select level 16 (i.e. copper on top).
Once this is done, the cursor is rolled as
precisely as possible onto the lower left corner
of the board. The keyboard key Pos1 immediately
declares this corner as the relative zero point
of our system (coordinates 0 0) and we move the
mouse to the position 3mm 3mm . There we
press the dot on the keyboard twice in
succession (to set the via) and then cut off the
unwinding connecting wire with Escape. The
remaining 3 holes are created in the same way.
Your positions are3mm 27mm 47mm 3mm 47mm
27mm Please reset the screen grid to 0.1mm now!
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Design example ( microwave PCB ) 100 MHz low
pass
e3mm 27mm 47mm 3mm 47mm 27mm Please reset
the screen grid to 0.1mm now! he remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now! remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now!
Step 8 For both coils we choose the SMD housing
2220 and arrange them as shown in the picture
opposite. However, please show the air lines
beforehand ( level 27) and turn the components
so that the air lines match the wiring correctly.
And not the option populate SMD on top to
forget. Step 9 Now its time to connect the two
outer capacitors, which are placed below the coil
connections.
You also draw a narrower vertical auxiliary line
(width slightly smaller. Here 0.5 mm) as a
vertical axis of symmetry. This is how it looks
at the end. Step 7 Now you first place the
middle capacitor C2 in the center marked in this
way. Please do not forget to activate the Mount
SMD on top option when selecting the 1206
housing and then use the d key to turn the
component 90 degrees before placing it down. This
is how the center of the microwave PCB board
looks immediately before the capacitor is set
down.
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Design example ( microwave PCB ) 100 MHz low
pass
e3mm 27mm 47mm 3mm 47mm 27mm Please reset
the screen grid to 0.1mm now! he remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now! remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now!
Step 10 Now we can delete our two auxiliary
lines and pull three pieces of cable with a
width of 1.83 mm as microstrip wiring from the
left to the right edge. First like this
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Design example ( microwave PCB ) 100 MHz low
pass
then like this!
e3mm 27mm 47mm 3mm 47mm 27mm Please reset
the screen grid to 0.1mm now! he remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now! remaining 3
holes are created in the same way. Your positions
are3mm 27mm 47mm 3mm 47mm 27mm Please
reset the screen grid to 0.1mm now!
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Design example ( microwave PCB ) 100 MHz low
pass
Step 12 And because this is already working
well, we lay out two small carpets in the upper
half for earthing the coil shielding cups. Step
13 From the drawing tools ( button with the
pencil) we get the filled rectangle and press
o for the options. The rectangles must be on
level 16 ( copper on top) and should combine all
five vias of a ground connection. Fortunately,
the holes in the vias are automatically kept free
by the program we dont have to do anything
about that.
Step 11 Now we give each capacitor a nice field
of 5 vias for its ground connection.Do you
remember? You have to move the cursor to the
intended position and then press the dot on the
keyboard twice in succession. Then the additional
connecting wire is cut with ESCAPE. (A bore
diameter of 0.6 mm, an aura of 0.3 mm and a
diameter of 1.5 mm were selected).
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Design example ( microwave PCB ) 100 MHz low
pass
Step 15 And to make things round, we go behind
the button with the magic wand to activate the
mass area filling option. We release the lower
side (level 2 copper below) and select the
signal GND. Then the program is started. Thats
how it looks like. Last step To print the top
of the board, we only switch to levels 16 (
copper on top), 23 ( outline) and 24(
Boreholes). Then we can take a closer look at
what the microwave PCB board will look like.
Step 14 You should never forget thata suitable
label on the upper copper side (level 16) must
be, because otherwise the microwave PCB
manufacturer does not know what is up or down and
we may get A mirrored board may have been
supplied.We also find the text option behind the
button with the pencil.
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Qualification and performance specifications of
Microwave PCB
Microwave PCB Material The market for microwave
technology has significantly fewer users than
conventional PCB technologies. There are only a
small number of PTFE suppliers, the Teflon
material that is often used for micron wave
substrates. The is in high contrast to the many
enterprises, the wire plate on the basis of FR-4
laminates. However, when it comes to the use of
materials, the term small number quickly
becomes relative in the huge electronics
industry. Numerous microwave PCB boards are now
in use.
IPC-6012, qualification and performance
specification for rigid printed circuit boards
and IPC-6013, qualification and performance
specification for flexible PCB. Typically, the
IPC tries to update these three qualification and
performance specifications at the same time.
IPC-6018 was published in January 2002 issue A.
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Qualification and performance specifications of
Microwave PCB
Microwave PCB Producing Guidelines The additions
deal with numerous changes regarding the circuit
board substrate materials and the conductor
tracks on them. The conductor tracks in the
microwave range have significantly different
performance parameters than those used for
conventional circuit boards. Many traces of a
typical microwave PCB board can be designed
according to the IPC requirements for rigid and
flexible circuit boards. In the areas in which
high-speed microwave signals are present,
however, completely different parameter values
??apply for conductor width, thickness and
spacing. There is therefore no question that a
different guideline must be used when procuring
microwave printed circuit boards.
Microwave PCB Application This technology is
used in many commercial applications such as
cellular base stations and military products
today, said Michael Luke, chairman of the IPC
D-22 subcommittee that developed the IPC-6018
directive. As semiconductor chip speeds continue
to increase, microwave technologies will also be
required in other areas.
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Qualification and performance specifications of
Microwave PCB
When these holes are drilled, resin residue known
as resin smear may remain when the hole wall is
formed. The IPC-6018B guideline contains special
criteria for the removal of resin residues (resin
smear), which take into account the special
properties of high-frequency circuit board
laminates. Its a big issue with PTFE circuit
boards, said Perry.
There are also differences in the substrates. In
contrast to the FR-4 substrates of conventional
printed circuit boards, most microwave PCBs are
based on PTFE (Teflon). PTFE laminates have their
own properties when individual layers are
laminated. The dimensional stability is
completely different, i. H. Designers and
manufacturers need to take this into account when
layouting circuit boards and positioning buried
holes or blind holes or other elements that
require drilling.
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Since the completion of Issue A in early 2002,
numerous other changes have occurred. The
developers of the directive have added reference
information on passive resistors and capacitors
to section 3 REQUIREMENTS. The new version has
also improved the requirements for soldering edge
breaks, which can arise whenever holes are not
drilled in the middle of the pads. The subject of
thermal stress has also been revised to take
account of the progress made by convection reflow
processes for thermal stress tests on ground
specimens or samples from production printed
circuit boards.
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