Printed Circuit Board Guidelines
Introduction :
The Poly-Grames research center has the ability to manufacture printed circuits on all
types of microwave substrates. Our present fabrication capabilities are limited to two
layers (double sided) and hole (via) metallization are limited to circuits no larger than
4’’x 4’’ for copper. Metal bases are machined on site for circuit/connector support upon
request. Circuit assembly is performed using SMT (and FlipChip) soldering guidelines
including wire bonding and silver epoxy applications for metal base supported circuits.
Conception :
Before designing a circuit you must know the process limitations and the component
assembly techniques. An important point to remember is how the circuit is going to be
measured (i.e. equipment and/or test fixture limitations). Microwave circuits can be
mounted on the Anritsu test fixture provided that the circuit is less than 3.9’’ long or else
connectors will be needed. Figures 1 & 2 demonstrate the most commonly used
connectors at used for circuit fabrication. Figure 1 being a PCB mount female SMA
connector and figure 2 being a female SMA panel mount connector. Make sure the
connectors you need as well as other components are available when planning your
circuit.
Figure 1
1
Figure 2
When conceiving your circuits remember to provide a minimum adjacent distance of 0.5
inches between SMA connector center pins, this is needed for mechanical assembly. The
fabrication process has a minimum line gap and width of .006 inches (6 mils.). An over
etch chemical correction factor of 1 mil must be included in your design. For example, if
you desire a line width of 10 mils then you must draw an 11 mils wide line. Similarly, if
you want a 10 mils channel you need to draw a 9 mils wide channel. If components are
to be mounted on the circuit, produce the required foot print and proper dimension for
each component,(for example a 0603 type resistor/capacitor foot print has a 25 mil gap
between both pads. See PCB and MHMIC assembly guide). Also, if amplifiers and
transistors are to be used their ground pins must be as short as possible; in certain cases
they must be soldered directly to the ground plane as in Fig. 3.
Solder directly
to ground plane
Figure 3
2
Via hole Metallization :
Rivet :
One of the procedures to metallize via holes is the use of copper rivets. The 30 mils
diameter rivet has been in use at the Poly-Grames for several years. Its simple use has
been its advantage particularly for circuits that count less than 10 holes. Allocate a
minimum 60 mils diameter pad when planning to use via rivets. Only one mask will be
required for this type of circuit: the conductor mask. Make a 10 mils guide dot were you
want the 30 mils via hole to be. Fig. 4 illustrates this with circular pads.
10 mils
OUT
IN
60 mils
R=65
Figure 4
Plated Holes:
The other metallization procedure is to electro-deposit copper on walls of the hole. This
technique requires several steps and takes considerably more time to fabricate. Two
masks are necessary to make a plated hole circuit; a perforation mask and a conductor
mask. Fig.6 shows the type of masks necessary for this kind of process. The first step
uses the drill mask for circuit and hole alignment. Ounce the holes are perforated (using
LPKF or Laser machining) and cleaned the second step adds a plasma deposited copper
primer prior to the copper thickening electroplating process. Finally, the third step
consists of aligning the conductor mask with the circuit alignment holes, then expose and
develop the PCB before etching it in the Fe2Cl3 solution. To obtain a proper electro
deposition the D/H ratio must be >1 as in Fig.5, this defines the minimum hole size.
Maximum substrate thickness can be no thicker than 60 mils.
3
Height H
Diameter D
Figure 5
In Fig.6 both hole and circuit masks have external alignment holes. Depending on circuit
size, the inner diameter of the alignment holes can vary between 31 mils to 60 mils and
the outer diameter should vary proportionally.
Alignment
Hole/Via Mask
20 mil
70 mil
Alignment
Alignment
Alignment
R=240
Conductor Mask
GD
R=86
+12
C=.01
C=100
C=100
C=100
ERA3
R=86
ERA3
Drill dots are now blacked out
ERA3
IN
Out
C=100
C=100
C=100
C=43
31 mil
100 mil
Alignment hole
and corner cutout marks
Alignment
Figure 6
4
Doubled sided circuits:
For double sided circuits, asymmetric alignment marks/circles permits only one way to
position both top and bottom conductor masks together without ambiguity. The bottom
portion of the circuit must be printed after it has been mirrored since the bottom mask is
always reversed in the fabrication process. Fig.7 shows an example of the original
artwork both top and bottom layers viewed from above, while fig.8 shows the next step
by mirroring the bottom layer only.
Both layers are seen from above
Top circuit
OUT
Alignment frame
+6
R=65 Ohm
Positioning holes
Bottom circuit
Alignment frame
Figure 7
5
Top circuit
OUT
+6
R=65 Ohm
Bottom mirrored circuit
Figure 8
Fabrication Parameters
Line width
Line gap
Overetch correction factor
Via hole diameter
Size of donut for Via with Rivet
Minimum hole diameter (for plated holes)
Maximum slide displacement for Anritsu fixtures
Anritsu fixture port offset
Minimum space between SMA connector center pins
Mask scale
Maximum circuit size
Available via/hole drill size dia. (mil)
6
6 mils
6 mils
1 mil
30 mils
60 mils hole of 10 mils
Aspect ratio D/H > 1
3.9 inches
+- ½ inch.
½ inch
3X
8’’ X 8’’ (single sided)
6’’ X 6’’ (Double sided)
4’’ X 4’’ (plated hole)
12, 16 , 20 , 23 , 28 , 31 , 44,
55 , 60 ,
Information to be included with artwork:
Fig.9 shows the information required in a typical microwave circuit design print.
Line width > 6 mil
ERA 1
Student name
OUT
IN
R=65
Professor’s name
Substrate (type and Er)
Thickness: (in mils)
Scale (1X, 2X, or 3X)
Base type (optional)
Connector type (optional)
Component value
Cutout mark
(Interior corner line defines circuit perimeter)
Fig.9
Required informations are:
-
Designer’s name and tel/ext. number.
Professor’s name
Project’s name
Substrate type
Substrate thickness
Print scale (Maximum 3X )
Connector type and or base if required.
Circuit cutout references ( Corner interior alignment)
Type of via to be used (rivet or plated…)
Date
Note: If component assembly is required, please provide all components other than
resistors and capacitors.
7
Microwave oscillator circuit with plated holes.
Rev.1, (translated by Roch Brassard 2005-02-03 )
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