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 R=65 60 mils IN 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 Fe 2 Cl 3 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. Hole/Via Mask 20 mil 70 mil Conductor Mask R=240 R=86 C=.01 +12 R=86 GD Drill dots are now blacked out IN ERA3 ERA3 ERA3 Out C=43 31 mil 100 mil hole and corner cutout marks 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 frame +6 R=65 Ohm Positioning holes Bottom circuit frame Figure 7 5
Top circuit OUT +6 R=65 Ohm Bottom mirrored circuit Figure 8 Fabrication Parameters Line width 6 mils Line gap 6 mils Overetch correction factor 1 mil Via hole diameter 30 mils Size of donut for Via with Rivet 60 mils hole of 10 mils Minimum hole diameter (for plated holes) Aspect ratio D/H > 1 Maximum slide displacement for Anritsu fixtures 3.9 inches Anritsu fixture port offset +- ½ inch. Minimum space between SMA connector center pins ½ inch Mask scale 3X Maximum circuit size 8 X 8 (single sided) 6 X 6 (Double sided) 4 X 4 (plated hole) Available via/hole drill size dia. (mil) 12, 16, 20, 23, 28, 31, 44, 55, 60, 6
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 R=65 IN 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 ) 8