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Wireless Issues

432 Chapter Ten

into a liquid plastic and laid out to dry. Both of these techniques, and more,
are used to protect the circuit and to displace excess heat from the hybrid
assembly.
A completed hybrid will take advantage of different circuit construction
techniques, such as thin- and thick-film resistors (which can be printed and
trimmed directly on the hybrid’s substrate), SMD IC chips, SMD capacitors,
flip-chips, trace coils, and microstrip transmission lines. In fact, hybrid man-
ufacture is basically a method of constructing a circuit on a specialized PCB by
employing any desired electronics technology that is available so that we may
decrease both the size and the cost of a circuit, while increasing reliability and
performance.
Most of the actual ICs placed on a hybrid board are in their unpackaged
form, even though many hybrids can and do use packaged SMD ICs. But the
ability to exploit ICs in their bare die form means that the size and cost of the
entire hybrid circuit can be significantly improved. The bare die is first
attached to the hybrid PCB with an epoxy that is optimized for thermal con-
ductivity and/or electrical conductivity, or the epoxy may insulate the bare
chip both thermally and electrically from the PCB, depending on the applica-
tion. The chip’s bond wires are attached to the hybrid PCB’s metallization lay-
er by a wire-bonding process. This process will rely on the type of bonding wire
used out of the bare die. Gold wires require thermosonic bonding, while alu-
minum wire will need an ultrasonic procedure. Both methods demand a spe-
cial machine that utilizes a chiseled point that contacts the wire, pressing it
against the board’s surface metallization while vibrating at ultrasonic fre-
quencies. Thermosonic bonding for gold wires, as its name implies, also adds
heat to this process.
A superior bare die attachment technique does not use wire bonding at all
and obtains much decreased inductive effects. Instead, small solder balls are
attached to the IC’s connection points. The chip, now called a flip-chip, is
affixed to the board by turning the IC chip over so that its solder balls are in
contact with the pads of the board’s metallization layer. The entire assembly
is then heated, causing the flip-chip to be permanently melded to the PCB,
both mechanically and electrically. There are other methods of attaching a
bare die to a PCB, such as tape automatic bonding and adhesive-bonded
microbumps. For more detailed information on these processes, consult
Hybrid Microelectronics Handbook by Sergent and Harper.
All of the other SMT components, such as the transistors, packaged RFICs,
capacitors, resistors, and inductors, can be attached to the substrate’s metal-
lization layer by the reflow soldering process, which employs a solder paste
that is “printed” onto the PCB and heated, causing the solder to reflow, elec-
trically and mechanically connecting the components to the board.

10.5.2 Board and conductor materials
The composition of the substrate and conductor materials are an important
consideration in any high frequency PCB design, and hybrids are no different.

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