Page 99 - Power Electronics Handbook

92 Power semiconductor control components
bonding techniques can be used. In flip chip bonding contact bumps are
formed on the die or the tracks and the die is flipped or inverted and then
connected to the bumps by soldering, ultrasonic, or thermocompression
bonding. In beam lead bonding metal leads or beams overhang the
semiconductor die and these are attached to the substrate contact areas. In
both flip chip and beam lead techniques no separate die bonding is required
since the electrical contact and mechanical support are both provided by the
beams and balls.

3.5.2 Multichip modules
A multichip module (MCM) is often mistaken for a hybrid circuit, although
it should more correctly be considered to have grown out from hybrids
(Iscoff, 1993; Sherwain et al., 1995). MCMs tend to have a thin film
structure although some may use thick film techniques. They also usually
contain many semiconductor chips on a substrate, compared to relatively few
used in a hybrid circuit (Lall and Bhagath, 1993). Apart from these
differences the techniques used to make hybrid circuits and MCMs are
similar. They also show similar advantages compared to discrete compo-
nents, i.e. smaller size and enhanced performance since the inductance of
leads is reduced, especially when flip chips are used (Losch et al., 1993).
MCMs are often classified by the technology used to make their
substrates. Four types are in use: MCM-L, MCM-C, MCM-D and MCM-Si
(Doane and Franzon, 1994; Tasker and Sekulic, 1994).
An MCM-L (laminate) is generally based on advanced forms of printed
circuit boards, which are usually multilayer structures. It consists of copper
foil conductors on plastic laminate-based dielectrics such as FR-4 epoxy
glass. An MCM-L typically has between six and eight layers and up to about
ten semiconductor die. The problem with MCM-L, as with all MCMs and
hybrids which use unpackaged die, is obtaining known good die (KGD),
since the die must be ready for mounting without further tests on the chip
itself. With improvements in semiconductor technology this problem is
reducing. MCM-Ls are used in the IT industry, within computers and
workstations.
An MCM-C (ceramic) is formed using thick film technologies, such as
screen printing paste, on cofired ceramic or glass ceramic multilayer
substrates. The term cofired is used to indicate that the paste and the substrate
are fired in an oven at the same time. The conductors are applied as pastes
containing gold, silver, palladium and copper, and an MCM-C is therefore
very similar to a thick film hybrid circuit. MCM-Cs are used in defence and
in medical electronics, as well as in computers, where reliability and small
size are important.
An MCM-D (dielectric) uses dielectrics of polymers or inorganic
compounds as the substrate. The conductors, such as aluminium or copper, are
deposited on this substrate by sputtering, evaporation, or plating, and the
patterns then formed by photolithography followed by wet or dry etching. An
MCM-D is therefore similar to thin film hybrid circuits. The technology gives
the highest interconnection density and the reduced signal paths result in lower
loading and noise, so that it can be used in speed critical applications.
Generally it tends to be more expensive than MCM-L or MCM-C.