Page 131 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

HYBRID AND MCM TECHNOLOGIES 111

Table 4.13 Comparison of MCM interconnection technologies. Adapted from Doane and Franzon
(1993)

Property Thick film HTCC Thin film Laminate
MCM class MCM-C MCM-C MCM-D MCM-L
Dielectric material: Glass-ceramic Alumina Polyimide Epoxy-glass
Dielectric constant 6-9 9.5 3.5 4.8
Thickness/layer (um) 35-65 100-750 25 120
Min. via diameter (urn) 200 100-200 25 300
Conductive materials: Cu (Au) W(Co) Cu (Al, Au) Cu
Thickness (am) 15 15 5 25
Line width (um) 100-150 100-125 10-25 75-125
Line pitch (um) 250-350 250-625 50-125 150-250
Bond pad pitch (um) 250-350 200-300 100 200
Maximum number of 5 to 10+ 50+ 4-to 10 40+
layers
Electrical properties:
Line resistance (£2 -cm) 0.2-0.3 0.8-1 1.3-3.4 0.06-0.09
Sheet resistance (mfi/sq) 3.0 10.0 3.4 0.7
Propagation delay 90 102 62 72
(ps/cm)
Stripline capacitance of 4.3 2.1 1.25 1.46
50 Q line (pF/cm)

MCM technology has several advantages for integrating arrays of microtransducers and
even MEMS (Jones and Harsanyi 1995). First, the semiconductor dies can be fabricated
by a different process, with some dies being precision analogue (bipolar) components and
others being digital (CMOS) logic components. Second, the cost of fabricating the MCM
substrate is often less expensive than using a silicon process, and the lower die complexity
improves the yield. Finally, the design and fabrication of a custom ASIC chip is a time-
consuming and expensive business. For most sensing technologies, there is a need for new
silicon microstructures, precision analogue circuitry, and digital readout. Therefore, fabri-
cating a BiCMOS ASIC chip that includes bulk- or surface-micromachining techniques
is an expensive option and prohibitive for many applications.
Figure 4.46 shows the layout of a multichip module (MCM-L) with the TAB patterns
shown to make the interconnections (Joly et al. 1995). This MCM-L has been designed
for a high-speed telecommunications automatic teller machine (ATM) switching module,
which, with a power budget of 150 W, is a demanding application.

4.6.3 Ball Grid Array

There are a number of other specialised packaging technologies that can be used as an
alternative to the conventional PCB or MCM. The main drive for these technologies is
to reduce the size of the device and maximise the number of I/Os. For example, there
are three types of ball grid array (BGA) packages. Figure 4.47 shows these three types:
the plastic BGA, ceramic BGA, and tape BGA. The general advantages of BGA are
the smaller package size, low system cost, and ease of assembly. The relative merits of