PCBs can also be formed on a ceramic board, and these may be referred to as ceramic PCBs. A ceramic board, such as alumina, offers a number of advantages over organic PCBs, because a ceramic board is much more rigid, tends to be flatter, has a lower dielectric loss, and can withstand higher process temperatures. In addition, alumina is a very inert material and hence is less prone to chemical attack than an organic PCB.
Ceramic PCBs can be processed in a number of different ways, such as thick-film, thin-film, co-fired, and direct-bond copper. The most important technology is probably the thick film. Circuit boards have been made for more than twenty years using this technology and are usually referred to as hybrid circuits.
In thick-film technology, a number of different pastes have been developed (known as inks), and these pastes can be screen-printed onto a ceramic base to produce interconnects, resistors, inductors, and capacitors.
Example:
1. Artwork is generated to define the screens or stencils for the wiring layers, vias, resistive layers, and dielectric layers.
2. Ceramic substrate is cut to size using laser drilling, and perforations that act as snapping lines are included after the process is complete.
3. Substrate is cleaned using a sandblaster, rinsed in hot isopropyl alcohols, and heated to 800 to 925 °C to drive off organic contaminants.
4. Each layer is then in turn screen-printed to form the multilayer structure. Each paste is first dried at 85 to 150 °C to remove volatiles and then fired at 400 to 1000 °C.
5. The last high-temperature process performed is the resistive layer (800 to 1000 °C).
6. A low-temperature glass can be printed and fired at 425 to 525 °C to form a protective overlayer or solder mask.
Thick-film technology has some useful advantages over other types of PCB manufacture.
The process is relatively simple - it does not require expensive vacuum equipment (like thin-film deposition) - and hence is an inexpensive method of making circuit boards.
Figure 4.44 shows a photograph of a thick-film PCB used to mount an ion-selective sensor and the associated discrete electronic circuitry (Atkinson 2001). The thick-film process is useful here not only because it is inexpensive but also because it forms a robust and chemically inert substrate for the chemical sensor. The principal disadvantage of thick-film technology is that the packing density is limited by the masking accuracy - some hundreds of microns. Photolithographically patterned thin-film layers can overcome this problem but require more sophisticated equipment.
4.6.2 Multichip Modules
Increasingly, PCB technologies are being used to make multichip modules (MCMs). A multichip module is a series of monolithic chips (often silicon) that are connected and
HYBRID AND MCM TECHNOLOGIES 109
Figure 4.44 ISFET sensor and associated circuitry mounted on a ceramic (hybrid) PCB. From Atkinson (2001)
250 200
100
50 0
PCB SMT
brids
50 microns HDMI 25 microns HDMI
10 microns HDMI
0 WSI 50
Silicon efficiency (%)
Figure 4.45 Silicon efficiency rating and line width of different interconnection and substrate technologies. After Ginsberg (1992)
packaged to make a self-contained unit. This module can then be either connected directly to peripheral ports for communication or plugged into another PCB. One important reason for using MCM instead of a conventional die-packaging approach is that the active silicon efficiency rating is improved (see Figure 4.45). In other words, the total area of the semiconductor die is comparable to the MCM substrate area. As can be seen from the figure, conventional PCB technologies and even SMT and hybrid are much poorer than the high-density MCM methods.
The ceramic-based technology is referred to as an MCM-C structure; other MCM-C technologies include high-temperature fired ceramic (HTCC) and low-temperature co-fired ceramic (LTCC). Table 4.11 lists the relative merits of different MCM-C technologies.
110 STANDARD MICROELECTRONIC TECHNOLOGIES Table 4.11 Relative merits of MCM-C technologies, with one being the best
Adapted from Doane and Franzon (1993), Property
CTE matched to alumina or silicon Smaller line and space
designs
Table 4.12 Properties of some commonly used MCM-C materials. Adapted from Doane and Franzon (1993)
Dielectric loss tangent at 1 MHz The choice of ceramic substrate is important and the >99 percent alumina
has a low microwave loss, good strength and thermal conductivity, and good flatness.
However, it is expensive and 96 percent alumina can be used in most applications. In cases in which a high thermal conductivity is required (e.g. power devices), beryllia (BeO) or aluminum nitride (A1N) can be used, although these involve a higher cost. Table 4.12 summarises the key properties of the ceramic substrates.
In addition, modules wherein interconnections are made by thin films are classified as MCM-D and those made by plastic (organic) laminate-based technologies are classified as MCM-L. Table 4.13 shows a comparison of the typical properties of the three main types of MCM interconnection technologies.
HYBRID AND MCM TECHNOLOGIES 111 Table 4.13 Comparison of MCM interconnection technologies.
(1993) Min. via diameter (urn) Conductive materials:
Thickness (am) Line width (um) Line pitch (um) Bond pad pitch (um) Maximum number of
layers
Electrical properties:
Line resistance (£2 -cm) Sheet resistance (mfi/sq)
Adapted from Doane and Franzon Thin film
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
112 STANDARD MICROELECTRONIC TECHNOLOGIES
Figure 4.46 Example of a high-density MCM-L substrate with TAB patterns. From Joly et al.
(1995)
plastic and ceramic PGA packages are similar to those already discussed for PCBs and MCMs. The tape EGA uses a TAB-like frame that connects the die with the next layer board.