Multichip Module Technologies:Multi-Chip Module Technologies

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Introduction

From the pioneering days to its current renaissance, the electronics industry has become the largest and most pervasive manufacturing industry in the developed world. Electronic products have the hallmark of innovation, creativity, and cost competitiveness in the world market place. The way the electronics are packaged, in particular, has progressed rapidly in response to customers’ demands in general for diverse functions, cost, performances, and robustness of different products. For practicing engineers, there is a need to access the current state of knowledge in design and manufacturing tradeoffs.

Thus arises a need for electronics technology-based knowledge to optimize critical electronic design parameters such as speed, density, and temperature, resulting in performance well beyond PC board design capabilities. By removing discrete component packages and using more densely packed interconnects, electronic circuit speeds increase. The design challenge is to select the appropriate packaging technology, and to manage any resulting thermal problems.

The expanding market for high-density electronic circuit layouts calls for multi-chip modules (MCMs) to be able to meet the requirements of fine track and gap dimensions in signal layers, the retention of accurately defined geometry in multilayers, and high conductivity to minimize losses. Multi-chip module technologies fill this gap very nicely. This chapter provides engineers/scientists with an overview of existing MCM technologies and briefly explains similarities and differences of existing MCM technologies. The text is reinforced with practical pictorial examples, omitting extensive development of theory and details of proofs.

The simplest definition of a multi-chip module (MCM) is that of a single electronic package containing more than one integrated circuit (IC) die [1]. An MCM combines high-performance ICs with a custom- designed common substrate structure that provides mechanical support for the chips and multiple layers of conductors to interconnect them.

One advantage of this arrangement is that it takes better advantage of the performance of the ICs than it does interconnecting individually packaged ICs because the interconnect length is much shorter. The really unique feature of MCMs is the complex substrate structure that is fabricated using multilayer ceramics, polymers, silicon, metals, glass ceramics, laminates, etc. Thus, MCMs are not really new. They have been in existence since the first multi-chip hybrid circuit was fabricated. Conventional PWBs utilizing chip-on-board (COB), a technique where ICs are mounted and wire-bonded directly to the board, have also existed for some time. However, if packaging efficiency (also called silicon density), defined as the percentage of area on an interconnecting substrate that is occupied by silicon ICs, is the guideline used to define an MCM, then many hybrid and COB structures with less than 30% silicon density do not qualify as MCMs. In combination with packaging efficiency, a minimum of four conductive layers and 100 I/O leads has also been suggested as criteria for MCM classification [1].

A formal definition of MCMs has been established by the Institute for Interconnecting and Packaging Electronic Circuits (IPC). They defined three primary categories of MCMs: MCM-L, MCM-C, and MCM-D. It is important to note that these are simple definitions. Consequently, many IC packaging schemes, which technically do not meet the criteria of any of the three simple definitions, may incorrectly be referred to as MCMs. However, when these simple definitions are combined with the concept of packaging efficiency, chip population, and I/O density, there is less confusion about what really constitutes an MCM. The fundamental (or basic) intent of MCM technology is to provide an extremely dense conductor matrix for the interconnection of bare IC chips. Consequently, some companies have designated their MCM products as high-density interconnect (HDI) modules.

Multi-Chip Module Technologies

From the above definitions, it should be obvious that MCM-Cs are descended from classical hybrid technology, and MCM-Ls are essentially highly sophisticated printed circuit boards, a technology that has been around for over 40 years. On the other hand, MCM-Ds are the result of manufacturing technologies that draw heavily from the semiconductor industry.

MCM-L

Modules constructed of plastic laminate-based dielectrics and copper conductors utilizing advanced forms of printed wiring board (PWB) technologies to form the interconnects and vias are commonly called “laminate MCMs,” or MCM-Ls [2].

Advantages

Economic Ability to fabricate circuits on large panels with a multiplicity of identical patterns. Reduces manufacturing cost. Quick response to volume orders.

Disadvantages

Technological More limited in interconnect density relative to advanced MCM-C and MCM-D technologies. Copper slugs and cut- outs are used in MCM-Ls for direct heat transfer. This degrades interconnection density.

MCM-L development has involved evolutionary technological advances to shrink the dimensions of interconnect lines and vias. From a cost perspective, it is desirable to use conventional PWB technologies for MCM-L fabrication. This is becoming more difficult as the need for multi-chip modules with higher interconnect density continues.

As MCM technologies are being considered for high-volume consumer products applications, a focus on containing the cost of high-density MCM-Ls is becoming critical.

The most usefull charateristic in assessing the relative potential of MCM-L technology is interconnec- tion density [3,4], which is given by:

Packaging efficiency (%) = Silicon chip area/Package area (9.1) The above formula measures how much of the surface of the board can be used for chip mounting pads

versus how much must be avoided because of interconnect traces and holes/pads.

MCM-C

These are modules constructed on co-fired ceramic or glass-ceramic substrates using thick-film (screen printing) technologies to form the conductor patterns using fireable metals. The term “co-fired” implies that the conductors and ceramic are heated at the same time. These are also called thick-film MCMs.

Ceramic technology for MCMs can be divided into four major categories

• Thick-film hybrid process

• High-temperature co-fired alumina process (HTCC)

• Low-temperature co-fired ceramic/glass based process (LTCC)

• High Tc aluminum nitride co-fired substrate (AIN)

Thick-film hybrid technology produces by the successive deposition of conductors, dielectric, and/or resistor patterns onto a base substrate [5]. The thick-film material, in the form of a paste, is screenprinted onto the underlying layer, then dried and fired. The metallurgy chosen for a particular hybrid construction depends on a number of factors, including cost sensitivity, conductivity requirements, solderability, wire bondability, and more. A comparative summary of typical ceramic interconnect properties is compiled in Table 9.1.

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