Multichip Module Technologies:Assembly Techniques and Summary of Multichip Module Technologies.

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Assembly Techniques
Surface-Mount Assembly

The surface-mount assembly technique can be categorized under lowest-cost, fastest turnaround assembly method, using pre-packaged components soldered to glass-epoxy board, flex circuit, or thick-film ceramic substrate. Many package styles are available, such as SIP, DIP. Pins may be attached as in-line leads, 90° leads, etc [19].

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Chip-and-Wire Assembly

In order to minimize interconnect electronic circuit parasitics, a high-density layout is one of the assembly techniques recommended as a solution. The highlight of this technique is epoxy attachment/wire bonding of integrated circuits and components (e.g., capacitors) to a glass-epoxy board (chip-on-board). Of course, another way is attachment to a thick- or thin-film ceramic substrate. Currently, many package styles are available and can be listed as follows [19,20]:

• Epoxy seal, using a B-stage ceramic lid epoxies to the substrate (quasi-hermetic seal)

• Encapsulation, in which bare semiconductor die are covered with a “glob top” (low-cost seal)

• Metal (typically Kovar) package with Kovar lid either welded or soldered to the package (hermetic seal)

• Leads are typically plug-in type, SIP, DIP, PGA, etc.

Mixed Technologies

Another category of assembly technique recognized as “mixed technologies” combines chip-and-wire with surface-mount assembly techniques on a single substrate, which may be a glass-epoxy board or ceramic substrate. Heat sink/heat spreaders are available in a variety of materials.

The Maxtek module shown in Figure 9.7 includes a 4-layer, 20-mil-thick glass-epoxy board mounted to a beryllium copper heat spreader.

Selectively gold plated for wire bonding pads and pads at each end for use with elastomeric connectors,

• Three methods of IC die attach

• Epoxied directly to glass-epoxy board

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• Epoxied directly to the BeCu heat spreader through a cutout in the board

• Epoxied to the head spreader, through a cutout, via a thermally conductive submount, to electri- cally isolate the die from the heat spreader

• Solder-mounted resistors and capacitors

• 50-ohm differential signal lines

• IC die may be “glob topped” or covered in either a ceramic or plastic lid for protection

Special Modules

Under special modules we can emphasize and highlight technologies of complex assemblies of mixed- technology substrates, flexible circuits, and/or electromechanical components. (See Figure 9.8.) Complex assemblies of mixed-technology substrates often utilize a B-stage epoxy lid or glob top over chip-and-wire circuitry. These technologies enable designers to provide an integrated solution com- plex system problems like in a module shown on page 22 which is CRT Driver System capable of displaying 4 million pixels with 1.5-ns rise and fall times. The circuit incorporates a thin-film MCM connected by a special high-frequency connector to an FR-4 board with thick-film ceramic low- inductance load resistor and flex circuit with an integral impedance-matching inductor, connecting directly to the CRT [19,20].

The design engineer of an MCM chip should work with customer to partition the circuit and optimize the design to be implemented in MCM technology. Application of technologies for placement, routing, via minimization, tree searching, and layer estimation will be important to assess at this point. The general function, purpose, and configuration of the active elements, interconnects, and assembly tech- nology should also be assessed, along with key materials and critical properties, representative manufacturing-process flows, potential failure mechanisms, qualification procedures, and design for testability.

Note that two concepts must be carefully examined for successful MCM production. An MCM design is initiated by selecting appropriate technologies from the many options available. The basic choices are for substrate technology and assembly techniques. Design trade-offs are analyzed, and a preliminary specification is completed. Following circuit simulation, prototypes are produced and tested. When the application requires it, an ASIC can be designed to be included in the MCM.

Summary

In summary, it is customary to give an answer to the fundamental question: What multi-chip modules do for you? . . . and here is the answer . . .

MCMs optimize critical 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, circuit speeds increase. The design challenge is to select the appropriate packaging technology, and to manage any resulting thermal problems [20].

MCM technologies found their way and are utilized in the wireless, fiber, and instrumentation markets; space and military programs; and in the real world, they stand in the forefront of best merchant-market technology.

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