A typical electrical subsystem package includes: (1) core devices, such as chips and wires; (2) housings, which include mounting points; (3) electrical connections and pass-throughs; (4) covers, which provide access and then sealing after closure; and (5) external features for mechanical attachment and thermal transfer. New packages for MEMS-based subsystems will also include unique, sometimes multichannel, other-than-electrical pass-throughs for mechanical, optical, fluid, thermal, chemical, and nuclear energy transfer. Systems of components that are based on SAS subsystems will also require additional packaging innovations related to information buses or other means of communication, less obtrusive fixation methods, local information processing nodes, and others specific to networked systems. Innovative assembly and testing procedures will be necessary to complete the manufacture of new products.
The definition of package requirements is also a complex process. Just a few necessary features include: geometry (size, shape, attachment), abuse tolerance (vibration, impact, acceleration), interconnection methods (regime, density, dynamics, delay), thermal (isolation, temperature range, control), life (maintenance requirements, fatigue, operational range), chemical effects (corrosion, intrusion), shielding (RF, nuclear, thermal), and economy (original and life cycle cost).
For these discussions, Figure 6.8 will be used to indicate areas of effort for PAT. The figure includes a review of procedures in terms of fabrication processes, packaging, assembly, and testing. Procedures are applied along the progression of systems from subsystems to components to complete systems. Note that in each area tests might be required to validate manufacturing success. Note also that the design of new PAT systems will be an integral part of the overall system design. Greater attention to analysis, modeling, simulation, and subtesting will be necessary.
Figure 6.8. PAT processes occur in all procedures and at all levels.
Fabrication processes, specifically intended for electronic chips and microsystems, are discussed in other chapters of this report. The range of processes available for micromechanical parts generation and joining is expanding rapidly since processes are the facilitators of progress in the MEMS area. New processes, as they develop, will also be used in PAT. Table 6.1 briefly reviews processes that have already been used in MEMS devices.
Packaging systems can be classified according to levels of sealing enclosure. Table 6.2 reviews levels in terms of open or closed -- rigid, flexible, sealed or exposed.
Assembly processes occur at subsystem, component, and complete system levels. Approaches can range from integrated techniques such as silicon micromachining to automated assembly to classical manual assembly. Many examples of integrated methods are contained in the other chapters of this report. Assembly procedures, manual or automated, include steps such as parts acquisition, transfer, insertion, placement, and connection. Most real examples of assembly are system-specific, with the greatest success in environments that require uniaxial placement of elements on planar substructures. Systems requiring three-dimensional placement and attachment are more difficult.
Testing for electrical properties in microsystems is a well developed art. Testing the mechanical properties of MEMS-based devices is not. Whether examining movements, surfaces or materials properties, the testing of micromechanical devices presents serious barriers. Objects are difficult to observe in operation. Depth-of-field restrictions in optical systems limit the observation of highly three- dimensional systems. Nonoptical viewing systems necessarily require environments that might prevent operation of the targeted microsystem. Small systems cannot be touched safely or interconnected without altering function or causing system damage. Signals produced are very low so measurements, again, can alter system function.