1996 Electronics Industry Environmental Roadmap - Civil and ...

Emerging Technologies
Multichip Modules: A significant trend affecting advanced electronics, packaging, and the PWB
industry is the emergence of a new class of devices known as multichip modules (MCMs).
MCMs are basically of three types [35]:
Ceramic multichip modules (MCM-C), which use ceramic materials as a substrate;
Deposited dielectric multichip modules (MCM-D), which use deposited organic and inorganic
dielectric films and metal conductors, often using processes similar to IC technologies;
and
Laminate multichip modules (MCM-L), which use existing PWB materials, technologies,
and facilities. MCM-L assemblies are essentially higher precision PWBs with a high
packaging density.
The emergence of multichip modules brings with it environmental issues that are fairly well understood
within the context of the emerging processes. For example, MCM-C, like other ceramic
processes, introduces issues of high-temperature processing and energy usage. MCM-Ds and
MCM-Ls share chemical and material issues with other IC or PWB technology processes.
6.2.4 Attachment
For many years, the attachment of advanced circuits used a through-hole approach in which small
wires from the packaged IC were inserted through holes in the PWB and connected to its back
side. The predominant attach technology today is surface mount in which devices are directly attached
to the surface of the PWB. In this process, principal issues concern the materials used to
complete the attachment, with a particular emphasis on the elimination of lead-based solders, and
the processes for cleaning the circuit board after attachment to remove waste materials.
Demands for increased density and performance, however, require acceleration of advanced approaches.
There is a fundamental transition underway from packaged ICs to unpackaged ICs that
are directly attached to boards. It is especially reflected in the emergence of multichip modules
of various types, as well as 3-D packages and new PCMCIA technologies. In the short run, these
demands will be met by the accelerated adoption of ball grid array packaging approaches. In the
longer run, direct chip attach approaches will become far more common and accepted.
NEMI explains that solder-based direct chip attach processes substrates will likely be used for
systems requiring high reliability, while adhesive-based direct chip attach processes and thin
PWBs or flex circuits will likely be used where cost is a dominant factor in system design [33].
The NEMI roadmap points out that:
“One key implication is that bare die will begin to dominate over time. This will be
driven by the movement to multi-chip modules and chip-on-board for a wide variety of
systems.”
Therefore, finding new materials that enable direct chip attach and other advanced methods must
be a high priority, since there is a strong consensus that the best environmental practices, as well
as regulatory requirements, may demand the elimination of lead-based solders. Anisotropic
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