Multichip modules march on
MCM use ramps up as more standard parts emerge and known-good die show signs of
improved availability
BY SPENCER CHIN
Associate Editor
Once little more than laboratory curiosities, multichip modules are well on
their way to becoming a mainstream packaging technology. Multichip module use
is being boosted by the increased availability of properly screened bare
die–commonly referred to as known-good die. (Known-good die, however, still
carry a considerably greater cost over unscreened die–though the difference is
often repaid in better die yield and thus lower rework costs . ) Manufacturers
are also refining substrate and chip-stacking technologies for MCMs.
Most of the early multichip modules (MCMs) were costly custom parts for
high-end applications like supercomputers and military and avionics equipment.
Now, however, more MCMs are available for the standard commercial market.
For instance, NEC Corp. (Mountain View, CA) has introduced the Series MR4401-75
RISC MCM for high-end PC servers, workstations, and multiprocessors running
Unix and Windows NT. This module, supplied in a 179-pin PGA package,
incorporates the company's 150-MHz 64-bit V4400SC microprocessor and ten
1-Mbyte static RAMs. It is pin-compatible with the company's VR 4400 CPU chip.
Production samples of the microprocessor are available now, with mass
production scheduled for September.
Kelly Microsystems (Irvine, CA) has released its PowerPlant 486 microprocessor
upgrades for 286- and 386SX-based computers. These upgrade MCMs (see Fig. 1)
snap either into the sockets of the original 286 CPU chips or on top of the
386SX CPUs. No additional slots are needed. The 486 processor upgrade comprises
a Texas Instruments 486 microprocessor with 1 Kbyte of internal cache, a
16-Kbyte cache controller, and an integrated 25-MHz clock generator.
Micro Module Systems (Cupertino, CA) has introduced a line of Intel Pentium
processor-based MCMs called Northstar. The modules incorporate one Pentium
processor, one Intel 82496 cache controller, and 18 Intel 82491 dual-port cache
SRAMs totaling 512 Kbytes of secondary cache. The modules support both 64- and
128-bit memory interfaces and operate from a single +5-V supply. Versions for
3.3-V operation are expected by midyear.
The availability of MCMs in standard IC packages enables users to gain the
density advantages of MCMs without requiring the user to alter the assembly
process radically. Motorola, for instance, has been using a 60-mm, 344-lead
ceramic quad flatpack to house several MCM-based products. The latest is a
68040-based processor, called the 168040, that crams in 5 Mbits of SRAM, a
field-programmable gate array, and a communication port.
Last year, the ball-grid array package was introduced as an alternative to the
quad flatpack (see Electronic Products, April 1993, p. 19). Now, several
companies are trying to use BGAs as the packaging format for MCMs (see page
xxx).
Known-good die available–at a cost
The use of MCMs has increased at a rate that coincides with the availability
of known-good die. These ICs are unpackaged, ready for assembly, pretested, and
guaranteed to have high-quality electrical performance and reliability. Because
MCMs are difficult if not impossible (and expensive) to rework, known-good die
are critical for them to be cost-effective.
Fortunately, many semiconductor manufacturers who build MCMs have made
known-good die a priority. For instance, Fujitsu Microelectronics (Santa Clara,
CA), has established a known-good die program for parts going into high-end
products like supercomputers, according to Dennis Stephenson, product marketing
manager for interconnect technology.
However, the availability of known-good die for use in less expensive,
commercial-grade MCMs is not as widespread. “Since the cost of implementing
known-good die typically adds at least $4 to $10 per part, parts used in
lower-end commercial applications do not always warrant the cost of supplying
the die in known-good die format to the user,” says Stephenson.
Roger Herbst, marketing manager of multichip technology for Cypress
Semiconductor (San Jose, CA), echoes Stephenson's statement. He asserts that
known-good die typically add $50 or more per board footprint to the cost of the
finished assembly–prohibitive for a cost-sensitive user. The company offers
known-good die for static RAMs on an as-needed basis.
National Semiconductor (Santa Clara, CA) has been actively making known-good
die available for the past 18 months. The company supplies, on customer
request, many of its parts as known-good die. National is also introducing some
standard parts as known-good die, according to Matt Penry, manager for the
company's Multichip Module Business Unit.
Motorola Semiconductor (Phoenix) does not formally supply known-good die yet.
However, the company offers die screened to most of the known-good die
requirements except burn-in and some electrical testing, according to John
Hornack, MCM operations manager for Motorola's Commercial Plus Technology
Operations Div.
The lack of standards is a stumbling block in making commercial known-good die.
The Microelectronics and Computer Technology Corp. (MCC) of Austin, TX, is
conducting a program to to determine the physical, electrical, and reliability
requirements for known-good die. MCC is now testing popular known-good die
carrier methods (such as temporary contact, soft connection, and minimal
packaging) for die damage and contamination, contact resistance, and
temperature cycling, and other parameters.
Diamond substrates
The three major substrate technologies used for multichip modules–MCM-L
(thick-film laminate), MCM-C (ceramic), and MCM-D (thin-film)–continue to find
niches in different applications. While materials like silicon, ceramic, and
aluminum nitride are most common, researchers are carefully studying another
material–diamond.
Sandia National Laboratories (Albuquerque, NM) is investigating the use of
synthetic diamond substrates for MCMs employed in the power electronics circuit
of a flight computer. Because of its high thermal conductivity, diamond allows
the substrate to dissipate heat effectively. Thus, more functions can be packed
into a given space, and three-dimensional MCMs can be more easily designed.
Several companies are developing the diamond materials, including E-Systems
(Dallas) and Norton Diamond Film (Northboro, MA). The problem with diamond is
its high cost–about 10 to 100 times costlier than silicon, according to Arjun
Partha, program manager of Norton Diamond Films. Nevertheless, Partha foresees
diamond working as a heat spreader atop a laminate in MCM-L applications.
nCHIP (San Jose, CA) goes another way by using a silicon circuit board
consisting of aluminum and copper metallization, silicon dioxide dielectric
layers, and a silicon substrate. The silicon technology provides an integral
decoupling capacitor. The capacitor is implemented using separate components
with the other substrate technologies. The silicon dioxide dielectric provides
high thermal conductivity.
Stacked chips
Stacking chips to attain higher density also continues to play a role in
building MCMs. Irvine Sensors (Costa Mesa, CA), recently received a research
and development contract from the U.S. Air Force's Phillips Laboratory to
demonstrate a three-dimensional multichip module that incorporates varying
sizes and types of chips and chip stacks.
Called the multiModular Computer Module, the MCM provides manufacturers and
users with a standard building-block structure for integrating the company's
chip stacks. The module shown in Fig. 2, for example, mixes an EEPROM, glue
logic, SRAM stacks, and a capacitor atop a processor. By mixing and matching
different chip stacks, a diverse range of electronic systems can be rapidly
fabricated. Moreover, the building-block structure eases field repair of the
multichip module.
Irvine Sensors has also announced that its Memory Short Stack technology (see
Electronic Products, July 1992, p. 21) is being used in a prototype
supercomputer being developed by Honeywell. The processor has a stack of six
memory chips, for a density of 10 GFLOPS/ft
Dense-Pac Microsystems of Garden Grove, CA, is also smoothing the user's road
to implementing MCMs with its ASIP (Application Specific Integrated Packaging).
With this technology, users can stack chips representing a mix of several
different IC technologies in the same footprint. They can also customize the
package width and depth. Technologies include SRAM, flash EPROM, and DRAM in
configurations ranging from 32-K x 8 to 4-M x 4 bits. Samples can be delivered
within 2 weeks with no NRE charges.
Future in bare chip?
Many current MCM packaging efforts involve placing several bare die in a
package and then attaching the package to a substrate. However, ever-rising
performance and density requirements lead most experts to project more use of
bare die attachment techniques in the future. By mounting the die on the
substrate through a flip-chip or other bare-die attachment technique, the
performance and space constraints imposed by the package are eliminated.
CAPTIONS:
Fig. 1. The PowerPlant 486 microprocessor upgrade MCMs from Kelly Microsystems
are MCMs that snap into the sockets of the original 286 CPUs, or on top of the
386SX CPUs.
Fig. 2. This demonstration module from Irvine Sensors mixes various chip
technologies in a single stack, making it possible to quickly assemble a wide
range of electronic systems using building-block functions.
The following companies supplied information for this article. For more
information, call the contact or circle the reader service number:
Cypress Semiconductor
San Jose, CA
Roger Herbst 408-943-2798
Dense-Pac Microsystems
Garden Grove, CA
in CA, 714-898-0007
E-Systems
Dallas, TX
Dean Schaefer 214-661-1000
Fujitsu Microelectronics
Santa Clara, CA
Dennis Stephenson 408-922-9214
Irvine Sensors
Costa Mesa, CA
Lynn O'Mara
Kelly Microsystems
Irvine, CA
Marci Schwabe
Micro Module Systems
Cupertino, CA
Howard Green 408-864-5986
Microelectronics and Computer Technology Corp.
Austin, TX
Joyce Graham
Motorola, Inc.
Tempe, AZ
Cousy Maher 602-897-4241
National Semiconductor
Multichip Module Business Unit
Santa Clara, CA
Jim Kath 408-721-7297
nCHIP
San Jose, CA
Jeff Denmin 408-945-9991
NEC Corp.
Mountain View, CA
Hotline 800-366-9782
Norton Diamond Film
Northboro, MA
Arjun Partha 800-336-6115
Sandia National Laboratories
Albuquerque, NM
David Andaleon 510-294-1552
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