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Vacuum Tubes > Transistors > Hybrid Circuits > Integrated Circuits
The Hybrid Microcircuit (1958-68)
A Hybrid Microcircuit is a miniaturized electronic circuit constructed of individual
semiconductor devices, as well as passive components, bonded to a substrate or circuit
board.
Some Memorabilia with Hybrid Microcircuits in them
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Interconnecting Electronic Circuits - The birth of the Micro-Module
Before the invention of the IC, electronic equipment was composed of discrete components like
transistors, which serve as both switches and amplifiers; resistors, which impede the flow of electrons;
and capacitors, which store them. These components, often simply called "Discretes", were
manufactured separately and were wired or soldered together onto Masonite-like circuit boards.
Discretes took up a lot of room and were expensive and cumbersome to assemble, so engineers
began, in the mid-1950s, to search for a simpler approach.
For almost 50 years after the turn of the 20th century, the electronics industry had been dominated by
vacuum tube technology. But vacuum tubes had inherent limitations. They were fragile, bulky,
unreliable, power hungry, and produced considerable heat.
It wasn't until 1947, with the invention of the transistor by Bell Telephone Laboratories, that the vacuum
tube problem was solved. Transistors were miniscule in comparison, more reliable, longer lasting,
produced less heat, and consumed less power. The transistor stimulated engineers to design ever
more complex electronic circuits and equipment containing hundreds or thousands of discrete
components such as transistors, diodes, rectifiers and capacitors. But the problem was that these
components still had to be interconnected to form electronic circuits, and hand-soldering thousands of
components to thousands of bits of wire was expensive and time-consuming. It was also unreliable;
every soldered joint was a potential source of trouble. The challenge was to find cost-effective, reliable
ways of producing these components and interconnecting them...
The Micro-Module (1958-1964)
One stab at a solution was the Micro-Module (also know as a Micromodule) program, sponsored by the
U.S. Army Signal Corps. The idea was to make all the components a uniform size and shape, with the
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wiring built into the components. The modules then could be snapped together to
make circuits, eliminating the need for wiring the connections.
The micromodule system called for piling tiny wafers of discretes on top of each
other like dishes. Connecting wires ran up the sides of the stacks through holes in
the wafers. Micromodules were not only somewhat easier to make than
conventional electronic systems, they were also a good deal smaller: a
six-component module was about the size of the sharpened cone of a pencil.
In the summer of 1958, a young engineer by the name of Jack S. Kilby went to
work for Texas Instruments, which by then had earned a reputation for itself as an
innovative manufacturer of transistors. Kilby was slated to work on TI's
micromodule program, but the army's system seemed to him to be unnecessarily
complicated. He wondered whether it would be possible, instead of stacking
discretes on top of each other, to fabricate all the electronic components -
transistors, resistors, and the like - out of the same piece of material. It occurred
to him that a properly engineered slice of germanium might be made to act as a
whole slew of components, much as a tapestry can be embroidered with any RCA's Micro-Module
number of designs and colors.
As computer systems grew more complex, engineers sought simpler ways to interconnect the
thousands of transistors they employed. Government agencies funded micro-module and multi-chip
hybrid circuit projects in search of a solution to this problem. In 1952, G. W. A. Dummer of England's
Telecommunications Research Establishment proposed "With the advent of the transistor and the work
in semiconductors generally, it seems now possible to envisage electronic equipment in a solid block
with no connecting wires."
As computer systems grew more complex, engineers sought simpler ways to interconnect the
thousands of transistors they employed. Government agencies funded micro-module and multi-chip
hybrid circuit projects in search of a solution to this problem. A hybrid integrated circuit, HIC, hybrid
microcircuit, or simply hybrid is a miniaturized electronic circuit constructed of individual devices, such
as semiconductor devices (transistors & diodes) and passive components (resistors, inductors,
transformers & capacitors), bonded to a substrate or printed circuit board (PCB). Hybrid circuits are
often encapsulated in epoxy. A hybrid circuit serves as a component on a PCB in the same way as a
monolithic integrated circuit; the difference between the two types of devices is in how they are
constructed and manufactured. The advantage of hybrid circuits is that components which cannot be
included in a monolithic IC can be used.
MICROMODULES: THE ULTIMATE PACKAGE (Army research)
Electronic Engineering Times, December 27, 1999, Rostky, George
It looked like a fountain pen, albeit a fat one, but it was really a radio. The U.S. Army liked it, not just
because it was very small for a radio, but because it introduced a new concept in circuit packaging
—small size and uniform construction. RCA's Surface Communications Division showed this pen-size
radio to the U.S. Army in October 1957, shortly after the Russians launched Sputnik, and
demonstrated that the world's leader in technology, the Americans, were way behind. At a time when
the American military was desperate to catch up, this ultimate in packaging density, called a
micromodule, could be just the answer.
The Army loved the concept. In April 1958, it awarded RCA a $5 million contract in what became
known as the Micromodule Program. Just a few months later, in January 1959, RCA's Aerospace
Communications and Controls Division built a stable inertial-guidance platform using micromodules.
And just two months after that, in March, at the annual show and convention of the Institute of Radio
Engineers (an IEEE predecessor), at a joint press conference with the Signal Corps, RCA announced
the commercial availability of Micromodule Designer's Kits for breadboards.
Then, in July 1959, the Army added $2.4 million to RCA's contracts—for new microelements, as they
were called—and for higher-temperature capabilities for some elements. In February 1960, the Army
added $8 million for demonstration helmet radios and miniature computers.
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The micromodule, as the package was called, was fundamentally akin to the short-lived Project
Tinkertoy, which the Navy funded in 1951. Tinkertoys were based on using 5/8-inch-square ceramic
wafers, each of them bearing a discrete component. That concept was the brainchild of Robert Henry
at the National Bureau of Standards.
The Micromodule Program was to use smaller ceramic wafers, 0.36-inch square, with three solder
ridges along each edge. These squares were stacked and interconnected by riser wires soldered to
those solder ridges, where appropriate. The entire module could be encapsulated and, depending on
the circuitry involved, might be from 0.4- to 0.8-inch high. Modules could be designed for almost any
system.
Production of a wafer element could be automated. Individual microelements might contain almost any
component. While RCA was the prime contractor, more than 60 producers participated in developing
individual microelement components and wafers. And here was another advantage. Individual
suppliers became component specialists and experts. Some companies would specialize in film
capacitors; others would do aluminum or tantalum electrolytic capacitors; still others would offer
thin-film or thick-film resistor networks; inductors; trimming potentiometers; and bare transistor chips
(called "dot" transistors). The top-most wafer could support a socket for a miniature 7- or 9-pin vacuum
tube, but that feature was never used, as transistors were already in widespread use.
It was more than obvious that the micromodule was the circuit package for the future. It was more
expensive than conventional printed-circuit packages, but follow-on costs, like maintenance and
logistics (including storage, handling and training) were lower.
The initial cost was indeed higher. In 1962, a 10-component micromodule cost $52 compared with
$20.45 for a printed-circuit-board equivalent. But micromodule costs were expected to plummet with
increased production in 1963 and 1964, with costs approaching or even slipping below conventional-
package prices, and declining rapidly beyond that.
Reliability soared. Based on millions of testing hours, RCA reported that the micromodule proved to be
six times as reliable as conventional military circuits and 60 times as reliable as tube circuits, still
widely used. Most wonderful, circuit density was beyond the wildest imagination of a few years earlier.
RCA's helmet radio had the equivalent of 210,000 components per cubic foot. And the future might see
packages with a staggering 600,000 parts per cubic foot. This was a remarkable contrast to 100,000
parts per cubic foot in the most advanced conventional circuitry.
For once, the future was clear. In August 1962, the Army's chief signal officer, Major General Earle F.
Cook, reported that the micromodule program had proved so successful that the Army's deputy chief of
staff for logistics had issued a directive to "take prompt and positive action to incorporate the
micromodule concept as appropriate."
Cook reported that the Army expected to commit $8 million for micromodule equipment in fiscal 1963,
about double the 1962 funding. Some $18 million had already been invested in the program.
In his 1962 report—at a joint press conference with RCA—Cook looked back fondly at the March 1959
Signal Corps-plus-RCA press conference when the micromodule was introduced to the world with
great expectations. As he listed a large number of micromodularized equipment that was to be
purchased, Cook expounded on the great benefits already realized as a result of the micromodule
revolution since that 1959 press conference.
The Future
Cook did note that the micromodule would accommodate advanced integrated circuitry, "logically,
through evolution, rather than by revolution. Such developments," he said, "normally mature bit-by-bit
over a protracted period of time, rather than suddenly appearing as an entire operational system. The
industry anticipates a long period of applications of the new devices in hybrid combinations with
conventional components."
Cook was clearly aware that the "advanced integrated circuitry," which micromodules would
accommodate through evolution over a protracted period of time, were beginning to attract some
attention.
But he may not have been aware of another press conference that took place in March 1959, possibly
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just a few doors from the Signal Corps-RCA press conference. That's when Texas Instruments
announced the work of an engineer, Jack Kilby, who had joined the company a year earlier.
Kilby had developed what TI called a "solid circuit," and what TI hailed as the ultimate in
microminiaturization. Kilby, it turned out, had a great deal to do with the fact that micromodules did not
accommodate advanced integrated circuitry. In fact, Kilby was a trigger for the demise of the
micromodule program.
Instead of mounting individual components on individual ceramic wafers, Kilby thought it would be nice
to manufacture all circuit components in one operation with one material. He was not the first to harbor
that idea. Others included G.W.A. Dummer of the Royal Radar Establishment in England who, in 1952,
spoke of the possibility of blocks of equipment with no connecting wires.
Solid circuits
At its March 1959 press conference, TI introduced two Kilby "solid circuits," a two-transistor flip-flop
and a one-transistor phase-shift oscillator, each at $450. (Competing thin-film modules cost less than
$50, so the solid circuit appeared to have a future only where cost was not a consideration.) Each solid
circuit was built on a single semiconductor chip. Within a year, TI had a line of six solid circuits—all in
flat packs—including gates and flip-flops. The dual-inline package, invented by Bryant C. (Buck)
Rogers at Fairchild, came later.
Though the name "solid circuit" didn't stick, "integrated circuits," as such devices came to be known,
did have a rather successful future, even surpassing TI's declaration in 1959 that these products
"represented the ultimate in microminiaturization."
Shortly after TI's introduction of these ICs, Fairchild became the second vendor, with IC's invented by
Fairchild co-founder Robert Noyce, based on the planar process developed by Fairchild's Jean Hoerni.
These IC's and those that followed from many vendors, all using the planar process, took over the
world and killed the micromodule program.
Yet the micromodule program was a huge success. While the program lasted, micromodule packages
accommodated 675 different circuits. In 1963, micromodule production at RCA alone reached a peak
of 10,000 units a month. And micromodules met or surpassed all the initial goals.
By late 1964, the initial enthusiasm for micromodules was gone and there were no further major
commitments. The early hesitancy in adopting integrated circuits was replaced by eagerness as prices
plummeted. ICs began to flourish as micromodules began to vanish.
RCA Micro Modules mounted on a PCB Sub Assemblies of the RCA MicroModule
Some Memorabilia with Micro-Modules in them
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RCA MicroModule assemby paperweight
Transistorized Modules (early 1960s)
Some computer makers such as Burroughs used plug-in type
Modules that allowed multiple Transistors, Resistors and
other components to be efficiently arranged inside. An array
of these small modules were plugged into printed circuit
boards allowing for even greater density than the earlier
technology which mounting the components individually and
spaced apart on a circuit board.
A 4 transistor module from a Burroughs B5500 Computer
IBM's Solid Logic Technology - SLT (1964-1968)
The Solid Logic Technology (SLT), introduced in 1964 by IBM in
System/360, was the industry's first high-volume, automatic,
microminiature production of semiconductor circuits. Mounted on
1/2-inch-square ceramic modules, the SLT circuits were denser,
faster and required less power than the previous generation of
transistor technology. In 1968 IBM's widely used Solid Logic
Technology modules achieved a reliability rate one thousand times
greater than its vacuum tube predecessors.
IBM developed Solid Logic Technology (SLT) for the System/360
computer family in 1964 prior to the ability of monolithic IC's to meet
the cost and speed demands of large computers. Transistor chips
IBM System 360 SLT Ad (1964)
and passive components mounted on 0.5" square ceramic modules
with vertical pins consumed less power and space while offering
faster speed and superior reliability compared to printed-circuit boards with packaged transistors. IBM
produced hundreds of millions of SLT modules in a highly-automated, specially-built plant in East
Fishkill, NY.
Solid Logic Technology was IBM's method for packaging electronic circuitry introduced in 1964 with
the IBM System/360 mainframe computer series and related machines. IBM chose to design custom
hybrid circuits using discrete, flip chip mounted glass--encapsulated transistors and diodes, with silk
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screened resistors on a ceramic substrate. The circuits were either encapsulated in plastic or covered
with a metal lid. Several of these were then mounted on a small multi-layer printed circuit board to
make an SLT module. Each SLT module had a socket on one edge that plugged into pins on the
computer's backplane (the exact reverse of how most other company's modules were mounted). SLT
was a revolutionary technology for 1964 with the reliability improvements over other assembly
techniques helping propel the IBM/360 mainframe family to overwhelming success during the 1960s.
SLT research produced ball chip assembly, wafer bumping, trimmed thick film resistors, printed
discrete functions, chip capacitors and one of the first volume uses of hybrid thick film technology.
Some Memorabilia with Solid Logic Technology in them
Transistor chips being placed on an IBM SLT Logic Chip using Automated IBM SLT Chips
Manufacturing (1964)
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Evolution of IBM 1960's Logic Chip Packaging Technology
Solid Logic Technology (SLT) introduced in 1964
IBM's first generation of Solid Logic Technology (SLT) chips used
only one circuit per module and the had 12 pins.
The Next Generations of the IBM SLT
The same basic packaging technology (both device and module) was also used for the
devices that replaced SLT as IBM gradually transitioned from hybrid integrated circuits to
monolithic integrated circuits:
Solid Logic Dense (SLD) introduced in 1967
Solid Logic Dense (SLD) increased packaging density and circuit
performance by mounting the discrete transistors and diodes on top of
the substrate and the resistors on the bottom. The SLD had two to five
circuits per module and 16 pins.
Advanced Solid Logic Technology (ASLT) 1967
Advanced Solid Logic Technology (ASLT) increased packaging density and circuit
performance by stacking two substrates in the same package.
Monolithic System Technology (MST) introduced in 1968
Monolithic System Technology (MST) increased packaging density and
circuit performance by replacing discrete transistors and diodes with
one to four monolithic integrated circuits (resistors now external from
the package on the module). The MST was introduced with six circuits
per module but eventually had up to 40 circuits per module. It had 16
pins.
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