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RELATED APPLICATIONS
The disclosure of co-pending patent application Ser. No. 08/053,505 filed
Apr. 27, 1993 and entitled "Integrated Circuit Package With Direct Access
to Internal Signals" is hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated circuit packages. More
particularly, the present invention relates to a method and apparatus for
socketing semiconductor devices and attaching semiconductor sockets to
printed circuit boards and similar structures.
2. The Prior Art
The package for an integrated circuit ("IC") semiconductor device typically
fulfills three functions. It provides environmental and physical
protection for the silicon chip, it provides a means to facilitate
handling of the chip, and it provides electrical connection from the chip
to the system in which it is installed. Packages are usually soldered to
their circuit boards to physically and electrically connect the package to
the circuit board.
An additional level of packaging--a socket--is sometimes used to
electrically and physically connect the IC package to its printed circuit
board. Sockets for IC packages are usually fitted with pins which are
soldered to a circuit board; pressure contacts can also be used, in which
case the socket is pushed against the circuit board with screws, springs,
or some other type of mounting hardware. In the latter configuration
pressure contacts must have enough compliance or conformance to compensate
for non-planarities present in the circuit board and/or the mounting
surface of the IC package.
With the increasing cost and complexity of some integrated circuits and the
high penalties often associated with equipment down-time, there is an
increasing need for component repairability and an ability to change
system functionality with little or no impact on the rest of the system.
It is well understood that determination of a defect's cause is made
easier by minimizing perturbations of the defective system. Removing a
device from its conventional socket may cause changes in the socket/IC
package system which will prevent adequate failure analysis. While
allowing the removal and replacement of components, neither type of socket
described above can be removed from a circuit board easily. Thus, a
defective part can be easily replaced, but not a defective socket. Also,
the time it takes to determine the cause of a failure--whether in the
device itself or in the socket--may result in an unacceptable amount of
down-time.
There are numerous examples of the first type of socket described above
sold to various standard through-hole and surface mount footprints by a
variety of vendors. Nepenthe Distribution, Inc. (U.S.A.) of Palo Alto,
Cal., for example, sells sockets for various pin grid arrays and quad
flatpacks such as the 225-pin PGA socket (Part Number NEPS-225-RS15) and
the 68-pin PLCC socket (Part Number 1-068-0000). Both of these are
intended to be soldered to a circuit board. U.S. Pat. No. 5,176,524 to
Iseki et al. entitled "IC Socket Structure" teaches the use of sockets for
surface mount devices ("SMDs") that are themselves surface mountable.
These are also intended for soldering to a printed circuit board.
Due to the fact that both through-hole and surface mounted sockets are
permanently attached to their circuit boards by solder, repair and
replacement of such sockets from their circuit boards is a serious
problem, usually requiring removal of the circuit board for a time from
its system. The socket must be removed by localized or general heating of
the circuit board past the melting point of solder. Further cleaning and
preparation are then required before a new socket can be installed on the
circuit board. Such delays and the accompanying potential for additional
damage to expensive circuit boards due to heating makes these types of
sockets unsuitable for many applications.
Of the second type of socket, U.S. Pat. No. 5,127,837 to Hopfer et al.
entitled "Electrical Connectors and IC Chip Tester Embodying Same" teaches
the use of an insulator ring permanently affixed to a printed circuit
board wherein the insulator ring is fitted with resilient wadded
conductors which provide electrical connection from an integrated circuit
package to a circuit board. In this case, the socket is not soldered to
the circuit board, but rather is attached by mounting hardware such as
screws and is not adapted to be removed from the circuit board without the
IC package first being removed from the socket. Thus the socket and IC do
not form an integral package.
Similarly, U.S. Pat. No. 4,692,790 to Oyamada entitled "Structure for
Connecting Leadless Chip Carrier" teaches a means of aligning an IC
package to a printed circuit board and use of an elastomeric interconnect
medium. A hinged cover with a spring in its center applies a downward
force against the IC package, creating an electrical connection between
the IC package and the circuit board. Although the interconnect medium and
the IC package can be replaced, there are no provisions for removing the
entire socket from the circuit board. Other sockets suffer from the same
drawbacks. See, e.g., U.S. Pat. No. 5,100,332 to Egawa entitled "IC
Socket" and U.S. Pat. No. 4,954,088 to Tsutomu et al. entitled "Socket for
Mounting an IC Chip Package on a Printed Circuit Board".
U.S. Pat. No. 5,161,983 to Ohno et al. entitled "Low Profile Socket
Connector" teaches attaching a sheet of elastomer to a molded plastic base
by means of plastic pegs. Several problems are presented by such a
structure. Elastomers as described are prone to tearing, particularly at
stress concentrations such as those applied when stretching elastomers
over pegs. Further, elastomers typically require low pressures to engage
electrically two opposing point contacts. Localized deformation of the
elastomer--typically a low-to-medium durometer silicon rubber--enable such
low contact force engagements. Over larger areas, such as those found
across land-grid array ("LGA") packages, there is no localized deformation
of the elastomer. In such cases the bulk elastomer material must be
compressed, requiring significantly higher forces to engage electrically
the contacts on either side of the elastomer. Although this patent teaches
contact on LGAs with three rows of contacts on each of the four sides of
the LGA's bottom surface, substantial force would need to be applied to
the LGA to compress the bulk elastomer material enough to create
electrical connection between the LGA and the circuit board given the
clamping mechanisms described.
Accordingly, there is a need for a demountable IC socket with high pin
density.
BRIEF DESCRIPTION OF THE INVENTION
In accordance with the present invention, a demountable socket for a high
pin density integrated circuit chip is provided which, together with the
IC chip forms an integrated structure. The chip is permanently mounted to
a ceramic carrier or "spreader", the spreader is, in turn, compressively
held against a first array of compliant or conformal button-type contacts
which are, in turn, electrically connected to a second array of conformal
contacts on the outside of the socket which are adapted to contact a
land-grid array on a printed circuit board or similar structure. The
socket is adapted to be easily and rapidly mounted and demounted to and
from the circuit board with simple hand tools. The IC chip need never be
removed from the socket in order to affect mounting and demounting. Shock
and contamination protection for the chip is provided by the socket which
is adapted to be hand held without significant risk of damage to the chip.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded perspective view of an integrated circuit and socket
package according to a presently preferred embodiment of the present
invention.
FIG. 2 is a front cross-sectional view of a portion of the assembly shown
in FIG. 1.
FIG. 3 is a front cross-sectional view of a portion of another embodiment
of the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Those of ordinary skill in the art will realize that the following
description of the present invention is illustrative only and not in any
way limiting. Other embodiments of the invention will readily suggest
themselves to such skilled persons. The integrated circuit packaging
technique of the present invention is now described in further detail with
reference to the drawings which illustrate various embodiments within the
scope of the invention.
FIG. 1 illustrates a presently preferred embodiment of the present
invention. Referring to FIG. 1, the integrated circuit die 10 is mounted
by the solder-bump flip-chip/C4 method onto the upper surface of a
multi-layer ceramic carrier (or "spreader") 12 as is well known to those
of ordinary skill in the art. The ceramic material used in the fabrication
of spreader 12 is preferably 90% alumina and 10% glass available from
Kyocera Corporation of San Jose, Cal. Spreader 12 need not be fabricated
from a ceramic material. Any of a variety of materials used in the
fabrication of chip carriers and printed circuit boards may be used, such
as FR4, bismaleimide triazine ("BT"), polyimide, etc. as is well known in
the art.
The solder-bump flip-chip/C4 process for connecting die 10 to its substrate
or carrier 12 provides the highest available density of interconnects, but
is not required by the present invention. The interconnections can also be
achieved by wire bonding, tape automated bonding ("TAB"), elastomeric
interconnect, or any of a variety of methods that are well known to those
of skill in the art.
A shell or cap 14 fabricated from a ceramic or plastic material with
B-stage epoxy 16 pre-attached by the supplier is affixed to spreader 12
above die 10. Cap 14 protects the delicate die 10. Such caps are readily
available from a number of suppliers, such as Kyocera Corporation of San
Jose, Cal. (Part Number KA0523B w/NC0-1505). Those of skill in the art
will recognize that there are numerous other methods of protecting the
interconnects and the integrated circuit, such as B-stage epoxies ("glob
tops") as are frequently used to protect chips mounted directly to printed
circuit boards.
Spreader 12 in the presently preferred embodiment is comprised of multiple
layers, 21 in the presently preferred embodiment. These layers are
themselves comprised of ceramic with metal lines deposited on their
surfaces. Vias are used to form connections between layers. The metal
lines spread the array of contacts 18 from a pitch of 0.0165" at the upper
surface of spreader 12 where die 10 is attached to a pitch of 0.040" at
the lower surface of spreader 12 which is also the pitch of the circuit
board pads or land-grid array 20 to which the socket 44 is to be attached.
Although design and manufacture of multi-layered ceramic carriers is well
known in the art, the substrate material need not be ceramic, as discussed
above, nor must the metal lines be deposited--they may be etched into
metal clad substrate materials rather than deposited. The bottom surface
of spreader 12 has an array of contacts 22. In the presently preferred
embodiment, contacts 22 are bumps or lands deposited on the bottom surface
of spreader 12 through electroless processes as is well known in the art.
The outermost metalizaton is gold, although other noble metals may be
used. The core of contacts 22 can be any conductor such as copper, nickel,
tungsten, or, as presently preferred, molybdenummanganese. Those
knowledgeable in the art will realize that there are many alternatives to
bump metalizations which could be used herein.
Spreader 12 is placed in a molded plastic socket cover 24. Although cover
24 must be dimensionally stable over the normal operating temperature of
the device for which it was designed, there are few materials
restrictions. Any of a variety of moldable or machinable plastics can be
used with Ryton TM available from Phillips 66 Company of Bartlesville,
Okla. presently preferred. Alternatively, metal may be used if heat
dissipation from the substrate 12 is required.
Spreader 12 is held in place and registered to the next level of
interconnect by expansion of the side walls 26 of socket cover 24.
Alternatively, co-molded beam springs or shaped metal springs may be used
to locate spreader 12 within socket cover 24. Depending upon the
dimensional accuracy of spreader 12 and the next level of interconnect,
more or less rigorous alignment methods can be used.
The next level of interconnect may be selected from any number of types of
compliant or conformal connectors. The presently preferred embodiment
utilizes a molded plastic insulator or "socket base" 28 comprising an
array of apertures set at a pitch of 0.040" and fitted with wadded wire
contacts or "buttons" 30 and bullet shaped plunger contacts or "plungers"
32 as available from Cinch Connectors of Elk Grove Village, Ill. The
buttons 30 are preferably fabricated of 0.001" diameter gold wire formed
into a cotton-like material and protrude upward from the array of
apertures in socket base 28 by approximately 0.006"; the plungers 32 are
preferably fabricated from gold plated brass and are bullet shaped and
protrude downward from the array of apertures in socket base 28 by
approximately 0.006". Thus socket base 28 with contacts 30 and 32 can
compensate for significant non-planarities of both the spreader 12 and the
circuit board 34 to which the assembly 44 is mounted. The present
invention is not limited to the use of wadded wire/plunger technology.
There are numerous suppliers of directionally conductive materials that
may be used instead of the wadded wire/plunger approach described herein.
Fujipoly, Inc. of Cranford, N.J. supplies silicone rubber sheets with fine
wires oriented to conduct only along the material's thickness (z-axis)
(Part Number 06-10099A, for example). Other technologies such as
ISOCON.andgate. available from Rogers Corporation of Rogers, Conn. utilize
conductive spring members suspended in closed-cell neoprene or silicone
carriers. Any of a variety of such uni-directional connectors may be used.
The presently preferred cover 24 is molded with multiple posts 36 along its
periphery. These posts 36 fit into shaped holes or apertures 38 in base 28
to form a plastic rivet attachment means. In order to affix cover 24 to
base 28 one pushes base 28 against spreader 12 and pushes against the ends
of posts 36. This causes the plastic posts to deform, locking the posts
36, and therefore the entire assembly, in place. There are numerous other
permanent methods of attaching cover 24 to base 28, such as metal or
plastic rivets, spot welding, or adhesive (glue). Although the assembly of
the cover 24 to base 28 in the presently preferred embodiment is
permanent, there are a number of non-permanent methods available for
removably attaching base 28 and cover 24 together, if desirable. Threaded
fasteners, thermoplastics and spring clips could be used for this.
Posts 40 on the bottom surface of base 28 are adapted to fit into
corresponding holes or apertures 42 in the circuit board 34 in the
presently preferred embodiment. This provides alignment of the socket
assembly 44 to the printed circuit board 34. Although posts 40 are part of
base 28, metal pins could also be used, whether co-molded with the base 28
or added after molding. Similarly, pins can be fitted in the printed
circuit board which are adapted to align and mate with holes in the base
28. If the alignment of base 28 to printed circuit board 34 is not
critical, alignment pegs 40 and alignment holes 42 may be omitted.
Alignment can then be achieved with the placement of screws 46 through
cover 24 and circuit board 34, finally tightening in support or backing
plate 48, or even another socket assembly 44 on the opposite side of the
circuit board.
Force must be applied to buttons 30 by spreader 12 in order to make a
reliable electrical connection between the two pieces 12, 28. The assembly
of cover 24 to base 28 pushes base 28--and therefore buttons 30--against
contacts 22 on spreader 12. Force must also be applied by plungers 32
against contact pads 20 on printed circuit board 34 in order to make
reliable electrical connection between the two pieces 28, 34. Screws 46,
passing through the corners of cover 24 and printed circuit board 34 are
adapted to screw into a support plate 48. The support plate 48 may be made
of any suitably rigid material that can hold threads. This system squeezes
circuit board 34 between support plate 48 and socket base 28. The normal
force needed to ensure adequate contact between plungers 32 and circuit
board 34 is typically about 1 to 3 ounces per contact. The presently
preferred embodiment utilizes standard #2-56 stainless steel screws. The
torque applied to each screw is preferably 8-12 inch-ounces. If circuit
board non-planarities are severe, larger screws or higher torque may be
used. If the board is sufficiently rigid, nuts may be used instead of
support plate 48. Those knowledgeable in the art will realize that there
are other methods of mounting the socket assembly 44 to circuit board 34
that do not use screws (e.g., bolts), and that the screws and required
torque are unique to the presently preferred embodiment.
To conserve board space or to provide unique electrical functions, it is
possible to mount socket assemblies on both sides of the circuit board.
Such an arrangement is depicted in FIG. 3 a pair of socket assemblies 44a
and 44b are shown attached to opposite sides of circuit board 34a with
screws 46a. As shown, screws 46a pass through the cover 24a of the top
socket assembly 44a, through the circuit board 34a, through the cover 24b
the bottom socket assembly 44b, and fasten to a support ring or plate, or
a nut. In FIG 3, support plate 54 is threaded to receive screws 46a.
According to a presently preferred embodiment of the present invention
socket cover 24 is provided with a large square aperture 50 therethrough
and cap 14 may be provided with printing 52 which is visible through
aperture 50. In this manner, chip identification information, for example,
may be printed on cap 14 and seen through socket cover 24, obviating the
need to print chip identifying information on socket cover 24.
While embodiments and applications of this invention have been shown and
described, it would be apparent to those skilled in the art that many more
modifications than mentioned above are possible without departing from the
inventive concepts detailed herein. The invention, therefore, is not to be
restricted except in the spirit of the appended claims.
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