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Description  |
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This invention relates to electronic device mounting and testing apparatus.
More particularly, it relates to socket apparatus for holding and making
electrical contact with the input/output terminals of ball grid array
devices during testing, burn-in and the like.
Advances in microelectronics technology tend to develop device chips which
occupy less space while performing more functions. As a result, the number
of electrical interconnections between the chip and external circuitry
required for the circuit in the chip to communicate with the outside world
increases and the physical size of each such interconnection must
decrease. In order to provide electrical communication between the chip
and external circuitry, circuit chips are usually contained within a
housing or package which supports interconnection leads, pads, etc., on
one or more of its external surfaces. In order to reduce overall lead
length from chip to external circuitry and to provide adequate spacing
between input/output terminals on the package, high pin count devices are
usually mounted in packages in which the input/output terminals are
arranged in a grid pattern on one face of the package. The terminals may
be in the form of pins extending from the package (usually described as a
pin grid array or PGA) or contact pads or lands (usually described as a
land grid array or LGA) on the surface of the package. To physically
secure the chip to a substrate and provide electrical connection between
its terminal pads and similar interconnect pads on the surface of a
substrate such as a circuit board or the like on which the package is to
be mounted, a small drop or ball of solder or the like is secured to each
terminal pad on the device package. Since the solder drop forms a
ball-like protrusion extending from the terminal pad, such devices are
ordinarily described as ball grid array (or BGA) devices.
While the term "ball grid array device" is usually applied to a device
package which has substantially spherical contacts extending from one face
thereof, the term is also applied to other structures. For example, bare
(unencapsulated) chips are sometimes provided with a grid array of
ball-shaped contacts for mounting in a package. However, at some point
during fabrication, the bare chip with ball-shaped contacts is fairly
described as a ball grid array device. Similarly, finished chips are
sometimes provided with terminal pads on one surface with ball-like
deposits of solder forming interconnections on the terminal pads. The chip
is then inverted and attached directly to a corresponding pattern of
interconnect pads on a substrate. When heated, the solder balls reflow
forming electrical and physical connections. This process (sometimes
referred to as "flip-chip" technology) obviously uses devices which may be
described as ball grid array devices. Accordingly, for purposes of this
disclosure the term "ball grid array device" means any structure,
including device packages, flip chips and bare dies, carrying a plurality
of substantially ball-shaped interconnections on one face thereof which
are arranged in a substantially grid-like pattern. The ball terminals are
substantially spherical and are arranged on one surface of the device
package in a predetermined pattern. Since the ball terminals are
substantially spherical and uniform in size, each ball terminal has a
geometric center which is spaced from the surface of the device package
from which the ball terminal depends and the geometric centers of the ball
terminals lie substantially in a plane (or planes) parallel with the
surface of the device package from which the ball terminals depend. This
plane (or the corresponding plane for each individual ball terminal) is
referred to herein as the center, centerline or extended centerline of the
ball terminal.
Many electronic devices are subjected to testing and burn-in at some point
during or after the fabrication process. For burn-in and testing, the
device must be removeably mounted on a test fixture which provides
electrical connection with each of the input/output terminals while the
device is functionally tested and evaluated. In many cases the device is
subjected to harsh environmental conditions (such as heat, etc.) as well
as electrical stresses to evaluate and assure full functionality of the
finished device. In order to provide for effective testing and burn-in,
the fixture in which the device is mounted for testing and burn-in must
permit rapid and easy insertion and removal without damage to the device,
the device package or the delicate ball terminals. However, the very
features of the ball grid array device which make it attractive as a
device structure (i.e., closely grouped very small contacts arranged on a
hidden face) make it extremely difficult to reliably mount in a test
socket without damaging the device structure.
In conventional test structures the ball grid array device is positioned on
an interconnect substrate having interconnect pads corresponding to the
ball grid array pattern. The ball grid array device is positioned on the
substrate so that the terminal balls are individually in contact with
interconnect pads on the test substrate. However, to maintain the ball
grid array device in the proper position and orientation for testing, a
lid or cover must be used which entraps the device and maintains the ball
grid array in register and contact with the interconnect pads.
Unfortunately, the entrapping lid interferes with proper circulation of
cooling air around the device and precludes use of heat sinks even though
the device may be designed to operate only in connection with a particular
heat sink. Such lids or covers are also difficult to manipulate, may cause
damage to the device and generally prevent automated loading and unloading
of the test sockets.
The present invention avoids the difficulties of the prior art by providing
a mounting housing or socket with an open top. No lids, covers, etc., are
required. Thus the top face of the device under test is available for
attachment of a heat sink or open to cooling air or the like. Furthermore,
since the top of the socket or mounting housing is open, devices to be
tested can be inserted and removed by automated gravity fed processes
without fear of damaging the devices or the mounting apparatus.
The socket or mounting housing of the invention comprises a top support
member having a top face with a plurality of windows arranged therein to
receive the array of interconnection terminal balls depending from the
face of the ball grid array package. The socket also includes a base
member in which a plurality of axially elongated finger-like contact
members or pins are anchored. One end of each contact member extends
through the base to provide an attachment tail which may be soldered to a
burn-in board or the like. The opposite or free end of each finger
projects into one of the windows and is positioned adjacent one side of
the window. The central portion of each finger (between the free end and
the base) extends through an aperture in a bending or biasing plate
mounted substantially parallel with and between the base and the support
member. The biasing plate may be moveable laterally with respect to the
support member to move the free ends of the contact fingers with respect
to the windows but is ordinarily secured in a fixed position with
apertures therein offset with respect to the anchored position of the tail
and the windows in the support member. The offset bends or biases the
central portion of each contact member to produce a curve in each finger
as it extends between the top support member and base member.
A cam plate having a plurality of openings therein and adapted for lateral
movement is positioned between the support member and the biasing plate.
The free ends of the contact fingers project through openings in the cam
plate and extend into the windows in the support member.
The contact members are mounted so that when the socket is in the open
configuration the free end of each contact finger is adjacent one side of
a window. When a ball grid array device is positioned on the top face of
the support member, the terminal balls project or depend into the windows.
In the preferred embodiment, a cam is used to move the cam plate
laterally, thus simultaneously and uniformly moving the free ends of all
the contact fingers in the same direction. The free ends are thus urged
into contact with the terminal balls occupying the windows. The extreme
end portion of each finger is positioned adjacent the top of the window.
Thus, when the free end of the contact member is moved by the cam plate,
it contacts the terminal ball between the horizontal centerline of the
terminal ball and the face of the ball grid array device from which the
terminal ball depends. The fingers thus provide individual electrical
contact to each ball and, since they contact the balls above their
centerlines (between the center of each ball and the device face from
which it depends), they retain the balls in their respective windows and
thus entrap the ball grid array device. Since the ball grid array device
is held in place by the free ends of the contact members, the size of the
balls may vary within limits without affecting the trapping effect of the
contact fingers.
Because of the simplicity of design and operation, the socket devices of
the invention may be made from a wide variety of available materials.
Since the top of the socket is open, automated processes may be employed
to load and unload the socket without damage to the devices or the socket
and the top surface of the device is exposed for cooling and/or attachment
of a heat sink. Other features and advantages of the invention will become
more readily understood from the following detailed description taken in
connection with the appended claims and attached drawing in which:
FIG. 1 is an exploded perspective view of the assembly of a ball grid array
device with a preferred embodiment of the mounting housing of the
invention;
FIG. 1A is an exaggerated fragmentary view of the top surface of the
mounting housing of FIG. 1;
FIG. 1B is an exaggerated fragmentary view of the ball grid array surface
of the ball grid array device of FIG. 1;
FIG. 2 is a partial sectional view of the mounting housing of FIG. 1 taken
through line 2--2 showing the position of the contact fingers when the
mounting housing is in the open condition;
FIG. 2A, 3A is an elevational view of the pivoting bar and cam assembly
used to activate the cam plate in the mounting housing of FIG. 1;
FIG. 3 is a partial sectional view of the mounting housing of FIG. 1 taken
through line 2--2 showing the position of the contact fingers when a ball
grid array has been inserted into the socket and the socket is in the
closed position;
FIG. 4 is a partial sectional view of the mounting housing of FIG. 1 taken
through line 4--4 of FIG. 2;
FIG. 5 is a diagrammatic representation of the relationship between the
free ends of the contact fingers and terminal balls of various nominal
ball sizes extending from the surface of ball grid array device;
FIG. 6A is an enlarged fragmentary view of an alternative embodiment of the
mounting housing of the invention illustrating the open position; and
FIG. 6B is an enlarged fragmentary view of the same portion of an
alternative embodiment illustrating the closed position.
The terms "mounting housing" and "socket" are used synonymously herein to
describe a device or apparatus for holding a ball grid array device while
providing electrical contact to terminal balls depending from one face
thereof. For clarity of illustration, like numerals are applied to like
parts throughout the drawing.
Operational arrangement of a ball grid array device 10 with the mounting
housing of the invention is illustrated in FIG. 1. The ball grid array of
device 10 has a bottom face 11 on which are formed a plurality of
spherically-shaped terminals 12. The terminals 12 are formed by depositing
solder at predetermined locations on mounting pads or the like (not
illustrated) on the face 11 of the device. Various methods for forming
such terminal balls are known and form no part of this invention. However,
the various processes are known to produce substantially spherical bodies
(see FIG. 5) which depend from the lower face 11 of the ball grid array
device. The terminal balls 12 are usually solder which has been deposited
and heated so that it contracts into a substantially spherical shape by
surface tension. Regardless of the method of manufacture, for reference
purposes the ball-shaped terminals extending from the face of the ball
grid array device are referred to herein as terminal balls or ball
terminals.
Terminal balls 12 are arranged on the lower face 11 of ball grid array
device 10 in a predetermined grid-like pattern. To accommodate the ball
grid array device, the mounting housing of the invention employs a top
support member 22 which has a plurality of windows 23 extending
therethrough. The windows 23 are arranged in a grid pattern matching the
grid pattern of the ball terminals 12. To accommodate ball grid array
devices of different dimensions, the top face 24 of support member 22 may
be provided with removeable spacers 35 of various sizes and
configurations. The spacers 35 define the periphery of each particular
ball grid array device and position the ball grid array device to prevent
movement thereof laterally with respect to top face 24. Spacers 35
therefore assure that each ball grid array is aligned with the ball
terminals 12 depending from the lower face 11 thereof in proper registry
and orientation with windows 23 and may be changed as required for each
size and shape of ball grid array device package.
In the preferred embodiment illustrated in FIG. 1 the socket of the
invention is formed of a plurality of plate-like components contained
within a unitary box-like housing 100 having an open top and open bottom.
As best illustrated in FIGS. 2, 3 and 4, the housing contains a base
member 21 which has a plurality of apertures therein. Each aperture 30 has
an internal shoulder 31 (see FIG. 4). An elongated contact member 40 is
positioned in each aperture 30. In the preferred embodiment, each contact
finger 40 defines an axially elongated body of resilient electrically
conductive material such as nickel-coated steel or the like. The
mid-section 43 of each contact member 40 is substantially widened to form
shoulders 45 and 46 on opposite ends thereof. Accordingly, when contact
members 40 are inserted in the base member 21, tail portions 41 project
through apertures 30 and shoulders 46 rest on shoulders 31. Trap plate 25
having apertures 32 and shoulders 33 in registry with and corresponding to
apertures 30 is secured to base member 21. The central portion 44 of each
contact member 40 extends through an aperture 32 and the shoulders 33
contact shoulders 43 on the expanded mid-sections 43 of the contact
fingers 40. Accordingly, the contact members 40 are securely entrapped and
held in place in the base member 21 by trap plate 25. Obviously, other
arrangements may be used to secure the contact members 40 in the base 21.
The lower ends of the contact fingers 40 extend from the lower face of base
support 21 to define input/output tails 41. Tails 41 may be secured in a
suitable circuit board, burn-in board or the like. Alternatively, other
means for making electrical contact to the circuitry of the supporting
medium may be used.
The central portion 44 of each contact finger 40 which extends above the
mid-section 43 projects through an aperture 54 in a bending or biasing
plate 28 and terminates with the extreme free end 42a in a window 23 in
top support member 22. In the preferred embodiment, the central portion 44
of each finger 40 is resiliently biased or bent into a curved shape
resulting from a lateral offset between the apertures 54 in the biasing
plate 28 and corresponding apertures in the base member 21 and windows 23
in top support member 22. A cam plate 55 adapted for reciprocal lateral
movement is positioned between the top support member 22 and the biasing
plate 28. The cam plate 55 has a plurality of apertures 56 therein
corresponding with the windows 23 in top support member 22. The extreme
end 42a of fingers 40 project through apertures 56 and into the windows 23
but do not extend beyond the top face 24. For best results, extreme ends
42a should be as near the top face 24 as possible. It is necessary,
however, that the extreme end 42a be above the centerline of the ball
terminal which it contacts.
The cam plate 55 is urged laterally by a cam lobe 51 projecting laterally
from a pivoting bar 50. The cam plate 55 is positioned immediately
adjacent top support member 22 and adapted for reciprocal movement
laterally with respect to support member 22. Since mid-sections 43 of
fingers 40 are securely anchored between the base member 21 and trap plate
25, and since the central portions of fingers 40 are curved by virtue of
the offset between aperture 54 in the biasing plate 28 and window 23 in
the top support plate, lateral movement of cam plate 55 in the direction
of curvature of the contact member 40 causes the extreme ends 42a of the
contact fingers 40 to move both laterally and slightly downwardly.
The pivoting bar 50 includes a first end 52 and a second end 53 projecting
in substantially opposite directions from an axial pivot 200 and is
adapted to pivot thereabout between a first position and a second
position. A cam lobe 51 projects from the face of the pivoting bar 50.
When the pivoting bar 50 is positioned with the first end 52 above the
second end 53 (as illustrated in FIGS. 1, 2 and 2A) the cam lobe 51 is
positioned above the cam plate 55 and the socket is in the open position.
When downward vertical force is applied to the first end 52 pivoting bar
50 rotates about pivot 200 until the second end 53 is above the first end
52. As the pivoting bar 50 rotates, the cam lobe 51 engages the end
surface 29 of the cam plate 55 and urges the cam plate 55 laterally.
Movement of cam plate 55 forces the extreme ends 42a in the same
direction, causing them to traverse the windows 23. It should be
recognized that a pivoting bar 50 with a cam lobe 51 is only one preferred
means for moving cam plate 55. Other camming mechanisms such as rotating
cam shafts, wedge plates, ratchets, plungers, rack-and-pinion arrangements
and the like may be designed to perform the relative movement function of
the cam. Thus the terms "cam" and "cam plate" are used herein to describe
any mechanical arrangement which moves the extreme ends 42a of the contact
fingers 40 laterally with respect to the windows in the top support member
22 or vice versa.
The position of the contact members 40 when the socket is in the open
position is illustrated in FIG. 2. A slanted shaping surface 32a in trap
plate 25 permits central portion 44 of contact member 40 to bend toward
the corresponding laterally offset aperture 54 in biasing plate 28.
Slanted shaping surfaces 23a in top support member 22 and 56a in cam plate
55, respectively, also permit the contact members 40 to bend toward the
corresponding laterally offset apertures 54 in trap plate 28 and window
23. Apertures 54 progressively widen with distance from the center to the
upper and lower surfaces of biasing plate 28, thus allowing contact
members 40 to resiliently flex around a bending point 54a. Elastic forces
in contact 40 firmly bias the extreme ends or tips 42a against the shaping
surface 23a in window 23. Contact members 40 may be pre-shaped to conform
substantially to the configurations illustrated in FIGS. 2 and 3. However,
the contact members 40 are preferably normally straight sections of thin
metal ribbon or the like which are drawn into the configurations
illustrated by placement in the manner described. Where the contact
members are normally straight, the curvature of the contact members as
they pass through apertures 30 and 54 may not be as sharp as illustrated.
Regardless of the manner in which the contact members 40 are formed or
mounted, it is only necessary that, when assembled, the free ends 42a
deviate from the major axis of the contact member so that when the free
end 42a is urged into physical contact with a terminal ball 12, the free
end contacts the terminal ball between the centerline of the terminal ball
and the face from which it depends.
If desired, a spring (not shown) may be positioned between the housing 100
and the end of the cam plate 55 opposite end 29 to ensure that the cam
plate 55 is fully retracted. Accordingly, a ball grid array device may be
positioned with the ball terminals 12 depending into windows 23 by simply
positioning the ball grid array in the proper position. Since the free
ends 42a are adjacent the sides of the windows 23, the ball terminals 12
simply depend into windows 23. Thus, no pressure is applied to any portion
of the ball grid array device 10 or the depending ball terminals 12.
Furthermore, no force is applied (other than gravitational) to any portion
of the socket by the electronic device package or the ball terminals.
When the ball grid array device is securely in place, pivoting bar 50 is
rotated to urge lobe 51 into contact with the end surface 29 of cam plate
55. As cam plate 55 is moved (to the left as shown in FIGS. 2 and 3) by
lobe 51, the extreme ends 42a of the contact fingers 40 uniformly and
simultaneously move into contact with the ball terminals 12 depending into
the windows 23.
As best shown in FIG. 1A and graphically illustrated in FIG. 5, the extreme
ends 42a of contact fingers 40 are positioned to extend into windows 23
near surface 24 but do not extend beyond surface 24. Furthermore, since
the contact members 40 are biased to form a curve by the relative
positions of the bending plate 28, the base 21 and the top support member
22, the extreme ends 42a deviate from the vertical toward the ball
terminal 12. As illustrated in FIG. 5, the extreme end 42a must extend
above the centerline of the ball terminal 12. For representative purposes,
FIG. 5 illustrates the relative position of extreme end 42a in contact
with a ball terminal when the nominal ball size is 0.030 inch. Nominal
ball sizes of 0.030 inch may vary from about 0.035 to about 0.024 inch in
diameter. Thus the point of contact on the ball may vary slightly with
variations in ball size. However, as shown in FIG. 5, where the extreme
end 42a extends at least 0.001 to about 0.002 inch above the extended
centerline (the horizontal line passing through the center of the ball
terminal 12), the point of contact between the extreme end 42a of contact
finger 40 will be above the extended centerline of the ball terminal 12.
Thus, since the ball grid array device 10 is trapped and prevented from
horizontal movement by spacers 35, pressure exerted against the ball
terminals 12 by extreme ends 42a of the contact fingers 40 have both a
lateral force component and a small downward force component. The ball
grid array device 10 is thus trapped and secured against the top face 24
of the support member 22 by the lateral and downward pressure exerted on
each ball terminal 12 by each contact finger 40. The pressure exerted on
each ball terminal 12 will be dependent, of course, on the length and
inclination of end section 42 of the contact member 40 from the vertical
major axis of the contact member 40, the resiliency of contact member 40,
and the movement of cam plate 55. These parameters may be controlled as
desired.
The relative positions of the components of the mounting housing and the
ball grid array device when the housing is in the closed condition is
illustrated in FIG. 3. Note that lobe 51 on pivoting bar 50 has forced cam
plate 55 to the left as shown in FIG. 3. The extreme ends 42a of contact
fingers 40 have moved in the same direction until they contact the
surfaces of ball terminals 12. As the cam plate 55 moves further to the
left, the free end 42 of each contact finger 40 is bowed until a contact
pressure of approximately thirty-five (35) grams is applied to each ball
terminal. Since the extreme end 42a of the contact finger 40 is above the
centerline of each of ball terminal 12, this pressure securely locks the
entire ball grid array device adjacent the top surface 24 of the mounting
housing and each contact finger 40 is in electrical contact with a ball
terminal 12 for electrical function testing, etc. However, a pressure in
the range of about thirty-five (35) grams is insufficient to damage or
dislodge the ball terminals 12. Since each terminal ball is contacted
above its horizontal centerline, damage to the balls below the centerlines
is avoided, thus enabling the balls to from proper solder joints.
Furthermore, additional flexing of the contacts 40 after the tip 42a
initially contacts the solder ball results in a scrubbing action which
penetrates surface oxidation on the ball terminals, thus providing better
electrical continuity.
After testing, burn-in or other procedures have been applied to the ball
grid array device 10, the device is released by merely rotating pivoting
bar 50 in the opposite direction, permitting the contact fingers 40 (and
springs, if included) to urge cam plate 55 in the opposite direction and
permitting the extreme ends 42a to return to and lie adjacent the angled
side 23a of window 23. The apparatus of the invention therefore provides a
totally zero insertion force open top socket for mounting ball grid array
devices for testing and burn-in. The test device may be simply inserted
into the open top of the mounting housing by gravity. No force (other than
gravity) is applied to the device package or the ball terminals 12 during
insertion or removal.
When the socket is in the closed position, the extreme ends 42a of the
contact fingers 40 each exert a lateral and downward force on each ball
terminal at approximately five degrees (5.degree.) above the plane of its
centerline. The pressure exerted by each individual finger is limited so
that there is no risk of damage to the ball terminals 12. Likewise, when
the contact fingers 40 are withdrawn to the open position, the ball grid
array device 10 may be removed simply by gravity or with a vacuum pencil
or the like. It is particularly noteworthy that the invention not only
permits total zero insertion force and withdrawal force, the housing 100
also contains all reactive forces applied to open and close the socket.
Thus no opening or closing forces which could damage the electronic device
package or the board on which the socket is mounted is ever transmitted to
the device package or the board. As illustrated in FIG. 3, an edge of the
device package 10 abuts edge 80 of spacer 35 which restrains movement of
the device package with respect to the housing 100. (Alternatively, each
ball could individually mate with a side of the window 23 in which it is
located. In either case, no pressure whatsoever is applied to the top of
the device package.) Furthermore, the entire top surface of the ball grid
array device is exposed since no lid or cover is employed. Cooling air may
be circulated thereover or a heat sink may be applied thereto.
Additionally, since test devices may be gravity loaded simply by vertical
movement, the test apparatus of the invention may be easily loaded and
unloaded by automated equipment.
It is to be understood that lateral movement of the free ends 42a with
respect to the ball terminals 12 may be accomplished in a variety of ways.
For example, the socket may be constructed such that the extreme ends 42a
of contact fingers 40 are held stationary while the device package 10 is
moved laterally. As illustrated in the alternative embodiment of FIGS. 6A
and 6B, top support member 22 is adapted for lateral movement by cam lobe
51 and cam plate 55 is held stationary. In the open position (illustrated
in FIG. 6A) the terminal balls 12 are positioned in windows 23 as
described above. As top support member is moved laterally (to the right as
shown in FIG. 6B) the terminal balls 12 are urged into contact wit the
free ends 42a while the end portions 42 of the contact members 40 are
maintained stationary and in the desired curved condition by the slanted
sides 56a of apertures 56 in cam plate 55. Alternatively, biasing plate 28
may be adapted to move laterally with respect to cam plate 55 and the
apertures 56 in cam plate 55 appropriately shaped to act as a fulcrum so
that movement of biasing plate 28 in one direction causes the extreme ends
42a to move in the opposite direction. Various other arrangements will be
apparent to those skilled in the art.
It will be readily recognized that the materials used for manufacturing the
invention may be varied as desired, depending upon the application.
Similarly, the physical size and shape of the components may be arranged
to accommodate any particular ball grid array device. For example, the
contact fingers 40 are shown as axially elongated metal strips and may be
cut or stamped from flat ribbon stock. However, the fingers 40 could be
formed from wire stock and may be formed into various configurations
without departing from the principles of the invention. Similarly, the
fingers may be anchored in the socket as desired by any suitable means. If
the socket is to be used for burn-in purposes, heat resistant materials,
of course, should be employed. The design is particularly attractive for
use in hostile environments since, in its preferred embodiment, very few
moving parts are employed and the opening and closing functions can be
readily automated. Thus the preferred structure is extremely reliable and
functional in extended repetitive use.
From the foregoing it will be recognized that the principles of the
invention may be employed in various arrangements to obtain the benefit of
the many advantages and features disclosed. It is to be understood,
therefore, that even though numerous characteristics and advantages of the
invention have been set forth together with details of the structure and
function of the invention, this disclosure is to be considered
illustrative only. Various changes and modifications may be made in
detail, especially in matters of size, shape and arrangements of parts,
without departing from the spirit and scope of the invention as defined by
the appended claims.
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