Note: Descriptions are shown in the official language in which they were submitted.
0146
This invention relates to the hermetic and non-hermetic
(plastic) packaging of devices, such as active and passive electronic
devices, and is particularly concerned with the provision of a considerable
degree of commonality in the packaging processes, and is also concerned with
a new lead frame for use in such common processes and with packaging members
for use in the hermetic process.
In the packaging of passive and active devices two methods can
be used, hermetic and non-hermetic or plastic. Hermetic packaged devices are,
as the name states hermetically sealed. Plastic packaged devices are not
hermetically sealed but are suitable for many operating environments.
Hermetic packaging is expensive. Conventionally, individual
chips or dies are mounted by hand, bonded to a base, a cap applied and then
fused.
Non-hermetic packaging is usually done in strips, the chips
or dies applied to pads in a lead frame, the chips usually applied by hand.
Bonding may be manual or automatic. The final device is then molded within
plastic.
Thus, in a device packaging plant, the chips or dies proceed
on two separate paths after die separation and sorting, with differing forms of
apparatus. In particular, the hermetic process is slower and more expensive
and more labour intensive.
The present invention provides for the hermetic and non-
hermetic packaging of passive and active electrical and electronic devices,
and is particularly suitable for semiconductor devices, using a common lead
frame in strip form, with, at the limit, only the molding or sealing steps
being carried out separately, although generally the wire bonding will also
be carried out separately when gold wire is required for the plastic molding
and aluminum wire for the sealing (fusing).
11;~0146
Broadly, in accordance with the invention, after wafers have
been tested, followed by die separation and sorting, the dies are attached
to a common lead frame. Then if gold wire bonding is used with plastic
molding and aluminum wire bonding used with hermetic sealing, the lead frame
separate, going to either aluminum wire bond and then to plastic molding or
to gold wire bond and then to capping and sealing. After either of these
steps the lead frames come together for plating of leads, trimming and forming,
and final test. If common wire bonding can be used then the lead frames
separate only after wire bonding, to go to either plastic molding or capping
and sealing.
The lead frame is formed with the normal leads extending
from a central die pad, the number depending upon the particular lead count
for the devices being packaged. The central die pad is also connected to the
frame with lead-like members projecting radially from each corner of the die
pad.
The invention will be readily understood by the following
description in conjunction with the accompanying drawings, in which:-
Figure 1 is a flow line diagram of conventional packaging
sequencies for hermetic and non-hermetic devices;
Figure 2 is a plan view of a novel lead frame strip
in accordance with the present invention, having ten frames or positions;
Figure 3 is an enlarged view of one of a single frame or
position of the lead frame strip of Figure 2;
Figure 4 is a flow line diagram in accordance with a
feature of the present invention;
Figure 5 is a plan view of a lead frame strip, after hermetic
sealing or plastic encapsulation;
Figure 6 is a plan view of one of the members which can be
used for the hermetic sealing;
)'1 46
Figure 7 is a cross~section on the line VII VII of
Figure 6;
Figure 8 is a cross-section through a hermetically sealed
package using members as in Figures 6 and 7i
Figure 9 is a cross-section through a non-hermetic sealed
package;
Figure 10 is an enlarged view of a single frame of a lead
frame strip, illustrating a modification thereof.
Figure 1 illustrates diagrammatically a typical flow line
system for hermetic and non-hermetic packaging. After wafers have been
processed, in the conventional manner, to produce a large number of chips or
dies, each having circuitry thereon, the dies are tested by testing apparatus
which usually comprises a series of probes which move down into contact with
contact pads on the dies, the testing apparatus including test circuitry for
checking the circuitry on the dies. The dies are generally moved sequentially
beneath the probes, and any dies which are faulty are marked by coloured ink.
This is indicated at 10 in Figure 1. After testing the wafers are broken up
into the separate dies and sorted - at 11. From here, in the conventional
systems, the dies proceed along two entirely separate paths depending upon
whether they are to be hermetically sealed or non-hermetically sealed. In
Figure 1 the hermetic seal path is designated 12 and the non-hermetic 13.
Taking first path 12 - hermetic packaging, the dies are
handled individually and attached individually to the die pad of a single
lead frame at 14. The leads are then wire bonded, using aluminum wire, to
respective pads on the die at 15. At this stage the lead frame is resting
on a bottom ceramic member which forms half of the hermetic package or seal.
The ceramic member has a bead of glass or similar fusible material on its
upper surface around the periphery thereof. A ceramic cap is then positioned
11'~0146
on the bottom ceramic member and lead frame. This cap also has a bead of
glass or similar fusible material adjacent its periphery, the two beads in
opposition. The assembly is then passed through a furnace where the beads
fuse and hermetically seal the two ceramic members together, the leads
extending out from between the members and hermetically sealed to them.
This is indicated at 16.
Following sealing, the leads are tin plated - at 17 - and
then the frame itself trimmed off leaving the leads extending from the package
at 1~. The leads are then bent down, the package tested - at 19 - and then
the hermetic sealed units packed for transport.
Turning now to path 13, non-hermetic or plastic packaging,
the dies are attached to the die pads of a number of lead frames in strip
form - step 21. The leads are wire bonded, using gold wire, at 22, and then
the strip of lead frames with dies passed to a molding die where the
individual dies and associated leads are embedded in plastic, at 23. The
leads extend from the plastic and these are tin plated, at 24. The frames
are then trimmed off leaving the leads extending and the leads are then bent
down - at 25. The packaged dies are tested, at 26, and then the plastic
embedded on non-hermetic units are packed for transport, at 27.
Thus two completely separate assembly or packaging lines are
provided with separate apparatus. For non-hermetic or plastic packaging
lead frames in strips can be used, but for hermetic packaging the devices
are handled individually.
With respect to the present invention, a lead frame strip
of a particular formation is used for both forms of packaging and the majority
of packaging steps are carried out on the same apparatus. The result is
cheaper packaging, particularly for the hermetic packaging, with the opportunityof more effective use of space, labour and apparatus.
11;~0146
Figures 2 and 3 illustrate a form of lead frame as used in
and forming a feature of the present invent;on. Figure 2 illustrates a strip
30 of ten lead frames indicated generally at 31, a single frame being
illustrated in Figure 3 to a much larger scale. The strip is provided with
locating holes 32 along each side. The strips of lead frames can be
formed by any conventional process, for example by photolithographically
etching from a strip of metal such as Kovar (TM). This metal has a coefficient
of expansion closely matching silicon. Other materials can be used.
In Figure 3 can be seen the die pad 33 to which is attached
the die, as by epoxy resin. The die pad 33 is attached to the frame 31 by
four radially extending lead-like members 34, one from each corner of the pad
33. In the particular example, each member 34 divides into two parts, one
part 34a, extending to the side member 31a of the frame 31 and the other
part 34b extending to a cross member 31b. A notch 35 can be formed at the
outer end of the radial portion of each member 34. In the example illustrated
twenty-four leads 36 are provided, the leads extending in from the side and
cross members 31a and 31b of the frame, with the inner ends stopping just
short of the die pad 33. The shear line for the leads is indicated by chain
dotted line 37.
Figure 4 illustrates diagrammatically a flow line, in
accordance with the present invention. After processing of the wafers, the
individual dies are tested as normally at test station 40. The dies are then
separated and sorted at 41. Then irrespective as to whether the dies are to
be hermetically or non-hermetically packaged, the dies are attached to the die
pads, 33 of a lead frame strip 42, the lead frame strips 30 indicated on
Figure 4. Following die attachment, in one embodiment of the invention, the
lead frame strips can follow one of two patns 43 or 44 for hermetic or
non-hermetic packaging.
0146
Following path 43, the leads are wire bonded to contact
pads on the die, using aluminum wire. The lead frames rest on a ceramic
bottom member at this stage - 45 - and then a ceramic cap is applied and
the assembly passed to the sealing of fusing station 45. Both the ceramic
bottom member and the cap have a peripherally extending bead of glass or
other fusible materials and these beads fuse and hermetically seal the
package.
Following path 44, the leads are wire bonded to contact pads
on the die, using gold wire, at stage 47, and then the lead frames pass to the
molding apparatus at 4~, the lead frame strips, with packaged dies, rejoin to
a common path 49. Passing along this path, the leads are tin plated, at 50,
then the frames trimmed off and the leads bent or formed down at 51. At this
stage, the devices, in whatever form of package, that is hermetic or non-
hermetic, are separated for the first time. After testing at 52 the devices
are packed at 53, for transport.
In an alternative arrangement the testing can be carried out
prior to complete separation from the lead frame.
Thus the testing as indicated at 52 in Figure 4, can be
carried after the leads are sheared and formed, at 51 in Figure 4, then
testing carried out at 52. The testing apparatus can mark such devices as
are not acceptable and then at a final separation step, indicated in dottPd
outline at 76 in Figure 4, only those devices which have passed the test are
separated, the separated devices then being fed to a "stick" in which the
devices are stacked for loading into apparatus for attachment to circuit
boards. The final separation takes place by shearing at the notches 35.
It will be appreciated that the production rate at the various
steps in Figure 4 can vary from step to step. Thus it is likely that certain
items of apparatus may be required in plurality. For example, the molding
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11'~0146
apparatus, with a large die having a large number o~ cavities may have a
production capacity higher than other steps. For example, a molding machine
can have a die with 180 cavities, each cavity taking a strip of 10 dies,
that is 1800 devices per shot, and can operate at 20 shots per hour, that
is 36000 devices per hour. This can be rather higher than the production
rate of the die attachment apparatus, or wire bond apparatus, or other steps.
This is met by providing additional machines at the slower steps all feeding
into the common production line. This is likely to be more economical than
reducing output of the molding machine, but this can be done, of course.
Due to the temperatures used conventionally in fusing the
glass or similar sealing material in the hermetic sealing at 46, it is normally
necessary to wire bond with aluminum at stage 45, to avoid gold contamination
of the die. However, lower sealing temperatures are now being contemplated,
using different hermetic sealing materials and it is now being considered that
gold wire bonding can be used, or will shortly be able to be used, for hermetic
packaged devices. In this case it will be possible to use a common wire
bonding stage for both hermetic and non-hermetic packaging. Then, the lead
frame strips, instead of separating along paths 43 and 44 after the die
attachment at 42, will proceed along one path to the wire bond station
indicated in dotted outline at 55. The lead frame strips then separating
along paths 56 or 57 to either the hermetic seal station 46 or molding station
48, and then coming back together again for tin plating at 50. Thus there
will only be one stage different for the two forms of packaging.
By the use of a common lead frame strip, it becomes possible
to use common apparatus and methods for many of the various steps for both
forms of packaging, hitherto not possible.
Figure 5 illustrates, in plan form, a strip of lead frames
after attachment of dies and wire bonding, after sealing of the ceramic members
11'~()146
together or encapsulation by plastic material, and prior to trimming of the
outer frame. It is a Feature of the invention that, if desired, instead
of the outer frames being trimmed off and then the leads formed, the leads
36 can be sheared from the frame and then formed, that is bent down, while
the packaged die is still held in the lead frame by the corner members 34.
The packaged devices can then be tested while in the strip. After testing
the frame can be trimmed off by shearing of the corner members 34 at the
periphery of the ceramic members. Thus, as illustrated in Figure 4, instead
of trimming and forming at stage 51, only forming would be carried out and
trimming would be done between stages 52 and 53.
Figures 6 and 7 illustrate a form of ceramic member 60
which can be used for the hermetic packaging. The member 60 is a flat member
having a raised rim 61, on which is deposited or formed a bead of glass or
other fusible material 62. The rim 61 defines a recess 63. In use one of
the members 60 is laid down at each of several positions corresponding to a
die pad on the lead frame strip. The lead frame strip is laid down, the
die pads over the recesses. The dies have previously been wire bonded to
the leads. A further member 60 is then positioned over each die, with the
rims 61 in opposition. The assembly is then heated and the beads fuse,
sealing the two ceramic members 60 together. Figure 8 illustrates the
sealed assembly after trimming off of the outer frame of the lead frame 31
and forming of the leads 36. A die is indicated at 64 with the wire leads
between die and frame leads indicated at 65. The recess 63 in the cap 60
provides clearance for the die and wire leads. The recess 63 is not strictly
necessary in the bottom member 60 but it conveniently enables the use of one
form of member for top and bottom. If desired the bottom ceramic member can
be a plain flat member, or of other suitable form.
Figure 9 is a cross-section, similar to that of Figure
ll'~V146
8, illustrating a plastic encapsulated arrangement. The frame 31, with the
die 64 bonded to the die pad 33 and wire leads 65 bonded to the die and frame.
The whole assembly is encapsulated in plastic material 70.
Figure 10 illustrates a modification to the lead frame
which can be provided to decrease the occurrence of excessive flash material
during plastic encapsulation. A "dam" bar 72 is added to the lead frame,
at each frame, extending between the leads 36 at a position which is just
outside the periphery of the encapsulating material. The dam bar is either
sheared out as indicated by chain dotted lines 73, at the same time as the
frame is sheared from the leads or is sheared out before shearing off of the
frame to enable testing to be carried out in the strip form. The dam bar
forms no part of the assembly for hermetic sealing but the same frame is used
and the dam bar sheared out after sealing.
Figure 10 also illustrates a modification in which the
leads 36 are extended, as compared to the leads of the lead frame illustrated
in Figure 4. Such long leads would be useful when the devices are mounted
with the leads passing through holes in a circuit board. Also, with the
provision of the dam bar 72 the cross-members 31b can be omitted and the
ends of the leads remote from the pad 33 can be unattached to the lead frame.
However, the dam bar 72 can be provided with the form oF lead frame
illustrated in Figure 3.
The predominant advantage of the present invention is
that there is effectively a single, common, production line for the
production of either form of packaged device, with only a divergence for
the hermetic or non-hermetic final form. This provides various advantages.
Thus it is possible with only a diversion one way or the other after die
attachment to provide the device in either of two forms, with the minimum
of extra apparatus. In an attempt to reduce labour content, most steps are
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llZ0146
now automated. Thus machines are available for attaching the dies to the
lead frame - or to individual lead members for conventional hermetic sealing,
and for wire bonding, sealing or encapsulation. However this machinery is
expensive and occupies a relatively large floor space. By the present
invention the die attachment machinery is common. Also by combining into
a single line, plating, trimming and forming and testing can be carried out
on the same machinery. Thus, when non-hermetic devices are being formed only
the wire bonding and sealing apparatus for hermetic devices is idle, and
similarly, for hermetic device production only the wire bonding and
encapsulation or molding apparatus is idle. Even further economies are
made if common wire bonding machines are used.
A further advantage arises in that the introduction of
new devices follows a series of steps. Generally the devices are produced
in an hermetic form as this gives the highest confidence factor for final
test. These devices are in fact often initially made by being hermetically
sealed using an eutectic sealing material - instead of the fused glass.
This is very expensive but the devices can still be made using the invention
up to and including the die attachment apparatus, the strips then being side-
stepped to a specialized sealing apparatus. The sealed devices can then be
returned to the normal production line, for example at 50 in Figure 4. Once
stabilized the new design is then hermetically sealed using the fused glass
system and can thus be incorporated into the production line as in the
present invention. This form is still expensive, as the individual ceramic
parts are expensive but any possible breaking or distortion of the wire leads
or bonds is avoided and thus the final test is of the device design itself.
Once production increases, with the device design finalized, plastic
impregnation is used and it will be realized that it is possible to feed in
what are in effect experimental batches into the system and arrange for the
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)146
strips to take whichever path is required:- hermetic sealing or non-
hermetic, or even only use those parts of the system as desired. Thus
the provision of spare apparatus is considerably reduced or even obviated.
While it is feasible for the leads 36 to be inserted
through holes in a circuit board, it is more likely that the packaged
devices will be surface mounted. That is the ends of the leads will be
soldered to contact pads or areas on the circuit board. At present devices
are provided for surface mounting but instead of leads, as in Figures 8
and 9, contact pads are provided on the base of the device, the pads resting
on and being soldered to contact pads or areas on the circuit board. However,
by providing leads, some clearance is provided between the device and the
circuit board. This enables cleaning to be provided to ensure no foreign
matter is trapped between device and board. This can be very important as
the device is often positioned over circuit patterns under the device. The
surface mounting of devices as produced by the present invention can be
handled by existing apparatus.
For surface mounting, it would be useful to have "feet"
formed at the ends of the leads. This can be provided by a two step
forming at the time the leads are sheared from the lead frame. Thus on
shearing, the extreme ends of the leads can be bent at 90 and then a second
forming step will bend down the leads. Feet are indicated in dotted outline
at 75 in Figures 8 and 9.
By using the invention, devices are produced with extremely
close spacing of beads and a very small overall size. Very high device
density can be provided. As stated, the term device is intended to include
dies or chips which carry both active and inactive devices. The dies can
carry integrated circuits, which may include various forms of electrical
and electronic devices, or the dies may carry such items as thick film
0146
devices, transformers, coils and other devices which can be formed on
the dies, while in wa~er form, and then a~ter die separation are
hermetically sealed or non-hermetically or plastic encapsulated.
- 12 -
