Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
This invention relates to a leadframe for leaded
semiconductor chip carriers.
In the manufacture of semiconductor chips, the chips are
mounted on a die pad, and connections made between contact areas on the
chip and leads which will eventually be used to connect the final device
into a circuit, as by insertion on a printed circuit board (PCB). The
chip is packaged, in one of a variety of ways, for example hermetic and
non-hermetic encapsulation.
With the extremely large numbers being produced and the need
to reduce labour content and to maintain high standards of accuracy and
minimal rejection rates, it is favourable to handle assembly in strip
form, a leadframe having a number of chip positions, with leads extending
from the frame towards a chip mounting position. One such form of
leadframe is illustrated and described in U.S. patent no. 4,214,364,
issued July 29, 1980~ in the name of the present assignees.
Several problems arise in leadframes, and their use.
Ideally they should be suitable for both hermetic and non-hermetic
encapsulation. It would be convenient if the leadframes could be
processed with the leads unformed, that is flat and in the same plane as
the frame, or with the leads preformed, that is with leg portions bent
down. Electroplating is usually carried out on the leg portions.
Protection of the chip from electrostatic discharge during processing is
important. Precision forming and cropping is also important and becoming
increasingly so with the continual reduction in lead width and closer
spacing of leads. Eventually reel to reel automated processing is
envisaged and the leadframe should be easily and cheaply manufactured.
The present invention provides a leadframe which provides
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for the above problems. A leadframe in accordance with the present
invention has parallel, spaced apart, side members and parallel, spaced
apart, transverse members. The transverse members extend between, and
connect to, the side members, to define a plurality of areas spaced along
the frame. A rectangular die or chip pad is positioned centrally in each
area with its sides parallel to the side members and transverse members of
the frame. A support lead extends from each corner of the die pad to the
corresponding conjunction of side and transverse members. A U-shaped
support bar extends between each adjacent pair of support leads, the bar
including parallel spaced leg portions connected at inner ends to the
support leads and connected at outer ends by a lead support portion, the
lead support portions parallel to related side and transverse members. A
plurality of leads extend from each lead support portion inwards towards
the die pad, with inner ends adjacent to but spaced from the die pad.
The invention will be readily understood by the following
description of a particular embodiment, by way of example, in conjunction
with the accompanying drawings, in which:-
Figure 1 is a plan view of part of a strip of leadframes,
with different process steps illustrated at different frames;
Figures 2, 3 and 4 are cross-sections on the lines II-II;
III-III; and IV-IV respectively on Figure 1;
Figure 5 is a cross-section, similar to that of Figure 4,
but with the leads formed to extend downwards;
Figures 6a, 6b and 6c are diagrammatic partial
cross-sections through one form of hermetic encapsulation, illustrating a
problem therewith;
Figures 7a and 7b are similar partial cross-sections, with
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an alternative design which overcomes the problem illustrated in Figure 5.
Illustrated in Figure 1 is a leadframe indicated generally
at 10, with three chip or die positions indicated at 11, 12 and 13. The
leadframe has parallel, spaced apart, side members 14 and parallel spaced
apart transverse members 15. Members 15 extend between and connect to the
side members 14 and define areas 16. At a central position of an area, as
seen particularly in position 11, is a die or chip pad 17, the pad being
supported from the frame by support leads 18 extending from each corner 19
of the die pad, to each conjunction 20 between a side member 14 and
transverse members 15.
Extending between each adjacent pair of support leads 18 is
a U-shaped support bar 21, having leg portions 22 connected at their inner
ends to the support leads 18 and connected at their outer ends by a lead
support portion 25. Extending inward from the lead support portions 25
are leads 26. The inner ends of the leads 26 are adjacent to but spaced
from the periphery of the die pad 17. Thus it will be seen that the die
pad 17 and the leads 26 and support bar 21 are supported from the frame by
the support leads 18. Perforations 27 and 28 extend along each side of
the frame, one set of perforations, 27, acting as reference or positioning
holes to ensure correct positioning of the frame at process stations.
At position 12, a die or chip 35 is seen positioned on the
pad 17. The chip is bonded to the pad, generally by an automated process.
On the upper surface of the chip are contact areas 36. At position 13,
wire bonding is shown. Wire connections 37 are made between the contact
areas 36 and the inner ends of leads 26.
From the position 13, the frame progresses to an
encapsulation apparatus. The frame may proceed to the encapsulation
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without preForming, or the leads 26 may be preformed. The bend or forming
line is indicated by the dotted line 38 at position 13 in Figure 1. Also
indicated in Figure 1, are small projections 39 on the leads 26. These
projections act as barriers to reduce outflow of plastic material in
plastic, or non-hermetic encapsulation.
Figures 2, 3 and 4 illustrate, in cross-section, the
leadframe 10 at the various positions 11, 12 and 13 respectively. Figure
5 shows the leads bent or formed, as would be the siutation if done before
encapsulation.
The use of the support leads 18 and support bars 21 provides
a number of advantages and overcomes many of the problems hitherto
experienced. Thus, for prebending, the leads are still interconnected at
their outer ends. This prevents distortion of individual leads and also
ensures that the leads bend down accurately. Guiding edges on the support
bar, indicated by thickened lines 40 in position 13 in Figure 1, ensure
that the leads bend down vertically. It is a feature of conventional
forms of leadframe that the leads are bent down with non-vertical bending
of the leads. While some inclination of the leads can be sometimes
accepted, for subsequent automatic assembly, with leads on only two
opposite sides of the die pad, this cannot be accepted for leads on all
four sides.
Also, whether prebent or bent after encapsulation,
electroplating of the leads is made much easier by interconnection of the
leads. Further, interconnection of the leads provides protection of the
chip from electrostatic discharge damage. After manufacture is complete,
the lead support portion is trimmed off. However, whether prebent or not,
the encapsulated device is still held in the leadframe, but the leads are
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now electrically isolated from it. Testing of the device can thus be
carried out while still in the frame. At the final stage the device is
removed from the frame by shearing of the support leads 18.
In many instances it would be convenient, and more economic,
to be able to process the frame and attached chip right through
encapsulation prior to bending or forming the leads downward. It is
easier to traverse a leadframe which is flat, relative to moving such a
frame with the leads bent down. Slides are easier to make, and maintain,
for a flat strip, whereas for a strip with bent down leads support can
mainly only be at the sides of the leadframe. If leads were only on two
sides, the lateral sides, no problems occur as grooves can be provided in
the sliding surface, but with leads on all sides this cannot be done.
An advantage does occur from pre-bending or forming, in that
the bent or formed leads provide self-jigging or alignment of the base
with the pad 17. Thus, turning the leadframe over, with the attached
chip, 35 underneath, a base can simply be dropped down between the leads,
and be in alignment. A further member of simi7ar form to a leadframe,
with pre-bent or formed leads, forms a holding and locating member for a
top. The leadframe with chip and base is then nested in the holder with
the cap, in fact a row of caps, and clips applied to hold the assembly
together. The assembly is then fixed to fuse the glass sealing the base
and cap together.
Bending after encapsulation can cause problems. In one
encapsulation process, for example, the chip with leads is hermetically
encapsulated by being enclosed between two housings, for example of
ceramic, which are sealed together by fused glass. Due to the brittle
nature of the seal, leads are either preformed or bent before sealing, or
the leads are clamped betwen the seals and the bent point, but this
increases the overall size. The problem is illustrated in Figure 6, (a),
(b) and (c). In Figure 6(a), the chip 35 is mounted on the base 46, with
a cap 47, of the same external dimensions as the base resting over the
base and chip. Prior to assembly a bead of glass exists around the
periphery of both base and cap. After attachment of the chip 35, to the
pad 17 and attachment of chip pad 17 to the base 46, the cap 47 is
positioned on the base. Heating of the assembly causes the glass b~ads to
melt and fuse, seali ng the cap and base together and also sealing around
the leads 26. This is indicated at 48 in Figure 6(a), and seen enlarged
in Figure 6(b). It will be seen that a glass meniscus 4ga and 49b extends
between base 46 and leads 26, and between cap 47 and the leads 26
respectively. If the leads are then bent or formed, an incipient crack
can form at the join face on the top of the lead, at 50, and chipping
occurs at the join face at the bottom or underside of the lead, at 51.
This can lead to eventual failure of the seal.
If the lead is clamped external to the base and cap, the
lead being bent down outside the clamp, the above problems can be avoided
but the extra room for the clamps, typically .035" between seal and bend,
20 increases the area occupied by the finished package.
Figures 7(a) and 7(b) illustrate a way of overcoming this
problem. The cap 47 is made smaller than the base 46. The bead of glass
on the base is moved inward slightly and when cap and base are sealed
together the meniscus 49b on the top of the lead is spaced inward from the
periphery of the base. Typically this distance, X, should be about 1~ to
2 times the lead thickness. The meniscus 49a at the underside of the lead
does not reach to the periphery of the base. This is illustrated in
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Figure 7(a). Figure 7(b) illustrates the situation after bending of the
leads. The top meniscus 49b is unaffected by the bending and similarly
the bending does not affect the lower meniscus 49a either. This would
enable post-encapsulation bending of the leads and facilitate processing
of leadframes, particularly reel to reel processing.
While the invention has been described in relation to
encapsulation of the type in which the chip is hermetically sealed between
glass or ceramic bases and caps, the invention can also be used for
plastic encapsulation, where the chip and chip pad are encapsulated by
molding of plastic material, for example a synthetic resin around the
chip, chip pad, and inner ends of the leads and the wire bonding between
chip and leads.
