Plastic and nonmetallic article shaping or treating: processes – Mechanical shaping or molding to form or reform shaped article – To produce composite – plural part or multilayered article
Reexamination Certificate
2000-04-03
2001-11-13
Ortiz, Angela (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Mechanical shaping or molding to form or reform shaped article
To produce composite, plural part or multilayered article
C264S272140, C264S272150, C264S272170, C264S328500, C425S116000, C425S543000
Reexamination Certificate
active
06315936
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the plastic packaging of electronic components. More particularly, the present invention relates to plastic resin encapsulation of integrated circuits; to a methodology employing non-homogeneous molding compound preparations for reducing the delamination of the resin encapsulant from the electronic components, and to the encapsulated electronic components, including plastic-resin encapsulated integrated circuits, produced according to the method.
BACKGROUND ART
The following discussion of the art of the plastic packaging of electrical components, especially of integrated circuits is, in part, summarized from Chapter 18, “Packaging” in
Microchip Fabrication
, pp. 539-586, Peter van Zant, McGraw-Hill, 1997. This background information is herewith incorporated by reference.
After wafer fabrication, semiconductor chips undergo several processes to prepare the chips for eventual use. By way of illustration, but not limitation, some of these processes include: backside preparation; die separation; die pick; inspection; die attach; wire bonding; pre-seal inspection; package sealing; plating; trimming; marking; and final testing. Many of these processes can be categorized as part of the packaging, or enclosure process.
One form of enclosure common to semiconductor or integrated circuit (IC) manufacture is the molded epoxy package. Epoxy packages perform the four basic requirements of an electronic package for the circuit chip, or die (“chip”) they house: they support a substantial lead system for connecting the device to the system component which can utilize it; they provide physical protection of the device from breakage, contamination and abuse; they protect the device from environmental hazards such as chemicals, moisture and gasses which could interfere with device performance; and they provide a path for dissipating the heat generated by the functioning of the device. Epoxy packaging presents several major advantages over some other device packaging technologies: it is light in weight, low in cost, and high in manufacturing efficiency.
One method of epoxy packaging of semiconductor devices is illustrated in prior art
FIGS. 1 and 2
. Having reference to those figures, this methodology is explained as follows: after die separation (and in some cases, after some of the previously mentioned steps, the die,
1
′, concentric with device
1
, is attached and bonded to a composite lead frame,
2
. In the exemplar here presented, lead frame
2
includes horizontal rails,
5
, and vertical tie bars,
7
, and provides a plurality of lead systems for connecting to the semiconductor dice (not shown), thereby producing the useful individual device. In this example there are provided a plurality of individual device lead frames
2
, each having mounted thereon a further plurality of dice,
1
′. After die mounting, the lead frames having the dice mounted thereon are often given some form of pre-seal inspection.
After the pre-seal inspection, the lead frames are transferred to a molding apparatus. Commonly used in this procedure is a transfer molding process which encapsulates and surrounds each of the dice and at least a portion of the lead frame assembly with a plastic encapsulant, or molding compound. Commonly utilized molding compounds include, but are not limited to: epoxies, monomers, polymers, and other resins. In the exemplar here presented, a silica-filled epoxy is utilized as the molding compound, or encapsulant.
The lead frames are placed in a mold, here a two-part mold consisting of mold halves
20
and
21
. At least one mold half, often the bottom, has formed therein a gate,
10
. The mold halves are clamped together, typically with some force, and often a portion of the lead frames,
2
, completes the mold cavity,
16
. The vent,
24
, which provides a path for escaping air during the transfer molding process, is typically filled with the encapsulant during that operation.
After the mold has been clamped about lead frames
2
and dice
1
′, the ram assembly of the molding apparatus is charged with a quantity of molding compound, for instance as a homogenous pellet, through sprues
14
. A prior art homogenous molding compound pellet,
60
, comprising a quantity of silica-filled epoxy incorporating all the adjuncts desired for the package, is shown at prior art FIG.
6
. The epoxy material may have been previously softened by means of heating or chemical reaction. The transfer molding apparatus then induces pressure, usually by means of a ram in operative combination with sprue
14
, on the molten, viscous epoxy and it flows from sprues
14
through a series of runners,
12
, through tapered sections
11
of gates
10
, and thence into mold cavities
16
. As the ram (not shown) continues to apply pressure to the mass of liquid epoxy, it is then forced around the integrated circuit dice,
1
′, encapsulating the dice and forming the individual packages, or devices,
1
.
After the epoxy is at least partially set, the molds are separated, and the lead frame assembly is removed therefrom. This assembly may then be further cured by an oven or other heat means. Following final curing, the packages undergo further processing including, but not limited to: plating; runner removal; de-flashing; marking; and final testing. The finished packaged component is then ready for use.
Referring now to
FIG. 3
, a prior art plastic resin encapsulated integrated circuit device,
1
, formed in accordance with the previously discussed process is shown. Device
1
comprises an integrated circuit (IC) device,
1
′, for instance a silicon IC chip. Chip
1
′ is bonded to a copper die paddle
30
, in this exemplar by means of a layer of silver plating,
32
. Device
1
further comprises at least one, and more generally a plurality of leads
34
. Leads
34
are electrically connected in this example by means of wire bonds
36
. Wire bonds
36
are typically first bonded to the correct chip bonding pad and then spanned to an inner end of lead
34
. Lead
34
is commonly, but not exclusively, manufactured from copper, and may include a layer of plating, for instance silver plating
38
, to increase the reliability of the wire bonding process. Leads
34
and die paddle
30
are typically formed utilizing the lead frame technology previously discussed. The previously discussed components are encapsulated, in this prior art example, by means of a homogeneous mass of silica-charged epoxy,
40
, in the manner hereafter discussed.
Prior art molding compounds, for instance the homogenous pellet shown in prior art
FIG. 6
, typically consist of a homogeneous mixture of silica, epoxy (whether plain or brominated), and one or more molding compound adjuncts including, but not necessarily limited to: flame retardants, including antimony trioxide; cross-linking agents; inhibitors; ionic getters; and mold release agents.
Transfer molding compounds, including the previously discussed epoxy molding compounds commonly used for the encapsulation of silicon IC devices, have variable adhesion to the several elements encapsulated within the package. In particular, adhesion of prior art molding compounds to the silver plated lead frames, as well as to the gold bonding wires, is poor. This is partially due to the fact that both gold and silver are substantially noble metals, which is to say that neither is particularly chemically reactive and neither forms a tenacious oxide. The biggest part of the problem, however, is due to the composition of the molding compound itself, and most particularly to the homogenous inclusion of molding compound adjuncts, including mold release agents therein.
Release agents typically include waxes (commonly carnauba or its synthetic equivalent) and stearates (commonly as the calcium or zinc salts of stearic acid). These release agents are incorporated into molding compounds to permit the encapsulated IC to be removed easily from its mold. The mold release compounds are typically incorporated into the molding compound,
Black J. Courtney
Blish II Richard C.
Hatchard Colin D.
Advanced Micro Devices , Inc.
LaRiviere Grubman & Payne, LLP
Ortiz Angela
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