Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Utility Patent
1998-05-29
2001-01-02
Le, H. Thi (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S331000, C428S403000, C428S404000, C428S620000, C428S701000, C428S704000, C438S778000, C438S780000
Utility Patent
active
06168859
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to filler powders for use in plastics used to encapsulate components, such as semiconductor devices.
BACKGROUND OF THE INVENTION
Semiconductor devices are commonly encapsulated to protect them from hazards, such as air, moisture, chemicals, dust and light, and to provide them with greater physical strength. The encapsulating material must be an electrical insulator, such as a dense ceramic or more commonly an organic resin. The most common of these organic resins are thermosetting epoxy resins.
These organic resins almost always contain fillers. The most common filler is fused silica. Fillers, such as fused silica, decrease the coefficient of thermal expansion (CTE) of the resin to more closely match the CTE of the encapsulated device (e.g., integrated circuit) and leads attached to the device. This prevents damage to the device from thermal cycling and reduces forces on the surface of the semiconductor during manufacture and in use.
The filler generally comprises greater than 50 percent by volume of the encapsulant to achieve the above effects. Besides the organic resin, the encapsulant may also contain other components, such as resin, catalyst, waxes and opacifiers. To achieve a high filler loading, the filler must have a broad particle size distribution containing a significant amount of fine particles (i.e., less than about 1 micrometer).
During operation, semiconductors generate heat which must be dissipated. As these devices shrink and become more complex, heat removal becomes more and more of a problem. A major disadvantage of silica-filled encapsulants is their low thermal conductivity, which limits their ability to dissipate heat from the encapsulated semiconductor. For this reason, there is a desire to provide approaches to improve heat dissipation in these devices.
One approach incorporates metal heat sinks or metal heat spreaders to the encapsulated device. These provide a means for rapid heat dissipation, but may be limited by space requirements (e.g., in cellular phones), and require additional parts, steps or both to make the device.
Another approach encapsulates the semiconductor in a resin loaded with a filler having a high thermal conductivity. Examples of these fillers include aluminum oxide (alumina), boron nitride, silicon carbide, silicon nitride and aluminum nitride. All of these improve the thermal conductivity of the encapsulant compared to equivalent silica-filled systems. However, silicon carbide tends to be too abrasive because of its high hardness resulting in unacceptable wear to process equipment.
The nitrides suffer from hydrolytic instability which may bloat the encapsulant causing the device to fail. This instability also makes it difficult to form powders having a significant amount of fine particles. The nitrides also contain basic nitrogen containing groups that can catalyze the curing of the epoxy resins making them difficult to process. Alumina also contains basic hydroxyl surface groups that can catalyze the curing of epoxy resins and, because of its hardness, is also abrasive to processing equipment.
To overcome some of these problems, aluminum nitride and alumina, for example, have been coated with silica-type coatings. The coatings generally are formed by first coating the aluminum nitride or alumina powder with a silica source (e.g., organo-silicate) and then heating the coated powder. The coatings tend to be thick relative to the starting powder size, which generally decreases the thermal conductivity of the powder. These coatings also must often be applied more than once to achieve a sufficient coating, which increases the cost of the process and the powder.
Accordingly, it would be desirable to provide a filler powder that improves the heat dissipation of an encapsulant compared to a silica-filled encapsulant, while avoiding some of the problems of the prior art, such as those described above. It would also be desirable to provide an improved method for making the filler powder.
SUMMARY OF THE INVENTION
A first aspect of the invention is filler powder comprised of an alumina powder coated with a silicon containing coating, wherein the silicon containing coating at most partially covers the surface of said coated alumina powder and the filler powder, when mixed with a thermosetting epoxy resin, has an average spiral flow length that is at least about 1.1 times greater than a comparable filler powder containing uncoated alumina powder mixed with the thermosetting resin. Uncoated alumina powder is the coated alumina powder before it has been coated. Comparable filler powder is the same as the filler powder except that it contains uncoated alumina powder instead of the coated alumina powder.
The coated alumina powder of the filler powder avoids the problem of decreased thermal conductivity associated with thick silica coatings, while surprisingly still avoiding the problem, for example, of catalyzing the curing of an epoxy resin. This premature curing typically results in a reduced average spiral flow length.
In a particular preferred embodiment, it has been surprisingly found that the coated alumina may be blended with AlN without catalyzing the hydrolysis of the AlN. It is surprising because (1) the coated alumina of this invention is only partially coated with silica and (2) it is known that surface hydroxyl groups of alumina powder catalyze the hydrolysis of AlN. It is even more surprising that the amount of Si on the surface of the alumina may be, for example, less than 1000 ppm of the total weight of the coated alumina powder.
A second aspect of the invention is a process for making a filler powder comprising: heating, simultaneously, an alumina powder with a second filler powder coated with an organo-silicate to a temperature, for a time, sufficient to volatilize, deposit and pyrolyze at least a portion of the organo-silicate on the alumina powder to form a silicon containing inorganic coating thereon (i.e., form the coated alumina powder). The process has the advantage, for example, of processing an AlN that must be coated to be a useable filler simultaneously with an alumina powder. The process, consequently, can avoid separately processing and subsequent blending of the AlN and alumina powders.
The filler powder may be used in molding compositions to encapsulate electronic devices or used in other thermal management applications.
DETAILED DESCRIPTION OF THE INVENTION
The Coated Alumina Powder
The coated alumina powder is comprised of an alumina powder coated with a silicon containing coating, wherein the silicon containing coating at most partially covers the surface of the coated alumina powder.
The alumina powder coated with the silicon containing coating may be any alumina powder known in the art. Suitable alumina powders that can be coated include, for example, calcined alumina, tabular alumina, fused alumina, synthetic boehmite alumina and alumina platelets. Preferably the alumina is a calcined alumina.
The silicon containing coating only partially covers the surface of the alumina powder. Wherein “partially covers” means the particles fail to have a distinct coating that completely envelopes each particle, as determined by microscopic techniques (e.g., transmission electron microscopy or scanning electron microscopy). Surprisingly, the amount of silicon of the coating of the coated alumina may be less than about 5000 ppm by weight of the coated alumina powder. Preferably the amount of silicon present is less than about 1000 ppm, more preferably less than about 500, even more preferably less than about 250 ppm and most preferably less than about 100 ppm to generally at least about 10 ppm by weight of the coated alumina powder. The amount of silicon from the coating may be determined by known bulk analysis techniques, such as X-ray fluorescence of the coated powder and uncoated powder or surface analysis techniques, such as Auger Emission Spectroscopy, Secondary Ion Mass Spectroscopy and Electron Spectroscopy for Chemical Analysis.
The Filler Powder
Even though the filler pow
Knudsen Arne K.
Mills Lynne K.
Swedberg Eric J.
Le H. Thi
Nilsen Kevin J.
The Dow Chemical Company
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