Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2002-06-21
2004-05-11
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C106S001190, C029S854000
Reexamination Certificate
active
06733695
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrically conductive paste and a semiconductor device prepared by using the paste. More particularly, the present invention relates to a highly reliable electrically conductive paste which is used for adhesion of semiconductive elements such as IC and LSI to metal frames and for electrically conductive bonding of semiconductive elements with outer electrodes and to a semiconductor device prepared by using the paste.
2. Description of Related Art
As the electronics industry has made the remarkable progress, semiconductor elements have achieved the developments from transistors to IC, from IC to LSI and from LSI to super LSI. The degree of integration in the semiconductor elements has rapidly increased and the mass production of the elements has been made possible. As the application of the semiconductor products using the semiconductor elements expands, improvement in workability and reduction of cost have become important subjects in the mass production. On the other hand, bonding of semiconductor elements with metal frames and outer electrodes has been conducted by using solder containing lead, which is harmful to the human health. However, from the standpoint of protection of the global environment, it is required that the bonding be conducted by using other electrically conductive materials which do not contain lead. Heretofore, in general, a semiconductor element is bonded to an electrically conductive member such as a frame in accordance with the Au—Si cocrystallization method and the bonded portion is sealed with a hermetic seal to produce a semiconductor product. However, a sealing method with a resin has been developed to improve workability and reduce cost in the mass production and is widely used at present. As an improved method for the Au—Si cocrystallization method in the mounting step, a method using a resin paste, i.e., a method using an electrically conductive paste, is used. Moreover, as the semiconductor devices have higher densities and greater capacities, it is required that the resin paste itself have electric and thermal conductivities as excellent as those of conventional solder metals. In particular, in a power MOSFET, resistance in the condition that a switch is put on must be lowered as much as possible so that generated heat at the surface of the element is removed and the function of the switch is enhanced. Therefore, bonding to outer electrodes with a metal plate is studied as a method to replace the wire bonding.
For further improving the reliability of semiconductor elements, it is recently required that the reliability of bonding of semiconductor elements be not adversely affected even when a resin package is exposed to a high temperature and a high humidity or to cyclic cooling and heating.
As the electrically conductive paste having the required excellent reliability, a paste prepared by dissolving a thermoplastic resin in a solvent and adding a great amount of a metal filler such as silver powder to the dissolved resin is studied. However, when an excessively great amount of the metal filler is added to surely obtain the high electric and thermal conductivities, drawbacks arise in that viscosity increases to deteriorate workability and that an organic solvent added to decrease the viscosity is scattered during heating to cause formation of voids, a decrease in the thermal conductivity and an increase in the electric resistance in the bonding portion.
In general, the electrically conductive paste is applied to coat the bonding portion in accordance with a method such as dispense, printing and stamping and used after being cured by heating. It is preferable that the paste exhibits excellent workability in coating independently of the area of the portion to be coated. Recently, as the degree of integration of semiconductor elements increases, coating a portion having a smaller area is required. As the result, it is required that the coating can be conducted by using a smaller nozzle. For example, when the coating is conducted using a conventional electrically conductive paste, nozzles having a diameter of 0.3 mm or greater can be used but nozzles having a diameter smaller than 0.3 mm cause a problem in that clogging of the nozzles takes place due to contact between filler particles and a uniform coating cannot be achieved. It is required recently that the coating be conducted using a nozzle having a diameter of 0.1 mm. Moreover, it is also required that the paste which can be used for coating by a nozzle having a small diameter can be used also for coating by a nozzle having a great diameter.
As shown in Japanese Patent Application Laid-Open No. 2000-114445, a metal plate of a lead frame and the surface of a die are bonded with an electrically conductive paste to enhance the switching function of a power MOSFET. However, when a conventional epoxy resin-based paste or a conventional polyimide-based paste is used, drawbacks arise in that the electric and thermal conductivities are insufficient and that the resistance in the condition that a switch is put on increases when the product is exposed to a high temperature and a high humidity such as the condition in the pressure cooker test or to cyclic cooling and heating although satisfactory results can be obtained under the initial condition. It is desired that that above drawbacks are overcome. In Japanese Patent Application Laid-Open No. 2000-223634, a bonding method using a gold bump is disclosed. However, this method is disadvantageous from the standpoint of cost.
SUMMARY OF THE INVENTION
The present invention has an object of providing an electrically conductive paste which has thermal and electric conductivities approximately the same as those of solder, shows little change in properties under a high temperature and a high humidity or under cyclic cooling and heating, exhibits excellent workability in coating and can be applied by using nozzles having a wide range of diameters and a semiconductor device prepared by using the paste.
As the result of the extensive studies by the present inventor to achieve the above object, it was found that, when an electrically conductive paste comprised 80% by weight or more of silver powder and 20% by weight or less of a thermosetting resin, the silver powder had a particle size distribution having two peaks at specific positions, viscosities of the paste measured under a low shearing stress and under a high shearing stress were in respective specific ranges, the ratio of these viscosities was in a specific range and a cured product obtained by heating the paste had a tensile modulus of 0.1 to 2.5 GPa, the paste had thermal and electric conductivities approximately the same as those of solder, showed little change in properties under a high temperature and a high humidity or under cyclic cooling and heating, exhibited excellent workability in coating and could be applied to coating by using nozzles having a wide range of diameters. The present invention has been completed based on this knowledge.
The present invention provides:
(1) An electrically conductive paste which comprises (A) silver powder and (B) a thermosetting resin, wherein a content of the silver powder in the paste is 80% by weight or more, a particle size distribution of the silver powder measured by an apparatus for measuring a particle size distribution by laser diffraction has one peak between 0.5 to 2 &mgr;m and another peak between 2 to 10 &mgr;m, viscosity of the paste measured by an E-type viscometer is 35 to 135 Pa·s at 25° C. at a rotation speed of 0.5 rpm and 10 to 30 Pa·s at 25° C. at a rotation speed of 2.5 rpm, a quotient obtained by dividing the viscosity at 25° C. at a rotation speed of 0.5 rpm by the viscosity at 25° C. at a rotation speed of 2.5 rpm is 3.5 to 4.5, and a cured product obtained by heating the paste has a tensile modulus of 0.1 to 2.5 GPa at 25° C.;
(2) A paste described in (1), wherein the particle size distribution of the silver powder is:
particles smaller than 0.5 &m
Fukuizumi Akira
Nakajima Yoshihiro
Onami Kazuto
Takeda Toshiro
Kopec Mark
Smith , Gambrell & Russell, LLP
Sumitomo Bakelite Company Ltd.
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