Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2000-04-11
2002-08-06
Curco, Kamand (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S260000, C361S767000, C361S771000
Reexamination Certificate
active
06429382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mounting structure for mounting electronic components such as a semiconductor device, a method for producing the same, and an electrically conductive adhesive used for production of the mounting structure.
2. Description of the Related Art
Heretofore, in mounting a semiconductor device onto input/output terminal electrodes of a circuit board, one often utilizes the wire bonding method by soldering. In recent years, however, owing to scale reduction of a package of semiconductor devices and increased number of connection terminals, an interval between the connection terminals is becoming smaller, making it more and more difficult to perform the mounting with the use of a conventional soldering technique.
Therefore, in recent years, a method is proposed in which the semiconductor device such as an integrated circuit chip is directly mounted on the input/output terminali electrodes of the circuit board for scale reduction and effective utilization of a mounting area.
Above all, a flip-chip mounting method in which a semiconductor device is mounted facedown on a circuit board is regarded as being a useful method because a collective electrical connection can be established between the semiconductor device and the circuit board in addition to large mechanical strength after connection.
As a connecting material for the aforesaid flip-chip mounting method, a system using an electrically conductive adhesive is proposed or put into practical use.
This method is a promising method in view of both an improvement in reliability and a measure for environmental protection, due to the following two reasons.
Firstly, since the electrically conductive adhesive contains a resin material such as an epoxy resin, a soft connection is established against external force or thermal stress as compared with a solder, i.e. a metal material, thereby leading to an improvement in reliability.
Secondly, since the electrically conductive adhesive mainly contains particles of silver as an electrically conductive component, a clean mounting without the use of lead can be carried out.
On the other hand, for a conventional structure in which an electronic component such as a chip component or a package component is mounted onto a printed board, a structure is likewise proposed in which an electrically conductive adhesive is used in view of an improvement in reliability and a measure for environmental protection.
Thus, the mounting structure using an electrically conductive adhesive, will be a promising method in view of both an improvement in reliability and a measure for environmental protection in the future.
However, the mounting structure using an electrically conductive adhesive has the following two problems, thereby hindering the practical use thereof.
The first problem is a decrease in reliability of insulation caused by ion migration. The ion migration is a kind of electrolytic action, and is a phenomenon of insulation breakdown that occurs between electrodes through the following four steps when an electrolyte such as water is present between the electrodes to which a voltage is applied.
Step 1) Elution and ionization of positive electrode metal
Step 2) Migration of the ionized metal towards the negative electrode by application of a voltage
Step 3) Deposition of the metal ion migrated to the negative electrode
Step 4) Repetition of steps 1) to 3)
Through such an ion migration phenomenon, the metal grows like a dendrite between the electrodes and finally the electrodes are bridged with the metal to cause an insulation breakdown.
Silver used as a material of an electrically conductive filler of an electrically conductive adhesive has a tendency of being eluted, and is liable to cause ion migration. Further, in accordance with the further reduction in scale and weight of electronic apparatus in recent years, the pitch of electrodes disposed on a semiconductor device, an electronic component, or a circuit board is becoming further smaller, thereby all the more increasing the likelihood of ion migration. For this reason, it is essential to solve the problem of ion migration for practically carrying out the mounting with an electrically conductive adhesive.
Conventionally, for restraining the ion migration, the following three methods are proposed:
Proposal 1) Forming an electrically conductive filler with an alloy (silver-copper, silver-palladium, or the like),
Proposal 2) Sealing an electrically conductive adhesive with an insulating resin such as an epoxy resin,
Proposal 3) Capturing eluted metal ions and forming them into an insoluble substance by addition of an ion-capturing agent such as an ion-exchange resin or a chelating agent to an electrically conductive adhesive.
However, these proposals involve the following disadvantages. In the proposal 1, the filler metal will be extremely expensive, thereby increasing the costs for producing the electrically conductive adhesive. In the proposal 2, addition of the sealing step necessitates an increase in the number of steps and extensive installation of additional equipment, thereby raising the production costs. In the proposal 3, elution of metal ions from the electrically conductive filler decreases the contact property of the electrically conductive filler to raise the connection resistance.
Thus, though the aforementioned proposals produce an effect of restraining the ion migration, they involve other problems, so that it is difficult to put these proposals into practical use except for special fields. The second problem is a rise in the connection resistance by sulfurization. Sulfurization is a phenomenon in which a metal changes to a substance called metal sulfide having a lower electrical conductivity by reacting with a weakly acidic gas containing sulfur, such as hydrogen sulfide or sulfur dioxide. The sulfurization seems to occur through the following steps, though a lot of portions thereof still remain unresolved.
Step 1) Elution and ionization of a metal in a weakly acidic atmosphere
Step 2) Generation of metal sulfide by reaction of sulfur ions and metal ions
As described previously, the electrically conductive filler is mainly composed of silver as a major component. However, since silver is extremely liable to be sulfurized, the volume resistivity of the electrically conductive adhesive increases when silver is sulfurized and, in accordance therewith, the connection resistance rises. Currently, few solutions to this problem have been reported, and it has been impossible to apply a mounting structure using an electrically conductive adhesive to a product of electronic components that may be possibly used in an environment, such as the neighborhood of a spa or a volcano, where hydrogen sulfide or sulfur dioxide is present at a relatively high concentration. For this reason, the field of applying the mounting structure using an electrically conductive adhesive has been extremely limited.
SUMMARY OF THE INVENTION
Thus, a principal object of the present invention is to maintain reliability of a mounting structure using an electrically conductive adhesive even under a comparatively severe condition such as a condition of much humidity or a gas atmosphere containing sulfur.
In order to achieve the aforesaid object, the present invention provides, in short, a mounting structure comprising an electric structure and an electrically conductive adhesive layer disposed on said electric structure, wherein said electrically conductive adhesive layer contains an electrically conductive filler, and an elution preventive film is disposed on at least a part of said electrically conductive filler. Owing to this construction, the mounting structure of the present invention exhibits a good insulation reliability even under a high temperature and high humidity environment. This is because the elution preventive film prevents elution of metal in the electrically conductive filler even under a high temperature and high humidity condition, thereby fundamentally preventing the ion m
Amami Kazuyoshi
Bessho Yoshihiro
Shiraishi Tsukasa
Takezawa Hiroaki
Allala Jose H.
Curco Kamand
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