Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2003-07-07
2004-11-09
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C156S305000, C156S327000, C438S610000
Reexamination Certificate
active
06814893
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a conductive adhesive agent used in joining of an electronic element and a printed-circuit board in a field of packaging of the electronic element, a packaging structure using the conductive adhesive agent, and a method for manufacturing the packaging structure.
2. Description of the Related Art
Recently, high consciousness for environmental harmonies has started to restrict use of lead contained in a solder alloy in the field of packaging of electronic devices, thus leading to an emergent need for establishment of technologies of joining electronic elements using a material in which lead is not contained.
As a lead-free packaging technology is known the one using a lead-free solder and a conductive adhesive agent. Recently, however, more and more attention is attracted to a conductive adhesive agent expected to have merits such as flexibility of a joining portion and lower packaging temperature.
A conductive adhesive agent generally has conductive particles dispersed in a resin-based adhesive component. To package a device, first a conductive adhesive agent is applied on a board electrode, the device is attached thereon, and then the resin is hardened. By this process, the joining portions are adhered to each other by the resin and also the conductive particles come in contact with each other as the resin shrinks, thus ensuring continuity at the joint.
The resin of a conductive adhesive agent has a hardening temperature of about 150° C., which is very low as compared to a melting temperature of about 240° C. required for soldering, thus qualifying that agent for use even in packaging of such inexpensive devices that have a low heat resistance.
Also, the joining portions are adhered to each other by a resin, thus being able to flexibly accommodating a deformation due to heat or an external stress. This gives the conductive adhesive agent a merit that the joining portions adhered thereby is not liable to have cracks as compared to those adhered by solder which is an alloy.
For the above reasons, the conductive adhesive agent is expected as an alternative of solder.
Silver, generally used as conductive particles of a conductive adhesive agent, has such a characteristic that it is subject to easy ion migration or sulfurization, problem of which must be solved to put the conductive adhesive agent to practical use as an alternative material for solder.
First, ion migration is described as follows. A phenomenon of ion migration is a sort of electrolytic action, by which dielectric breakdown occurs between electrodes along the following four steps when an electrolyte such as water is present between the electrodes under application of voltage:
Step 1: An anode metal is eluted and ionized;
Step 2: The ionized metal migrates toward a cathode under application of voltage;
Step 3: The metal ions which have migrated to the cathode are precipitated; and
Step 4: Steps 1 through 3 are repeated.
Such a phenomenon of ion migration causes the metal to grow in a tree shape between the electrodes, finally bridging the gap between the electrodes, resulting in dielectric breakdown.
Silver used as a conductive filler of a conductive adhesive agent is easily eluted, thus bringing about ion migration. Further, a recent trend for further reduction in size and weight of electronic equipment has narrowed a pitch between electrodes formed in a semiconductor devices an electronic element or on a printed-circuit board, thus further easily causing ion migration. Taking this into account, the problem of ion migration must be solved indispensably to put to practical use the packaging technology by use of a conductive adhesive agent.
There have conventionally made such three proposals that inhibit ion migration:
Proposal 1: Alloying of conductive filler (e.g., alloying of silver and copper or silver and palladium);
Proposal 2: Sealing of conductive adhesive agent by use of insulating resin such as epoxy resin; and
Proposal 3: Capturing eluted metal ions and rendering them insoluble material by addition of ion capturing agent such as ion exchange resin or chelating agent to conductive adhesive agent.
Those proposals, however, have the following disadvantages. Proposal 1 requires a very expensive filler metal, thus increasing a cost of the conductive adhesive agent. Proposal 2 needs to add an extra step of sealing to thereby increase the number of required steps or greatly expand provisions, thus increasing the manufacturing costs. Proposal 3 causes a metal ion to be eluted from the conductive filler to thereby deteriorate contact-ness of the conductive filler, thus raising the connection resistance.
Thus the above-mentioned proposals have indeed an effect of inhibiting ion migration but also have various problems and so are difficult to put to practical use except in a special application field.
Next, a phenomenon of sulfurization is described as follows. Sulfurization refers to such a phenomenon that a metal reacts with a weal acidic air containing a sulfuric content such as hydrogen sulfide or sulfur dioxide to provide such a material with low conductivity that is called a metal sulfide. Although sulfurization is not know enough yet, it is considered to occur along the following steps:
Step 1: A metal is eluted and ionized in a weak acidic atmosphere; and
Step 2: the metal ions react with sulfur ions to generate a metal sulfide.
As described above, a conductive filler is mainly made up of silver but is liable to be sulfurized, so that when silver is sulfurized, volume-specific resistance of the conductive adhesive agent rises, which is accompanied by a rise in the connection resistance. Few solutions for this problem have been reported so far, so that a packaging structure using a conductive adhesive agent cannot be applied to a product having an electronic element which may be used in such an environment as surroundings of a hot spring or volcano, in which hydrogen sulfide or sulfide dioxide is present at a relatively high concentration. This greatly restrict application fields of the packaging structure using a conductive adhesive agent.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide such a packaging structure using a conductive adhesive agent that is capable of maintaining a reliability even under a very humid condition or such a severe condition as a gaseous atmosphere containing sulfur.
To this end, a conductive adhesive agent according to the invention has a binder resin, a conductive particle, and an elution preventing-film forming agent, which forming agent becomes reactive after electrical continuity through the conductive particle is established in this conductive adhesive agent when the binder resin is hardened, thus forming an elution preventing film on the surface of the conductive particle. This causes the following to occur.
When the surface of the conductive particle is coated with the elution preventing film, the conductive particle can be prevented from being eluted even if it is left in a hot and humid environment or in a gas containing sulfur. Therefore, the elution preventing film can prevent ion migration as well as the above-mentioned first step of sulfurization. Thus, the conductive adhesive agent according to the invention can be used to manufacture a packaging structure not liable to encounter ion migration and sulfurization.
When, in this case, the elution preventing film is made of an insulating material and if it is present at a site related to conduction (i.e., contact point between conductive particles and that between a conductive particle and an electrode or the like), it inhibits electrical continuity, thus leading to such a disadvantage that raises a connection resistance of the packaging structure.
By the conductive adhesive agent according to the invention, on the other hand, no elution preventing film is formed on the conductive particle until that adhesive agent is hardened, so that only after the conductive particles come in contact with each other
Ishimaru Yukihiro
Kitae Takashi
Mitani Tsutomu
Nishiyama Tousaku
Takezawa Hiroaki
Kopec Mark
Matsushita Electric Industiral Co., Ltd.
McDermott Will & Emery LLP
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