Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-01-28
2001-10-23
Harrison, Jessica J. (Department: 3713)
Metal working
Method of mechanical manufacture
Electrical device making
C029S830000, C029S832000, C029S841000
Reexamination Certificate
active
06305074
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for mounting an integrated circuit on a support and to the resulting support. It applies more particularly to the tape mounting technology, commonly called TAB (Tape-Automated Bonding) technology.
DESCRIPTION OF RELATED ART
Integrated circuits are actually thin, substantially quadrangular slices of semiconductor material, the largest of which has about 15 mm per side. They are provided with a large number of input-output terminals, on the order of 500. In TAB technology, each integrated circuit is mounted on a support constituted by an insulating substrate which carries a conductive metallic structure, generally called a spider. The input-output terminals of the integrated circuit are attached, by an operation called ILB (Inner Lead Bonding), to the respective ends of conductors of the spider. The substrate can exist in various forms. It can, for example, take the form of a flexible strip of the motion picture film type provided with lateral perforations for moving and positioning it, which carries integrated circuits disposed along its length. Another common form is comprised of a frame carrying one or more integrated circuits.
A first type of standard TAB support is made of a spider bonded to a substrate. This support is comprised of three layers: spider—adhesive—substrate. Ordinarily, the spider is a sheet of copper having a thickness of about 17 to 70 &mgr;m, the adhesive is made of acrylic or epoxy, spread to a thickness of about 25 &mgr;m, and the substrate is made of an organic material that is flexible and electrically insulating, whose minimum thickness is about 50 &mgr;m and usually on the order of 100 &mgr;m. The fabrication of the support is comprised, in standard fashion, of bonding a uniform sheet of copper to the substrate, and of etching the sheet of copper to form the spider. According to a first embodiment, the substrate is preformed so as to have the desired configuration of the openings it comprises. The preforming is done by mechanical cutting, such as punching. A second embodiment is comprised of cutting the substrate after the formation of the spider. This support and the mounting of the integrated circuits on this support have several drawbacks.
The ILB operation takes place through openings in the substrate and the adhesive. According to a first embodiment, the openings are small plated-through holes disposed underneath the ends of the conductors. The standard attachment of each end to the corresponding terminal of the integrated circuit is achieved by means of a ball bond which must have a diameter substantially greater than the height of the hole, which is a minimum of about 75 &mgr;m. These large balls can only be soldered to large terminals, and require the punching of large plated-through holes, on the order of 600 to 800&mgr;m, which conflicts with the desired density of the terminals of the integrated circuit and their connection. However, the formation of large plated-through holes can be achieved by mechanical cutting of the substrate before bonding it to the copper sheet. This cutting offers the advantage of being inexpensive and well suited to mass production, but it has the serious drawback of being inapplicable to the high densities that are currently available.
Several solutions have been proposed for adapting an integrated circuit support to high densities. One solution is comprised of piercing the substrate after the formation of the spider, either chemically or using precision cutting means such as a laser. However, chemical cutting requires numerous additional processing operations, such as the deposit of a photosensitive layer on the substrate, the sensitization and etching of this layer so as to form a mask having the desired configuration, the etching of the substrate, the removal of the adhesive, and finally, the removal of the mask. These operations add considerably to the cost of the support. The same is true for the use of laser. In practice, it has proven impossible to produce preliminary designs or prototypes on a small scale at costs that are not prohibitive. Moreover, a cutting operation of this type leaves residues which can interact with the solder during the ILB operation, thus forming a connection that is of poor quality or is not very reliable.
Another solution resides in a second type of standard support, called a two-layer support: spider—substrate. For the fabrication of this support, a thin uniform layer of copper is deposited under vacuum onto one side of a thin substrate of insulating material. Ordinarily, a polyimide substrate about 20 &mgr;m thick is used, and a very thin layer of copper is deposited on it under vacuum, generally through the medium of a priming layer made of chromium for example, and the copper is selectively grown electrochemically so as to form the spider. The thin layer of copper used as a base for the growth remaining outside the desired configuration of the spider is removed. The substrate is then etched chemically or by precision cutting means such as a laser, as in the preceding embodiment. In particular, it will be noted that the use of polyimide involves the etching of the substrate by oxygen plasma. The oxygen plasma does not attack the organic material, that is, the photosensitive material and the polyimide, but it has the drawback of having a relatively long penetration time, on the order of one micrometer per minute. It is understood that this process cannot be applied industrially to a thick substrate. The resulting support costs about three times as much as a standard three-layer support in which the substrate is perforated mechanically. Moreover, the thinness of the substrate can make it too flexible when it is large, more than 70 mm for example, thus rendering it difficult to use. But the resulting support has the advantage of being thin and of being able to use small ball bonds to carry out the ILB operation.
Yet another solution has been presented in the Applicant's patent EP-A-0356300. The support is mixed, in that it is essentially the standard three-layer type with a preformed substrate and in that the ends of the conductors carry a thin frame that is perforated for using small ball bonds during the ILB operation. Thus, it has the advantages of both of the above-mentioned standard types. The fabrication of the thin frame can be costly and its mounting is a delicate operation.
Another problem involves avoiding the use of ball bonds in the ILB operation. One solution was disclosed in the IBM document Technical Disclosure Bulletin, Vol. 25, No. 4, September 1982, “No-bump beam tape.” In this solution, the gold-plated copper conductors are supported on the edge of the integrated circuit by the insulating photodefinable polyimide substrate of the support, and a soldering tool presses down on their ends in order to bond them directly to the terminals of the integrated circuit by thermocompression. Each of these terminals is the bottom, ordinarily made of aluminum, of a plated-through hole in the passivation layer which covers and protects the corresponding surface of the integrated circuit. The height of a plated-through hole is on the order of one micrometer. Taking into account the small width of a plated-through hole and its proximity to the edge of the integrated circuit, it requires a thin insulating substrate in order to avoid excessive bending of the end of the conductor in order to lower it to the bottom of the plated-through hole. This explains why the support used is the second standard type mentioned above. Otherwise, it is necessary to bend the ends of the conductors before soldering them to the terminals of the integrated circuit. This bending has the drawback of adding a delicate operation and of using a relatively expensive bending device.
The same problems and drawbacks arise when using the process described in the patent U.S. Pat. No. 4,917,286. The process for mounting an integrated circuit on a support is very similar to that described above and is comprised of placing the substrate on the passiva
Bull S.A.
Harrison Jessica J.
Kondracki Edward J.
Miles & Stockbridge P.C.
Nguyen Binh-An D.
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