Low Temperature Flexible Die Attach Adhesive and Articles Using Same

This application is related to U.S. patent application Ser. No. 07/843,738; Filed Feb. 28, 1992, pending.

The present invention relates to a flexible die attach adhesive. The flexible die adhesive can be modified to be capable of developing very good adhesive strength at low curing temperatures. One embodiment of the invention relates to flexible card for processing in a computer that includes a semiconductor device enveloped therein adhered to a substrate by the aforementioned adhesive.

As uses for semiconductor devices continue to increase there is a growing demand for adhesive compositions and formulations capable of securing semiconductor devices to substrates under a variety of conditions. The wide variety of applications using semiconductor devices sometimes require that such adhesive formulations be flexible where the device must be flexible and sometimes that the adhesive formulation be capable of developing very good adhesive strength at low curing temperatures so that significant stress is not imparted to the die and the curing conditions do not adversely affect other components. To improve workability, a modified adhesive formulation as described above but of low viscosity is also desirable.

One application employing semiconductor devices which is gaining increasing importance is in the field of cards to be processed in computers where such cards incorporate one or more semiconductor devices and are therefore able to store more information than magnetic cards. Such cards may be useful over a broad spectrum of applications including, for example, credit and telephone cards. These cards, which are referred to as "smart cards", must be thin and must be able to bend without breaking, i.e., the card and card components must be flexible.

Typically, "smart cards" are constructed of a material such as polyester reinforced with glass fibers and have enveloped within the card one or more semiconductor devices. Desirably, such devices should be secured within the card by a die attach adhesive which is flexible but which provides very good adhesive strength and does not impart stress to the die during curing. It is also important that the adhesive be capable of being cured at relatively low temperatures to facilitate fabrication of the cards without damage to the card or the semiconductor devices.

Cards including semiconductor devices enveloped therein are disclosed in an article by Hans Steinegger, Microelectronics Manufacturing Technology, pp. 13-15, Dec. 1991; the disclosure of which is expressly incorporated herein by reference. As disclosed in the aforementioned article, the semiconductor devices have heretofore been bonded directly to a metal conductor or other substrate using an epoxy adhesive followed by encapsulation with epoxy resin. The obstacles to the production of such "smart cards" include problems arising in wire bonding because the basic material of the film is not as heat resistant as might be desired. It is pointed out in the article that for smart cards wire bonding must be performed at temperatures that do not exceed 180.degree. C. It is apparent therefor that there is a need for a flexible die attach adhesive which is capable of being cured at relatively low temperatures but has very good adhesive strength.

Adhesive formulations have been proposed which are rapidly curing, i.e., are capable of being cured in under 5 minutes at 200.degree. C. However, these formulations contain cyanate esters and are not flexible. The present invention provides a method of rendering cyanate esters flexible by reacting therewith at least one elastomeric and thermo plastic modifier from the group of hydroxyl (--OH), amine (NH) or epoxide reactive groups, or mixtures thereof by, for example, grafting such groups to cyanate molecular structures. Where flexible adhesive formulations with low viscosity are desired for better workability, etc., the cyanate esters are reacted with a polyhydroxyl polymer compound having the formula:

Advantageously, the polyhydroxyl polymer compound used has a molecular weight of between about 200 and about 5000 and contains at least two hydroxyl groups.

In addition to the foregoing, the invention includes adhesive formulation for bonding a semiconductor device to a substrate, which comprises a reaction product of cyanate ester and the elastomeric modifier, as discussed above, having a low glass transition temperature of less than about 25.degree. C. To impart rapid curability at low temperature, the adhesive may include an alkylphenol, a metal-containing curing catalyst, and/or silver in the form of flake or powder. Advantageously, the alkylphenol is nonylphenol, the metal-containing curing catalyst is from the group consisting of cobalt acetylacetonate and copper napthenate and the silver is present in amounts up to 90 wt. %. In one preferred embodiment the adhesive comprises the reaction product of 6 to 24 wt. % modifier and 4 to 16 wt. % cyanate ester, 0.01 to 0.06 wt. % metal-containing curing catalyst, 0.5 to 2 wt. % alkylphenol and up to 90 wt. % silver flake and/or powder. Where alkylphenol and/or metal containing catalyst are present as little as 4 to 20% silver may be sufficient. Where these curing catalysts are absent 60 to 90 wt. % silver may be needed to impart rapid curability to the adhesive formulation.

A novel card in accordance with the invention, for processing in a computer, comprises a semiconductor device enveloped within the card and adhered with a flexible adhesive to a substrate. The substrate is advantageously a flexible organic polymeric substrate and/or a lead frame or metal conductor. The adhesive comprises the reaction product of cyanate ester and modifier, as described above, having a glass transition temperature of less than about 25.degree. C., and which may include an alkylphenol, a metal-containing curing catalyst and/or silver in the form of flake and/or powder.

FIG. 1 is a graph showing the relationship between the percentage of modifier, i.e., flexibilizer and radius of curvature, ("ROC");

FIG. 2 is a graph of die stress against ROC;

FIG. 3 is a schematic view partly in section of a card within which one or more semiconductor devices are enveloped and showing such devices bonded to a substrate; and

FIG. 4 is a graph illustrating % conversion of the polyhydroxyl polymer flexibilizer as a function of time and temperature.

As has been previously explained, a flexible adhesive is needed for a number of applications where semiconductor devices with large surface areas are attached to flexible substrates, particularly where there is a significant difference between the thermal expansion coefficients of the silicon of the semiconductor device and the substrates. In such cases, a low-stress die attach adhesive is required for maximum performance. This is especially important where the semiconductor device is to be attached to lead frames, such as copper lead frames, and polymeric substrates such as polyester, polyamide films, etc.

If a conventional rigid adhesive such as epoxy referred to in the aforementioned Steinegger article is used in the above described applications, the thermally induced tensile stress imparted to the surface of the semiconductor die is so high that it usually results in a silicon fracture.

To assess the phenomenon involved it is necessary to measure the die stress. One such method is by determining the radius of curvature ("ROC"). A stress free die would be flat and would have a very large radius of curvature. The relationship between tensile stress imparted on the die surface and the radius of curvature is illustrated in FIG. 1. Since the tensile stress of silicon is about 100 MPa, an ROC value of about 1 meter or above is advantageous to eliminate silicon fracture with dies of customary size.

Die attach adhesive compositions including cyanate ester and silver do not give the flexible properties needed for the above applications. However, flexible properties can be achieved by reacting cyanate esters with at least one elastomeric and thermo plastic modifier from the group of hydroxyl (--OH), amine (NH) or epoxide reactive functional groups, or mixtures thereof by, for example, grafting such group to cyanate ester molecular structures.

To illustrate the foregoing, blends of low Tg, (Tg <25.degree. C.), flexible epoxy and cyanate ester resin undergo the following reactions: ##STR1##

The effect of flexible epoxy on ROC is shown in FIG. 2. As can be seen, a mixture of at least 50% epoxy gives acceptable performance and the cyanate ester in this mixture also acts as curing agent for the epoxy. However, at least about 15% cyanate ester is needed in the adhesive formulation to provide satisfactory curing in this system. Other elastomeric modifiers containing hydroxyl reactive groups, including hydroxyl terminated butadiene and butadiene-acrylonitrile copolymers, such as "HYCAR" sold by B. F. Goodrich and "VITEL-3600" which is a saturated copolyester resin with a Tg of -11.degree. C., and polyhydroxyl polymer compounds are useful as well as amine functional groups such as polyurethane, elastomer, urethane acrylate, silicone imides, etc.

All of the foregoing modifiers have proven to be miscible with cyanate ester in the uncured state. During curing, the system is phase-separated, forming a co-continuous structure of hard and soft domains composed of cyanate ester-rich and elastomer-rich agglomerates. This system exhibits many advantageous properties, such as increased flexibility due to the soft modifier, while maximizing adhesive strength at high temperature.

The following are examples of some flexible adhesive formulations, with amounts in wt. %:

A so-called "smart card" is shown in the schematic illustration depicted in FIG. 3. As can be seen, a card 10, usually made of plastic, includes a metal conductor 12 to which is bonded a semiconductor device 14. Glass fiber reinforced polyester pads 16 may be included and the entire device is encapsulated, such as within an epoxy resin 18. The epoxy resin may be ground flat to produce a thin plastic card.

The adhesive formulation of the present invention is especially suitable for bonding the semiconductor device to a substrate because it is capable of being cured at comparatively low temperatures of 100.degree. to 150.degree. C. and has very good adhesive strength while imparting low stress on the die. It is highly desirable for smart card application to employ a die attach adhesive capable of being cured at temperatures not substantially greater than about 150.degree. C., preferably not greater than about 130.degree..degree. C. The die may be attached to a lead frame, such as a copper lead frame in which case the low temperature curing adhesive minimizes oxidation of the copper and thereby eliminates the need of nitrogen or other inert atmosphere such as is often used in curing ovens.

The following examples are of flexible adhesive compositions used to bond semiconductor devices suitable for smart card manufacture.

The cyanate ester Arocy L10 is available from Rhone-Poulenc of Louisville, Ky. The modified bisphenol epoxy resin such as XB 4122 or XUGY 376 is a flexible epoxy supplied by Ciba Geigy. The catalyst system is a solution of copper napthenate or copper acetylacetonate in nonylphenol. The silver flake has the same physical properties described in the previous examples. Adhesive strength results appear in Table I.

The following is another example of a low curing adhesive formulation:

The catalysts in the foregoing systems and in the systems hereafter described may be copper napthenate or copper acetylacetonate, nonylphenol and silver. The formulation in Example 3 was cured at 100.degree. C. for 15 minutes and exhibited an adhesive strength of over 30 Kg for a semiconductor die 150 mil.sup.2 attached on a copper lead frame.

Where flexible adhesive formulations with low viscosity are desired, the cyanate esters are reacted with a polyhydroxyl polymer compound having the formula:

Preferred polyhydroxyl polymer compounds include polyhydroxyl polyester and polyhydroxyl polyether. Advantageously, the polyhydroxyl polymer compound used has a molecular weight of between about 200 and about 5000 and contains at least two hydroxyl groups. Such preferred low molecular weight polyhydroxyl polymer compounds result in low viscosity adhesive formulations of about 5 to 50 poise, the molecular weight being empirically related to viscosity. A molecular weight range of about 200 to about 5000 is preferred because below about 200 the material tends to be too volatile and above about 5000 the viscosity may be too high for processing conditions. Low viscosity adhesives which have improved workability enabling the formulation to be applied to a wide range of die sizes with varying substrate types, from flexible to rigid, and with reliable performance. Formulations employing polyhydroxyl polymers reacted with cyanate ester can be cured at temperatures between 30.degree. C. to 200.degree. C., as shown in FIG. 4, in the absence of a catalyst to form partially cross length (partially polymerized) B-stage, non-tacking films which can be subsequently hardened by heat to complete the polymerization in the actually applied process, such as during attaching of electronic devices to substrates. These formulations can be more rapidly cured, and at temperatures up to 200.degree. C., in the presence of catalysts by either heat or UV to form adhesive formulations comprising soft rubber-like layers mass or layer and when combined with a rigid, hard, or elastic layer, both layers would be inseparable from one another. Of course, the compositions may be cured at higher temperatures with shorter curing time.

It has been found that adhesive formulations made from the reaction of polyhydroxyl polymers of compounds having the formula described above with cyanate ester possess a high adhesion strength over a wide range of curing temperatures and on different substrates. They also possess high temperature heat resistance of up to 350.degree. C. and considerable flexibility, thereby providing low stress and low modulus characteristics which are useful with a wide range micro-electronic packages and devices. Suitable substrates for die attach adhesives include copper lead frames, printed circuit boards, laminates with "glob"-top coating for substrates varying from rigid to flexible.

The presently preferred low viscosity formulation comprises, in wt. %, 2-22% cyanate ester, 8-32% polyhydroxyl polymer compound, 0.01-0.06 wt. % metal-containing curing catalyst, up to 4.0% of an additive such as amorphous fume silica or clay, e.g., bentonite, for thixotropic behavior control, 0.5-2%, preferably 0.1-0.5%, alkylphenol and up to 90% silver flake and/or powder. The silver flake and/or powder preferably has a particle size within the range of 0.1-50 .mu.m, a surface area of 0.1-2 m/g and a tap density of 2-6 g/cc.

Low viscosity flexible adhesive formulations of the present invention can be applied to a wide range of semiconductor die size and shape. For example, die sizes from 10 mil. to 700 mil. (0.4 mm to 0.28 mm) either square or rectangular may be employed. A substrate, which may be rigid or flexible, from material such as alumina, gold coatings or silver coatings, plain copper lead frames and of varying thicknesses from 4 mil. and above, may be used. The curing temperature can be from 30.degree. C. to 300.degree. C., but can be reduced with curing catalysts to 200.degree. C. and the processing temperature may vary from 30.degree. C. to 350.degree. C., if required.

Other advantages of the low viscosity flexible adhesive formulation as described is that it is able to resist change even during high temperature thermal aging at 150.degree. C.-185.degree. C. and during thermal cycling from -60.degree. C.-50.degree. C. for at least 200 cycles. The formulations may be rapidly cured within 5 minutes at 200.degree. C. or for longer times at lower temperatures.

The following Table II summarizes a number of examples of low viscosity flexible adhesive formulations employing polyhydroxyl polymer compounds described by their trade names, "Multron R-18", "Desmophen 1700, 1915 U, and 1652", respectively. The material identified as "SD-2" is a bentonite clay additive which may be added to assist in improving thixotropic behavior, i.e., thixotropic characteristics. The physical properties of the examples in Table II are summarized in Table III.

To illustrate that the shear strength of the formulations described in Table II is maintained and does not degrade over time the composition of Example 8 was subjected to adhesion stability testing to compare die share against aging time. The average results of ten samples are reported in Table IV. For this test, the substrate used was a copper lead frame of 6 mil. thickness, a curing temperature of 150.degree. C., an aging temperature of 150.degree. C., and a die size of 5.08.times.5.08 mm-200.times.200 mil.

In order to determine whether the die stress remains at acceptable levels for the polyhydroxyl polymer containing adhesive formulations, test of flexibility tests were run to determine the radius of curvature change, if any, over time. The average results of ten samples of the Example 8 composition are reported in Table V which summarizes the tests conducted on the same substrate at the same curing and aging temperature described in connection with the tests reported in Table IV but with different die size. The die size for these tests were 15.24.times.15.24 mm-600.times.600 mil.

It is apparent from the foregoing that various changes and modifications may be made without departing from the spirit of the invention. Accordingly, the scope of the invention should be limited only by the appended claims wherein what is claimed is: