Background of the Invention Adhesives that provide anisotropic electrical conductivity between conductive pads on an integrated circuit die and matching conductive pads on a substrate, while also providing mechanical bonding between the die and substrate are known and are commonly used. One example is described in U. S. Patent 5,120,665, entitled"METHOD OF USING AN ANISOTROPICALLY ELECTROCONDUCTIVE ADHESIVE HAVING PRESSURE-DEFORMABLE ELECTROCONDUCTIVE PARTICLES TO ELECTRICALLY CONNECT CIRCUITS", issued to Tsukagoshi et al., on Jun. 9,1992. Such adhesives have been beneficially used to attach integrated circuit die to transparent substrates, such as a glass substrate used in one type of liquid crystal displays.

Intermittent malfunctions have become evident during the use of electronic devices having displays that are operated under conditions in which strong UV light impinges on the display, for example, during use outdoors on a sunny day. Such malfunctions include blackening of areas of LCD displays and resetting of logic circuits. These type of malfunctions in LCD displays have been traced to the conduction of UV light (light energy in wavelengths up to about 400 nanometers) through the transparent glass substrate of the display, then through the adhesive used to bond and electrically connect a circuit die to the glass substrate. The UV light reaches the circuit die where the adhesive attaches to the circuit die and penetrates into the circuit die, interfering with the normal operation of the transistors in the circuit die, causing the malfunctions. Other malfunctions of electronic devices that are used in conditions in which UV light impinges on the electronic device are likely caused by similar circumstances of construction. For example, some printed circuit board substrates might be semi transparent to UV light and could therefore conduct the UV light to adhesives that are used to bond circuit die to such substrates.

Since printed circuit boards are used less extensively in situations where they are exposed to UV light, malfunctions of circuits mounted to printed circuit boards in all likelihood occur less often, but it is likely that some malfunctions whose origins have not been traced are due to the problem of UV light getting into circuit die mounted on printed

circuit boards.

Thus, what is needed is a technique to provide a high reliability electronic circuit by preventing UV light from getting into circuit die mounted on substrates that are translucent to UV light.

Brief Description of the Drawings FIG. 1 is an isometric view of a radiotelephone having a display, in accordance with the preferred embodiment of the present invention.

FIG. 2 is an isometric view of the display of the radiotelephone, in accordance with the preferred embodiment of the present invention.

FIG. 3 is a cross sectional view of a connection pad of an integrated electrical circuit bonded to the glass of the display, in accordance with a prior art method of bonding FIG. 4 is a cross sectional view of a connection pad of an integrated electrical circuit bonded to the glass of the display, in accordance with the preferred embodiment of the present invention.

Description of a Preferred Embodiment Referring to FIG. 1, an isometric view of a radio telephone 100 having a display 110 is shown with a flip open door 120 in an open position, in accordance with the preferred embodiment of the present invention. The radio telephone 100 is a conventional StarTAC cellular telephone manufactured by Motorola, Inc., of Schaumburg, IL., except that in the preferred embodiment of the present invention some of the integrated circuit die in the radio telephone 100 are uniquely bonded in a manner that makes the circuitry of the radio telephone 100 more reliable than in radio telephones in which prior art bonding techniques are used to bond integrated circuit die to substrates.

Referring to FIG. 2, an isometric mechanical drawing of the display 110 is shown, in accordance with the preferred embodiment of the present invention. The display 110 is a liquid crystal display (LCD) having a front glass piece 230 that serves as a substrate for a plurality of integrated circuit die 220 that are bonded to the substrate 230 in a unique manner described more fully below. The plurality of integrated circuit die 220 form a portion of a display driving circuit. Three such integrated circuit die 220 are illustrated in FIG. 2, but it will be appreciated that some LCD displays have many more and some have less integrated circuit die. The front glass piece and substrate 230 has a viewing area 210 which is exposed to the user's view when the front door 120 is in the open position. The display 110 further comprises a back panel 232 and a inner separator 231. The inner

separator 231 has interstices within which portions of liquid crystal compound are held, in a conventional manner. The front glass piece (substrate) 230, the back panel 232 and the inner separator 231 are conventional parts, and although they are glass in accordance with the preferred embodiment of the present invention, LCDs having plastic panels will achieve similar benefits of improved reliability achieved by displays having glass panels, when the bonding adhesive of the present invention is used. A flexible cable 240 is connected to the substrate 230 in a conventional manner.

Referring to FIG. 3, a cross sectional diagram of a portion of a prior art display 305 is shown that includes one of the integrated circuit die 220. The prior art display 305 differs from the display 110 of the present invention in the nature of the adhesive base 340 used to bond the integrated circuit die 220 to the substrate 230. The adhesive base 340 used is a conventional polymer material, such as a hot melt thermoset or thermoplastic material. In the example shown in FIG. 3, spherical particles 360,361 are mixed into the adhesive base 340 before bonding is completed. The spherical particles 360,361 are conductive, consisting of such substances as solid metallic particles or metal plated particles. The adhesive base 340 is selected with viscoelastic characteristics such that when non-destructive pressure is applied to the integrated circuit die 220 and the display 110 during a bonding process in a direction to squeeze the opposing surfaces of substrate 230 and integrated circuit die 220 together, they move towards each other until a conductive portion 370 of the integrated circuit die 220 (e. g., a conventional circuit pad), and a conductive portion 380 of the substrate 230 (e. g., a conventional circuit pad) are held separated by the spherical conductive particles 361. (In the typical case when there are a plurality of such opposing conductive portions, they also are each held apart by the spherical particles 361). Accordingly, the circuit pads 370 and 380 are electrically connected by the spherical particles 361. The surfaces of the substrate 230 and the integrated circuit die 220 are essentially non-conductive, and the spherical particles 360, 361 are dispersed within the adhesive base 340 widely enough such that the odds of a "chain"of them touching each other to provide a conductive path between different pairs of opposing conductive portions 370,380 are vanishingly small. Thus, the combination of the adhesive base 340 and the spherical particles 360,361 form an electrically conductive anisotropic adhesive. It will be appreciated that a variety of electrically conductive anisotropic adhesives exist, each having differing characteristics suitable for a variety of substrates materials, a variety of bonding temperatures and pressures, and a variety of reliabilities, at different costs. Examples of two these electrically conductive anisotropic adhesives are type AC-7073Z-25 adhesive, manufactured by Hitachi, Ltd. of Tokyo,

Japan and Sony, type CP 7131 adhesive, manufactured by Sony Corporation, Tokyo, Japan.

Referring again to FIG. 3, light paths 350-354 illustrate ultraviolet (UV) energy that has entered the substrate (front glass panel) 230 through the viewing area 210 when the flip open door 120 is open. The UV energy that enters through the viewing area 210 scatters through the substrate 230 in many directions, reflecting off of material interfaces, such as the interface of the front glass panel 230 and inner separator 231 and the interface of the inner separator 231 and the back panel 232 in a manner well known to one of ordinary skill in the art. The light paths 350-354 are a very few examples of such reflected energy. The energy in light paths 350,351 is energy that goes between some of the spherical particles 360 and enters the integrated circuit die 220. The energy in light path 352 hits a surface 321 at such an angle that some of it is reflected as energy in light path 354, while another part 353 of it passes through the surface, through liquid crystal (LC) material 320 in the interstices of the inner separator 231, and also through separation pieces 330 of the inner separator 231, reflecting 322 back up into the integrated circuit die 220. When the UV energy form these and other paths enters the integrated circuit die 220 the energy impinges on a variety of transistor junctions, and the UV energy impinging on such junctions can be sufficient to cause abnormal performance of such junctions. Such anormal performance is more likely for integrated circuits operating at lower energy levels. The lowering of energy levels in conventional integrated circuits continues to occur as transistor junction sizes and voltage levels are reduced, which is occurring especially in integrated circuits designed for portable electronic devices. In this example, light has no direct (non-reflective) path through the display because the area of the display glass on which the die is located is behind an opaque portion of the housing of the flip open door 120. It will be appreciated that problems can also arise in which UV directly radiates onto the surface of a die that is opposite the bonding surface, but the problem can then be solved in a different manner (such as opaque tape) that is not amenable to the bonding side of the die.

It will be appreciated that the same problem exists with other prior art techniques in which conventional electrically conductive, anisotropic adhesives are used. Several examples of these are described in U. S. Patent 5,879,530, entitled"Anisotropic Conductive Film for Microconnections", issued to Caillat on March 9,1999, which is incorporated herein by reference.

Referring to FIG. 4, a cross sectional diagram of a portion of the display 110 is shown that includes one of the integrated circuit die 220, in accordance with the preferred

embodiment of the present invention. The display 110 is identical to the prior art display 305, except for the composition of the electrically conductive anisotropic adhesive that bonds the substrate 230 to the integrated circuit die 220. In the present invention, the adhesive base 440 has been uniquely altered to reduce the amount of UV light that passes through the adhesive base 440. As illustrated by the light paths 450-454 in FIG.

4, most of the energy is absorbed in the adhesive base 440 and does not reach the integrated circuit die 220. The alteration is accomplished by combining a UV absorbing component into a conventional adhesive base, either by mixing or by copolymerization.

The percentage of UV light that passes through the UV absorbing electrically conductive anisotropic adhesive to the integrated circuit die 220 is characterized herein by a UV transmissivity of the UV absorbing electrically conductive anisotropic adhesive. The UV transmissivity is expressed herein as a percentage of light passing through a stated thickness of the cured adhesive, for example 8% to 12% at 0.0010 0.0003 inches average thickness. Other expressions of the amount of UV energy passing through the adhesive could obviously be used equally well. (The UV absorbing component can be selected from conventional UV controlling components that are from the chemical families of benzophenones, zinc oxides, and polyethylene terephthalate, but some of these are better suited for the types of adhesives (the adhesives are typically epoxies and acrylates) typically used for die bonding. (Chemicals in the families of benzophenones are more compatible with epoxies and acrylates than zinc oxides and polyethylene terephthalate, in the sense that lower UV transmissivities are possible when combining an amount of such compound that does not significantly alter vital characteristics of the adhesive, of which one is bonding strength). One group of components from which the UV absorbing component can be selected are stabilizers in the chemical classes consisting of: (1) benzotriazoles, (2) benzophenones, (3) aryltriazines ; and (4) salicylic acid derivatives; and (5) oxanilides.

Specific examples of these that are UV absorbing stabilizers are: (1) 2- (2-hydroxyaryl) benzotriazoles, (2) 2-hydroxybenzophenones, (3) 2- (2-hydroxyaryl)-4, 6-diaryl-1, 3,5-triazines (4) salicylic acid derivatives, (5) 2-hydroxyoxanilides,

(6) blocked derivatives of (1) to (5), wherein the phenolic OH group is blocked with a suitable blocking group, and (7) mixtures of (1) to (6).

Combinations of some of these UV absorbing components are further described in U. S. Patent 5,621,052, entitled"AMINOPLAST-ANCHORED ULTRAVIOLET LIGHT STABILIZERS", issued to Szita et al., on April 15, 1997.

In accordance with the preferred embodiment of the present invention, one of the UV absorbing components is combined with an adhesive base of an electrically conducting anisotropic adhesive, the combination being chosen such that the combination, when used in the electronic device (represented herein by the radio telephone 100), is an economical choice that achieves a desired set of characteristics, including a predetermined UV transmissivity, sufficiently high bonding strength, sufficient electrical conductivity, a bonding temperature and pressure compatible with the substrate 230 and integrated circuit die 220, environmental friendliness, and time to accomplish bonding.

The combination is expressed as a volumetric proportion of the UV absorbing compound to a basis, the basis being the electrically conductive anisotropic adhesive or the adhesive base, with a statement as to the basis. Other expressions of the combination could be used. One method of determining the predetermined transmissivity needed in a particular application is experimentation to determine now much UV light energy can be tolerated by a particular product hardware configuration (i. e., the combination of the product housing, substrate mounting and other components in the housing that affect the amount of light that reaches the bonding area of the integrated circuit die, using an average thickness of the conventional adhesive that is expected during manufacturing), to achieve a sufficient electrical performance reliability, and comparing the tolerable amount of incident UV energy to the UV energy in a predetermined"worst case"situation, for example, operation at noon on a sunny summer day in Phoenix AZ to determine a transmissivity at the expected average thickness. Another method is experimentation using the product that has integrated circuit die bonded, using adhesive bases 440 having varying amounts of UV absorbing compounds combined therewith in a test chamber producing the"worst case"UV energy, to find a combination proportion that achieves a predetermined electrical reliability. The thickness of the UV absorbing electrically conductive anisotropic adhesive used for die bonding is typically in the range of 0.0002-0.0020 inches.

Another class of components from which the UV absorbing component can be selected are compounds described in U. S. Patent 3,399,173, entitled"LIGHT-STABLE

POLYADDITION COMPOUNDS"issued to Heller et al. on August 27,1968, of the formula wherein R'is a hydrogen atom, and compounds of the same formula, but wherein R'is methyl and R is alkyl having from 4 to 12 carbon atoms of which not more than 6 are arranged in a continuous unbranched (linear) chain, as described in U. S. Patent 4,612,3578, entitled "UV-ABSORBING MONOMER AND POLYMERS THEREOF", issued to Besecke et al. on Sept. 16,1986.

It will be appreciated that the object of combining the UV absorbing material to the electrically conductive anisotropic adhesive is different that that of prior art stabilizers that are used to protect the plastic material in which they are combined, because the object of the present invention is not to protect the adhesive material (inasmuch as mechanical failures of the adhesive material are not the symptom of the problem), but rather to prevent transmission of UV light through the adhesive. Thus the methods of determining the required proportion are different.

It will also be appreciated that while either a combination by mixing or by copolymerization of many of the above materials with the typical electrical conductive anisotropic adhesive can be used to achieve the desired transmissivity, copolymerization will typically result in an more highly efficient UV absorbing electrically conductive anisotropic adhesive, in the sense that less of the UV absorbing material will have to be added to achieve a desired transmissivity.

It will be appreciated that an alternative method of achieving the desirable result of preventing the UV is to coat the portions of the surface of the integrated circuit die 220 that are not the conductive portions 370 with a coating of UV absorbing material, but that this entails a separate step and requires selective coating, adding considerable expense to the resulting product.

It will be further appreciated that the present invention provides similar benefits for substrates of types other than plastic and glass, such as other ceramics or printed circuit board materials that are translucent to UV light energy. It will also be appreciated that the present invention can provide the same benefits for any other electronic device which is

exposed to UV light energy that can be conducted by UV translucent substrates to integrated electronic circuits that are bonded to the substrates by adhesives.

By now, it can be seen that by uniquely combining a UV absorbing component with an electrically conductive anisotropic adhesive, a UV absorbing electrically conductive anisotropic adhesive can be generated that will absorb undesirable UV energy that otherwise travels through substrates that are transparent or translucent to UV energy, causing malfunctions in integrated circuit die that are bonded to the substrate with anisotropic adhesives that are otherwise translucent or transparent to UV light energy.

The use of such UV absorbing electrically conductive anisotropic adhesive thereby contributes to the reliability of an electrical circuit that, in turn, improves the reliability of an electronic device in which it is used.