Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-12-20
2003-12-16
Mulcahy, Peter D. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S265000, C524S547000, C524S555000, C524S556000, C524S560000
Reexamination Certificate
active
06664318
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to encapsulant compositions used for packaging electronic components and devices. More particularly the invention relates to a thermomechanical-shock-resistant, cured composition for solventless, hydrophobic resin encapsulation of electronic components comprising a non-silicone oligomer including a flexible hydrocarbon backbone having reactive functionality. The composition includes up to about 40% of an adhesion promoter by weight of the total composition.
2. Description of the Related Art
Advances in electronic circuit design have increased the number of electronic components occupying the area available on a printed circuit board. Evidence of such increases can be appreciated by considering the mounting numbers and increased versatility of electronic devices such as personal digital assistants, cellular phones, portable compact disc players, personal computers, lap-top computers and other equipment relying on electronic devices for effective performance. Progressive board space consumption, by semiconductor integrated circuit components, raises problems with a large quantity of very small contacting locations that require interconnection to outside or external circuitry. An interconnection structure, typically positioned around a space occupied by e.g. a semiconductor chip, may be referred to as a lead frame. Lead frame technology has led to tape automated bonding (TAB) technology, referring to connections to a conductor pattern placed on a tape filament. Associated with lead frame technology is another connection scheme, known as wire bonding, wherein segments are bonded to the contact or pad on a chip at one end and to an external conductor at the other end. A problem with wire bonding technology occurs when wires break during injection molding of encapsulant to surround a semiconductor device. Individual connecting wires, of the wire bonding pattern, may snap upon impact of the advancing wave of injected encapsulant material. Therefore protection of wire interconnections is essential to maintaining the electrical integrity of an electronic device.
In addition to the protection of wire bonded connections, modern electronic circuits must be able to withstand a wide variety of environmental conditions, such as exposure to moisture, heat, mobile ion contaminants and abrasion. The search for suitable protection revealed several types of sealing or encapsulating materials, including a variety of flexible polymer compositions derived from polyurethanes, polysiloxanes, polyepoxides and other organics, plastics and the like. Such compositions are now well known for providing protective layers for circuits and attached semiconductor components. Despite improvement in performance of sealing and encapsulating materials, resistance to moisture, and ion penetration still remain as problems for most materials, while selected materials are subject to specific problems. For example, deficiencies of silicone-based encapsulants include poor solvent resistance and a relatively poor level of adhesion to substrate materials.
A review of encapsulant materials shows common use of silicone compositions. Despite the problems of solvent susceptibility and poor adhesion, these materials appear to offer a better balance of properties than other, previously used, types of material. U.S. Pat. No. 4,888,226 describes an electronic device encapsulated by a silicon dioxide-containing silicone resin gel that cures with a hard protective shell. The encapsulant material is an improvement over a silicone composition, in U.S. Pat. No. 4,565,562, which could fail when subject to thermal stress, abrasion or solvent attack. Non-silicone encapsulants in combination with silicone encapsulants provide performance improvement by contributing properties that silicones lack. An example of this is provided by U.S. Pat. No. 5,047,834 wherein one material gives protection around bonded leads of the lead frame while another provides environmental protection extending over the surface of the device and the leads. A silicone encapsulant provides environmental protection and covers a filled epoxy encapsulant that surrounds the bonded leads. The combined protection of silicone and epoxy materials overcomes the problems of each used individually.
The long term successful use of epoxy compositions as potting compounds and encapsulants for electrical industries may not apply directly in electronic packaging applications. Generally, epoxy resins alone are too rigid, brittle, moisture absorbing and susceptible to thermal shock to satisfy the requirements of bonded lead protection and electronic component encapsulation. The primary problem appears to be a lack of flexibility of cycloaliphatic polyepoxides, glycidyl ethers of bisphenol A and related epoxy resins. Evidence exists of numerous attempts to improve the flexibility of epoxy resins with the goal of developing suitable electronic encapsulant materials. One frequently addressed approach uses toughener addition to increase the flexibility of epoxy resin compositions. The toughened epoxy formulation may also include a reactive diluent. U.S. Pat. No. 5,726,216, for example, describes the addition of a toughener or flexibilizer to e.g. glycidyl ethers of various phenolic compounds. Materials as tougheners for such resins include oligomers of engineering thermoplastics which contain functional groups capable of reacting with the epoxy resin during curing. Suitable oligomers may be flexible molecules with terminal reactive substituents, such as epoxy groups, at one or both ends of a long, predominantly aliphatic hydrocarbon chain. Additional references, including U.S. Pat. Nos. 4,578,425; 4,778,851; 5,053,496; 5,420,202; 5,461,112; 5,499,409; Canadian published application CA 2,164,915, and European published application EP 0 565 206, provide further evidence of physical property modification to provide more flexible epoxy resins, either by addition of toughening components that rely on molecular structure or the introduction of core/shell particles into the epoxy resin composition. In all cases, preferred compositions contain less than 40% of toughener. U.S. Pat. 4,639,503 discusses the addition of a phenol adduct of a conjugated diolefin polymer to an epoxy resin produced by reaction of epichlorohydrin and bisphenol A or a novolak resin, alicyclic epoxy resins and epoxy resins derived from halogenated bisphenol A. Up to 300 parts of phenol adduct of a conjugated diolefin polymer may be added to 100 parts of an epoxy resin to provide encapsulant compositions which, upon curing, survive pressure cooker (125° C.×200 hrs) and heat cycle (100 cycles between −40° C. and 120° C.) testing. The glass transition temperatures (Tg) of cured compositions are in the range of 60° C. to 85° C. probably due to the relatively high amount of epoxy resin in the composition.
Despite efforts, an encapsulant has yet to be found to meet the needs previously outlined and in addition survive the thermomechanical shock and outgassing testing required of today's sophisticated electronic devices and related hardware. Towards this goal, it would be beneficial to provide a material capable of flowing about an electronic device, to encase it, with appropriate adhesion, electrical properties, thermal stability, moisture resistance, solvent resistance, and shock resistance after curing to yield an encapsulant hard enough to resist penetration but soft enough to cushion impact.
SUMMARY OF THE INVENTION
The invention provides novel encapsulant compositions containing major quantities of components unlike any of the polyurethane, polyepoxide and polysiloxane base resins previously investigated. One version of an encapsulant composition, according to the present invention, includes a linear hydrocarbon chain, usually in the form of a polyolefin oligomer providing a flexible backbone with reactive functionality attached to the oligomer chain. The polyolefin oligomer is hydrophobic, providing moisture resistance, while the li
Biernath Rolf W.
Bymark Richard M.
Kropp Michael A.
Mahoney Wayne S.
McKenzie Taun L.
3M Innovative Properties Company
Harts Dean M.
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