Package encapsulant compositions for use in electronic devices

Details Package encapsulant compositions for use in electronic devices Package encapsulant compositions for use in electronic devices

1999-06-18

2001-11-13

Pezzuto, Helen L. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S258000, C526S259000, C526S262000

Reexamination Certificate

active

06316566

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to package encapsulant compositions for electronic devices that protect the electronic component and its metallization from environmental corrosion and mechanical damage.
BACKGROUND OF THE INVENTION
Microelectronic devices contain millions of electrical circuit components, mainly transistors assembled in integrated circuit (IC) chips, but also resistors, capacitors, and other components. The integrated circuit component may comprise a single bare chip, a single encapsulated chip, or an encapsulated package of single or multi-chips. These electronic components are interconnected to form the circuits, and eventually are connected to and supported on a carrier or substrate, such as a printed wire board.
The various materials used to manufacture the integrated circuits and their related interconnect materials are susceptible to environmental, moisture, and mechanical damage. Protection is provided by encapsulation of the electronic component within a polymeric material. Encapsulation can be performed by a transfer molding process in which the component is loaded into a mold cavity, constrained, and the polymeric encapsulant transferred from a reservoir into the cavity under pressure. Typically, the encapsulant is a thermosetting polymer, which then cross-links and cures to form the final assembly. Encapsulation also can be performed by dispensing an aliquot of polymeric encapsulant onto the component, such as a chip or integrated circuit supported on a substrate, and subsequently curing the composition.
For most commercial and industrial end uses, particularly those utilizing chip-on-board packages and multi-chip modules, the encapsulation is accomplished with polymeric thermosetting materials. The preferred thermosetting package encapsulation materials must have a viscosity and a thixotropic index that allows easy dispensability by syringe, sufficient adhesion to the components, low ionic content to avoid corrosion of the metallization, adequate mechanical strength, high thermal and moisture resistance at application temperatures, and matched coefficient of thermal expansion to the materials it contacts.
For single chip packaging involving high volume commodity products, a failed chip can be discarded without significant loss. However, it becomes expensive to discard multi-chip packages with only one failed chip, and the ability to rework the failed component would be a manufacturing advantage. Today, one of the primary thrusts within the semiconductor industry is to develop not only a package encapsulant that will meet all the requirements for protection of the component, but also a package encapsulant that will be reworkable, allowing for the failed component to be removed without destroying the substrate.
In order to achieve the required mechanical performance and reworkability, relatively high molecular weight thermoplastics would be the preferred compositions for package materials. These materials, however, have high viscosity or even solid film form, which are drawbacks to the manufacturing process. Therefore, there is a need for new encapsulant compositions that are easily dispensable to conform with automated manufacturing processes, and that are reworkable.
SUMMARY OF THE INVENTION
This invention is a curable encapsulant composition for electronic components that comprises one or more compounds containing one or more maleimide functionality, or one or more compounds containing one or more vinyl functionality, or a combination of compounds containing maleimide or vinyl functionality, a free-radical initiator and/or a photoinitiator, and optionally one or more fillers.
A compound containing one maleimide functionality will be referred to hereinafter as a mono-functional maleimide compound. A compound containing more than one maleimide functionality will be referred to hereinafter as a poly-functional maleimide compound. A compound containing one vinyl functionality will be referred to hereinafter as a mono-functional vinyl compound. A compound containing more than one vinyl functionality will be referred to hereinafter as a poly-functional vinyl compound. The functionality is defined herein to be a carbon to carbon double bond.
In another embodiment, this invention is also a cured encapsulant composition that results after the curing of the just described curable encapsulant composition.
In another embodiment, this invention is an electronic component electrically and mechanically connected to a substrate, encapsulated in a cured encapsulant composition, in which the cured encapsulant was prepared from a composition comprising one or more mono- or polyfunctional maleimide compounds, or one or more mono- or polyfunctional vinyl compounds other than maleimide compounds, or a combination of maleimide and vinyl compounds, a free radical curing agent and/or a photoinitiator, and optionally one or more fillers.
DETAILED DESCRIPTION OF THE INVENTION
The maleimide and vinyl compounds used in the package encapsulant compositions of this invention are curable compounds, meaning that they are capable of polymerization, with or without crosslinking. As used in this specification, to cure will mean to polymerize, with or without crosslinking. Cross-linking, as is understood in the art, is the attachment of two polymer chains by bridges of an element, a molecular group, or a compound, and in general will take place upon heating. As cross-linking density is increased, the properties of a material can be changed from thermoplastic to thermosetting, which consequently increases polymeric strength, heat-and electrical resistance, and resistance to solvents and other chemicals.
It is possible to prepare polymers of a wide range of cross-link density, from tacky, elastomeric to tough glassy polymers, by the judicious choice and amount of mono- or polyfunctional compounds. The greater proportion of polyfunctional compounds reacted, the greater the cross-link density.
If thermoplastic properties are desired, the package encapsulants of this invention can be prepared from mono-functional compounds to limit the cross-link density. However, a minor amount of poly-functional compounds can be added to provide some cross-linking and strength to the composition, provided the amount of poly-functional compounds is limited to an amount that does not diminish the desired thermoplastic properties. Within these parameters, the strength and elasticity of individual package encapsulants can be tailored to a particular end-use application. The cross-link density can also be controlled to give a wide range of glass transition temperatures in the cured encapsulant to withstand subsequent processing and operation temperatures.
In those cases where it is necessary to rework the assembly, a thermoplastic composition should be chosen so that the electronic component can be pried off the substrate. Any residue package encapsulant can be heated until it softens and then be easily removed.
In the inventive package encapsulant compositions, the maleimide compounds and the vinyl compounds may be used independently, or in combination. The maleimide or vinyl compounds, or both, will be present in the curable package encapsulant compositions in an amount from 2 to 98 weight percent based on the organic components present (excluding any fillers).
The package encapsulant compositions will further comprise at least one free-radical initiator, which is defined to be a chemical species that decomposes to a molecular fragment having one or more unpaired electrons, highly reactive and usually short-lived, which is capable of initiating a chemical reaction by means of a chain mechanism. The free-radical initiator will be present in an amount of 0.1 to 10 percent, preferably 0.1 to 3.0 percent, by weight of the maleimide or vinyl compound, or a combination of both maleimide and vinyl compounds (excluding any filler). The free radical curing mechanism gives a fast cure and provides the composition with a long shelf life before cure. Preferred free-radical initiators include peroxides, s

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