Underfill encapsulants prepared from allylated amide compounds

Details Underfill encapsulants prepared from allylated amide compounds Underfill encapsulants prepared from allylated amide compounds

1999-06-18

2002-02-26

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...

C526S227000, C526S303100, C526S306000, C526S310000, C526S312000

Reexamination Certificate

active

06350840

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to underfill encapsulant compositions prepared from allylated amide compounds to protect and reinforce the interconnections between an electronic component and a substrate in a microelectronic device.
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. 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 integrated circuit component may comprise a single bare chip, a single encapsulated chip, or an encapsulated package of multiple chips. The single bare chip can be attached to a lead frame, which in turn is encapsulated and attached to the printed wire board, or it can be directly attached to the printed wire board.
Whether the component is a bare chip connected to a lead frame, or a package connected to a printed wire board or other substrate, the connections are made between electrical terminations on the electronic component and corresponding electrical terminations on the substrate. One method for making these connections uses metallic or polymeric material that is applied in bumps to the component or substrate terminals. The terminals are aligned and contacted together and the resulting assembly heated to reflow the metallic or polymeric material and solidify the connection.
During subsequent manufacturing steps, the electronic assembly is subjected to cycles of elevated and lowered temperatures. Due to the differences in the coefficient of thermal expansion for the electronic component, the interconnect material, and the substrate, this thermal cycling can stress the components of the assembly and cause it to fail. To prevent failure, the gap between the component and the substrate is filled with a polymeric encapsulant, hereinafter called underfill or underfill encapsulant, to reinforce the interconnect and to absorb some of the stress of the thermal cycling.
Two prominent uses for underfill technology are in packages known in the industry as flip-chip, in which a chip is attached to a lead frame, and ball grid array, in which a package of one or more chips is attached to a printed wire board.
The underfill encapsulation may take place after the reflow of the metallic or polymeric interconnect, or it may take place simultaneously with the reflow. If underfill encapsulation takes place after reflow of the interconnect, a measured amount of underfill encapsulant material will be dispensed along one or more peripheral sides of the electronic assembly and capillary action within the component-to-substrate gap draws the material inward. The substrate may be preheated if needed to achieve the desired level of encapsulant viscosity for the optimum capillary action. After the gap is filled, additional underfill encapsulant may be dispensed along the complete assembly periphery to help reduce stress concentrations and prolong the fatigue life of the assembled structure. The underfill encapsulant is subsequently cured to reach its optimized final properties.
If underfill encapsulation is to take place simultaneously with reflow of the solder or polymeric interconnects, the underfill encapsulant, which can include a fluxing agent if solder is the interconnect material, first is applied to either the substrate or the component; then terminals on the component and substrate are aligned and contacted and the assembly heated to reflow the metallic or polymeric interconnect material. During this heating process, curing of the underfill encapsulant occurs simultaneously with reflow of the metallic or polymeric interconnect material.
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 an underfill encapsulant that will meet all the requirements for reinforcement of the interconnect, but also an underfill encapsulant that will be reworkable, allowing for the failed component to be removed without destroying the substrate.
Conventional underfill technology uses low viscosity thermosetting organic materials, the most widely used being epoxy/anhydride systems. In order to achieve the required mechanical performance, relatively high molecular weight thermoplastics would be the preferred compositions for underfill 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 underfill encapsulant compositions that are easily dispensable to conform with automated manufacturing processes, and that are reworkable.
SUMMARY OF THE INVENTION
This invention relates to a curable underfill encapsulant compositions comprising allylated amide compounds, a free radical curing agent and/or a photoinitiator, and optionally, one or more fillers or other additives. The composition optionally may also contain mono- or polyfunctional vinyl compounds.
The composition can be designed to be reworkable by choosing a major amount of mono-functional compounds for the composition.
The ability to process these compositions for underfill encapsulants is achieved by using relatively low molecular weight reactive oligomers or pre-polymers and curing them in situ after application to the electronic assembly. The relatively low molecular weight translates to a lower viscosity and ease of application to the substrate.
In another embodiment, this invention is a cured encapsulant composition that results after the curing of the just described curable underfill encapsulant composition.
In another embodiment, this invention is a microelectronic assembly comprising an electronic component having a plurality of electrical terminations, each termination electrically and mechanically connected by a metallic or polymeric material (the metallic or polymeric material also referred to herein as interconnect or interconnect material) to a substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component, and a cured encapsulant disposed between the electrical component and the substrate to reinforce the solder or polymeric interconnects, in which the cured encapsulant was prepared from curing a composition comprising one or more allylated amide compounds, a free radical curing agent and/or a photoinitiator, and optionally, one or more fillers. The composition optionally may also contain mono- or polyfunctional vinyl compounds.
In another embodiment this invention is a method for making an electronic assembly, the electronic assembly comprising an electronic component having a plurality of electrical terminations, each termination electrically and mechanically connected by a metallic or polymeric material to a substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component, and a cured reworkable underfill encapsulant composition disposed between the electronic component and the substrate, the method comprising: (a) providing a curable underfill encapsulant composition, (b) disposing the curable composition between the electrical component and the substrate; and (c) curing the composition in situ.
DETAILED DESCRIPTION OF THE INVENTION
The allylated amide, and vinyl compounds, used in the underfill 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,

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