Coating processes – Direct application of electrical – magnetic – wave – or... – Polymerization of coating utilizing direct application of...
Reexamination Certificate
1999-12-22
2001-01-30
Seidleck, James J. (Department: 1711)
Coating processes
Direct application of electrical, magnetic, wave, or...
Polymerization of coating utilizing direct application of...
C427S508000, C427S517000, C427S516000, C427S518000, C427S096400, C427S528000, C427S531000, C428S901000, C428S457000, C428S461000, C522S102000, C522S106000, C522S142000
Reexamination Certificate
active
06180187
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of making ar, electronic component using a reworkable underfill encapsulant composition. Underfill encapsulant compositions protect and reinforce the interconnections between an electronic component and a substrate.
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, that is, one that will allow the failed component to be removed mechanically 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.
SUMMARY OF THE INVENTION
This invention is a method for making an electronic assembly consisting of an electronic component, such as a flip-chip or a ball grid array, connected to a substrate and reinforced with a reworkable underfill encapsulant composition.
The underfill encapsulant composition 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 filler;.
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.
The composition is designed to be reworkable by choosing a major amount of mono-functional compounds for the composition. The ability to process these thermoplastic compositions for underfil 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.
The electronic component has 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 electronic component. The underfill encapsulant composition is disposed between the electronic component and the substrate to reinforce the metallic or polymeric interconnect.
The method of making the electronic assembly comprises the steps:
(a) providing a curable underfill encapsulant composition comprising
(i) one or more mono-functional maleimide compounds, or one or more mono-functional vinyl compounds other than maleimide compounds, or a combination of mono-functional maleimide and mono-functional vinyl compounds, in a major amount effective to provide thermoplastic properties, and
(ii) optionally, one or more poly-functional maleimide compounds, or one or more poly-functional vinyl compounds other than maleimide compounds, or a combination of poly-functional maleimide and poly-functional vinyl compounds, in a minor amount effective to provide strength to the composition when cured but ineffective to offset the thermoplastic properties of the cured composition,
(iii) a curing initiator selected from the group consisting of a free-radical initiator, a photoinitiator, and a combination of those;
(iv) optionally, one or more fillers;
(v) optionally, one or more
Ma Bodan
Tong Quinn K.
Xiao Chaodong
Gennaro Jane E.
McClendon Sanza
National Starch and Chemical Investment Holding Corporation
Seidleck James J.
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