Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
1998-09-04
2002-04-09
Martin-Wallace, Valencia (Department: 3713)
Metal working
Method of mechanical manufacture
Electrical device making
C029S841000, C228S180100, C228S180210, C228S180220, C228S214000, C228S215000, C156S182000, C156S305000, C156S330000, C156S307100, C156S331100
Reexamination Certificate
active
06367150
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to interconnection and encapsulation of electronic components, in particular to interconnection and encapsulation methods for flip-chip integrated circuits, and specifically to material selection for interconnection and encapsulation of flip-chip integrated circuits.
BACKGROUND OF THE INVENTION
Thermosetting resin compositions such as epoxy resins have been used as semiconductor device encapsulants for over 25 years as noted by reference to U.S. Pat. No. 3,449,641, granted Jun. 10, 1969.
U.S. Pat. No. 3,791,027, Angelo et al., describes epoxy fluxes for soldering. Angelo et al. teach that the fluxes may be formulated to be removable from the solder situs or may be formulated through cross-linking after the soldering process to form a thermoset epoxy polymer which remains at the solder joint and reinforces the strength of the solder joint.
Anhydride-cured epoxy resin encapsulants used in flip-chip manufacturing methods that are applied after electrical interconnection are described in U.S. Pat. No. 4,999,699, granted Mar. 12, 1991, and U.S. Pat. No. 5,250,848, granted Oct. 5, 1993.
The flip-chip method of attaching integrated circuits to substrate boards involves a series of metal solder bumps on the integrated circuit which form metallurgical interconnections with the metal bond sites on the board substrate. The active side of the integrated circuit is flipped upside down in order to make contact between the bumps on the chip and the metal bond sites on the substrate. An organic soldering flux is used to remove metal oxides and promote wetting of the solder when the assembly is heated above the temperature of the solder. This process is referred to as reflow soldering. The purpose of the flux is to clean the surface of the metals. The solder, or lower melting alloy, may be the composition of the board bond pads, of the bumps on the chip or both depending on the materials selected. Similarly, the higher melting alloy may be present on either the bond pad or the bumps on the chip. This process is derived from the controlled, collapse, chip, connect (C4) method developed by IBM in the 1960's.
The reflow soldering operation provides a gap of 0.025 mm to 0.17 mm between the chip and the substrate. Although this small standoff height significantly enhances the electrical performance of the mounted flip-chip, the residue from the flux is difficult to remove from the narrow gap. Thus, no-clean fluxes, in which flux residues are not removed from the board after reflow soldering, are the flux type of choice for most flip-chip applications. These no-clean fluxes may be dispensed onto the metal bond sites on the board prior to chip placement. These liquid no-clean fluxes are formulated to contain more than 94% solvent which evaporates during the reflow process and flux activators which sublime during the reflow step. Thus, minimal amounts of residue remains on the board after reflow. These liquid fluxes, however, have difficulty in holding the chip to the board prior to reflow. The high solvent content of the flux causes the small integrated circuit to skew and misalign before peak soldering temperatures are reached. An additional problem arises from the volatility of many solvents used in these fluxes which blow the chips out of alignment during reflow. Although tackifying agents can be added to overcome these problems, the no-clean, low-residue requirement of the flux dictates a high solvent content which leads to alignment problems during reflow.
In order to maintain alignment of the chip to the board prior to reflow soldering, a viscous tacky flux may be applied to the bumps on the chip. This method involves dispensing the flux onto a rotating disk or drum then applying a blade above the rotating drum. Thus, a desired thickness of flux on the drum can be achieved by adjusting the height of the blade. The integrated circuit, containing solder bumps, is then dipped into the flux on the drum to a set depth. Using this method a desired amount of tacky flux is applied to the surface of the bumps only. The chip is then aligned and placed onto the substrate so that the bumps, which contain tacky flux, make contact with the appropriate metal bond sites. The tacky flux is formulated to contain a higher solids content which aids in the adhesion of the chip to the substrate prior to reflow. The tacky flux acts as a temporary glue to hold the chip in proper alignment during placement of the assembly into the reflow oven. The tacky flux contains less solvent which prevents the phenomenon of blowing the chips off the board during reflow commonly seen using liquid fluxes. Since only a small amount of flux is applied to the bumps, minimal residue remains on the board after soldering.
The tacky fluxes commonly used are the solderpaste flux vehicles used in no-clean surface mount processes. Although the formulations of no-clean solderpaste flux vehicles vary, a typical composition contains 50% rosin, 40% solvent, 5-8% thickeners, and 2-5% flux activators such as organic acids and amines. The rosin, or a synthetic resin with similar characteristics, does not boil-off during the reflow profile and is necessary to act as a carrier for flux agents at peak soldering temperatures. The residue which remains after soldering is typically rosin or a similar resin with any remaining ingredients such as decomposed organic acids, amines, thickeners, or other organic constituents of the solderpaste. When these solderpaste flux vehicles are used to solder flip-chip devices using the described drum flux method they provide desirable properties such as rolling on the drum, forming thin films and leaving minimal residue.
The flip-chip assembly is formed by soldering the solder bumps of the integrated circuit to the appropriate metal bond sites of the organic substrate. The resulting flip-chip assembly has a gap between the integrated circuit and substrate. This gap is generally filled with an underfill encapsulant. The liquid underfill encapsulant is dispensed around the sides of the soldered flip-chip and allowed to flow under the assembly by capillary action. The purpose of the encapsulant is to relieve the thermomechanical stresses on the solder interconnections that are caused by the difference in thermal expansion coefficients between the silicon IC (CTE=2.5 ppm/° C.) and the organic substrate (CTE=15-20 ppm/° C.). Typical underfill encapsulants used in flip-chip assemblies are composed of epoxy resins, curing agents and inorganic fillers to yield a cross-linked thermosetting polymer when cured. The properties of the cured polymer, such as the CTE and elastic modulus, help relieve the thermomechanical stress on the solder joints during thermal cycling testing. Thermal cycling tests involve repeated exposure of the flip-chip assemblies to cycles of cold and hot environments. This repeated cycling induces thermal fatigue on the solder joints as the chip and organic substrate expand at different rates. A typical thermal cycle test involves repeated exposure of the flip-chip assembly to two different liquids at −55° C. and +125° C. with 10 minute dwell time at each temperature. Thus, the overall purpose of the underfill encapsulant is to enhance flip-chip assembly reliability by relieving the thermomechanical stress on the solder joints. Flip-chip assemblies on inorganic substrates, such as ceramic, do not generally use an underfill encapsulant as the CTE of ceramic closely matches that of the silicon IC.
Several process and material property characteristics dictate the material selection of the underfill encapsulant. First, the epoxy underfill encapsulant must flow quickly under the chip to achieve fast production cycle times. The viscosity, surface tension and particle size distributions can be optimized to achieve efficient flow under the chip during the encapsulation step. To further reduce the underfill time the substrate may be heated in order to reduce the viscosity of the uncured epoxy material. This heating significantly enha
Ladas & Parry
Martin-Wallace Valencia
Nguyen Binh-An D.
Northrop Grumman Corporation
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