Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2001-05-17
2003-05-27
Trinh, Ba K. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C549S546000
Reexamination Certificate
active
06570029
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to thermally reworkable epoxy resin compositions, and more particularly to thermally reworkable carbamate or carbonate epoxide resin compositions which degrade at temperatures significantly lower than traditional cycloaliphatic epoxy resins.
BACKGROUND OF THE INVENTION
Modern electronics manufacturing relies upon two general techniques to attach electrical components, such as integrated circuit chips (IC's), resistors, capacitors and the like, to circuit boards. In the older, traditional reverse-mounting method, the components include wire leads which are extended through holes in the circuit board and soldered to connections on the back side of the circuit board. In recent years, the reverse-mounting method has been largely supplanted by the technique of surface-mounting, in which the components are soldered to the same side of the board to which they are mounted. Surface-mounting offers may advantages over the reverse-mounting method, including reduced assembly time, lower cost, and the ability to interconnect very small structures at a much higher density.
Integrated circuit (IC) chips usually have a large number of connecting leads in a very small area to support their high associated I/O requirements. Accordingly, surface-mounting techniques are well suited from the attachment of IC chips to circuit boards. One surface mounting technique which has grown in popularity in recent years is the technique known as “flip-chip” mounting. In the flip chip method, small solder bumps are positioned at locations on the surface of the circuit board and/or the underside of the chip wherein it is desired to form interconnections. The chip is mounted by placing it in contact with the circuit board and then heating it to cause the solder to reflow. Upon cooling, the solder hardens to attach the chip to the board and to create the appropriate electrical connections.
As initially practiced, the flip-chip technique oftentimes utilized relatively high cost materials, such as high lead solder and ceramic substrate. However, the desire to reduce costs has prompted the use of less expensive materials, such as in the flip-chip on board (FCOB) method, which typically utilizes eutectic solder and organic printed wiring board (PWB). While reducing material costs, the use of FCOB method has led to problems because of coefficient of thermal expansion mismatches between the IC chip and the organic substrate of the FCOB, particularly when large IC chips having a fine pitch and low profile solder joints are utilized. Due to the large coefficient of thermal explansion mismatch between silicon IC chips (2.5 ppm/° C.) and organic substrates, i.e., FR-4 PWB (18-24 ppm/° C.), temperature cycle excursions experienced by the FCOB can generate tremendous thermomechanical stress at the solder joints. Over time, these stresses can result in performance degradation of the interconnections which may degrade or incapacitate device performance.
One method developed to minimize the thermomechanical stresses on the solder joints has been to introduce an underfill material into the spaces or gaps remaining between an IC chip and substrate. The undefill is typically an adhesive, such as an epoxy resin, that serves to reinforce the physical and mechanical properties of the solder joints between the IC chip and the substrate. The underfill improves the fatigue life of the packaged system, and also serves to protect the chip and interconnections from corrosion by sealing the electrical interconnections of the IC chip from moisture. The use of an underfill can result in an improvement in fatigue life of ten to over one hundred fold, as compared to an un-encapsulated packaged system.
Cycloaliphatic epoxies, typically combined with organic acid anhydrides as a hardener, have commonly been used as underfills in flip-chip packaged systems. They offer the advantage of low viscosity prior to curing, and have acceptable adhesion properties after curing. Other epoxies such as bisphenol A or F type or naphthalene type have also been used in the underfill formulations. Silica powder has sometimes been utilized as a filler in underfill formulations in order to adjust the coefficient of thermal expansion of the underfill to match that of the solder. When the coefficients of thermal expansion of the solder and the underfill match there is much less movement and fatigue between the underside of the flip chip and the solder connections, further improving device lifetime.
By way of example, the material properties represented in Table 1 typically are exhibited by typical epoxy underfill compositions.
TABLE 1
Typical Underfill Properties
Solids Content
100%
Form
Single component, pre-mixed
Coefficient of Thermal Expansion (&agr;
1
)
22-27 ppm/° C.
Tg
>125° C.
Cure Temperature
<165° C.
Cure Time
<30 min.
Working Life (@ 25° C., visc. Double)
>16 hrs.
Viscosity (@ 25° C.)
<20 kcps
Filler Size
95% < 15 &mgr;m
Filler Content
<70 wt %
Alpha Particle Emission
<0.005 counts/cm
2
/hr.
Hardness (Shore D)
>85
Modulus
6-8 Gpa
Fracture Toughness
>1.3 Mpa-m
1/2
Volume Resistivity (@ 25° C.)
>10
13
ohm-cm
Dielectric Constant (@ 25° C.)
<4.0
Dissipation Factor (@ 25° C., 1 kHz)
<0.005
Extractable Ions (e.g. Cl, Na, K, Fe, etc.)
<20 ppm total
Moisture Absorption (8 hrs. boiling water)
<0.25%
While the use of underfills has presented a solution to the problem of the coefficient of thermal expansion mismatch between chip and circuit board, it has created new challenges for the electronics manufacturing process. The new manufacturing steps required to apply the underfill, and to bake the assembly to harden the underfill, substantially complicate and lengthen the manufacturing process. Accordingly, it would be desirable to simplify the underfill manufacturing process for flip chips.
One method of simplifying the manufacturing process has been to dispense the underfill before placing the flip chip into contact with the circuit board using a process known as “no-flow” underfill. In the no-flow underfill process, the underfill is applied directly to the underside of the chip and/or circuit board before alignment of the chip on the board. Thus, when using a no-flow process it is no longer necessary to use a low-viscosity underfill material that can flow into the thin space between the chip and the circuit board. This allows the use of higher viscosity underfill materials that are easier to handle and apply than the low viscosity underfills used in more traditional flow based flip-chip manufacturing. The manufacturing process is further simplified because the heating steps for soldering and curing the underfill can be combined, eliminating several manufacturing steps.
The no-flow underfill method requires that the underfill material be adapted to allow solder interconnects to form. Generally, a fluxing agent must be applied to the solder bumps and/or the circuit pads on the circuit board to aid in interconnect formation by removing oxidation from the circuit pads and solder bumps. Accordingly, fluxing agents have been included in some prior no-flow underfill compositions to facilitate solder joint formation.
An additional disadvantage to traditional flip chip methods has been that the use of an adhesive underfill can make it difficult, if not impossible, to disassemble the components when a defect is discovered after assembly of an electrical component. Because the solder assembly and underfill steps occur simultaneously during the heating process, it is difficult to test the electronic assembly until the assembly is complete. Thus, if a defect is discovered, the underfill has already hardened, making removal and disassembly impractical. This results in increased production costs due to the waste of otherwise usable components. An effective way to address this problem is to make the flip-chip devices reworkable under certain conditions.
One method of making a reworkable flip-chip device has been to incorporate a non-stick release coatin
Li Haiying
Wang Lejun
Wong Ching-Ping
Deveau Todd
Georgia Tech Research Corp.
Schneider Ryan A.
Trinh Ba K.
Troutman Sanders LLP
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