Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Assembly of plural semiconductive substrates each possessing...
Reexamination Certificate
1999-04-09
2001-05-08
Picardat, Kevin M. (Department: 2823)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Assembly of plural semiconductive substrates each possessing...
C438S118000, C438S612000
Reexamination Certificate
active
06228679
ABSTRACT:
BACKGROUND OF THE INVENTION
Without limiting the scope of the invention, its background is described in connection with the packaging of integrated circuits, as an example.
Bumped silicon chips, also known as flip-chips, use a solder bump or ball to solder the electrical connections that interconnect integrated circuits. Flip-chip arrangements avoid the problems of breakage and lack of planarity encountered with conventional integrated circuit packages that are wire-bonded. Solder bumps or balls allow direct coupling between the pads on the silicon chip and matching contacts on the substrate. The flip-chip is aligned to the substrate and all connections are made simultaneously by reflowing the solder.
The use of solder bumps on the underside of silicon chips has led to the need to underfill the gap between the silicon chip and the substrate following reflow of the solder bumps. The underfill increases the mechanical integrity and reliability of integrated circuit packages, but requires a time consuming series of passes in which underfill polymer in liquid form is added to the periphery of the silicon chip. With each addition of underfill polymer, the underfill is built-up over the solder bumps in the gap by capillary action. The entire sequence can take up to 3 minutes per integrated circuit package.
Typically, an underfill polymer is dispensed onto each of the four adjacent sides of bonded flip-chip integrated circuits (ICs). The liquid underfill polymer is drawn into the gap by capillary action. The duration of each flow cycle can last as long as twenty minutes depending on the number of passes and the size of the gap. Furthermore, the sides on which the underfill polymer out-flow are substantially less homogeneous than the in-flow side. A non-symmetric underfill geometry causes stress gradients leading to premature failures of the integrated circuit package.
The inability to provide a uniform underfill has frustrated attempts to provide flip-chip mounting techniques that are reliable in a mass production environment. The types of flip-chip automation that would benefit from a reliable means of dispersing underfill polymer include: in-line automation, repeatable automatic process control, high throughput productions of flip-chips on substrates such as printed circuit boards (PCB), flip-chip multi-chip modules, or carriers to form flip-chip chip carriers, and high volume production of flip-chip chip carriers on PC boards and ceramic substrates.
Yet another solution to the problem of improving the speed of automation of underfilling the gap between the silicon chip and the substrate in flip-chip configuration has been the use of alternating cycles of vacuum and pressure. U.S. Pat. No. 5,203,076 issued to Banjeri, et al. discloses such a method. The Banjeri patent, however, fails to address the need to increase the efficiency in the automation process, as liquid underfill polymer must still be added in repeated cycles.
U.S. Pat. No. 5,073,816 (INMOS Limited) discloses an apparatus and method for packaging semiconductor devices aimed at reducing package size and providing improved heat dissipation over the packages in the prior art. While the specification describes an apparatus and method for flip-chip packaging, it fails to address the need to automate the underfill process.
A need has arisen for a simple, effective apparatus and method for providing a uniform liquid underfill polymer in a manner that avoids the difficulties of prior infiltration methods. A need has also arisen for increasing the speed and automation of the underfill process.
SUMMARY OF THE INVENTION
An object of the present invention is to a provide a simple, effective apparatus and method for dispensing a liquid underfill polymer under an integrated circuit. The present invention also increases the speed and automation of the underfill process. More particularly, the present invention comprises an integrated circuit package having a substrate and a silicon chip mounted on the substrate. The space between the substrate surface and the silicon chip defines a gap which is underfilled with a liquid underfill polymer. A liquid underfill dam is disposed on the surface of the substrate. The underfill dam serves to create a reservoir for the liquid underfill polymer.
The liquid underfill polymer can be, for example, an epoxy or a plastic polymer. In another embodiment of the present invention, the underfill dam is made of, for example, an epoxy or a plastic that is molded or extruded onto the substrate surface.
The present invention, disclosed herein, also comprises a method of attaching ah integrated circuit package to a substrate comprising the steps of, disposing an underfill dam on a substrate and electrically interconnecting a silicon chip on the substrate. The distance between the substrate and the silicon chip define a gap, which is filled by placing a bead of underfill material on the substrate about the periphery of the silicon chip and within the underfill dam.
In one embodiment of the present invention, the method of the present invention may further comprise the step of applying at least a partial vacuum to the integrated circuit package. Applying a vacuum removes air entrapped in the gap between the silicon chip and the substrate by bubbling the entrapped air through the underfill material. In yet another embodiment of the present invention, the step of providing a bead of underfill material comprises providing a sufficient amount of underfill material to completely fill the gap in one step. Once the gap has been filled with the liquid underfill material, the integrated circuit package having an underfill dam may be stored, handled or further processed by curing the liquid underfill polymer into a solid underfill.
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Collins D. M.
Galanthay Theodore E.
Jorgenson Lisa K.
Picardat Kevin M.
STMicroelectronics Inc.
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