Flip chip with integrated flux and underfill

Abstract

A flip chip having solder bumps and an integrated flux and underfill, as well as methods for making such a device, is described. The resulting device is well suited for a simple one-step application to a printed circuit board, thereby simplifying flip chip manufacturing processes which heretofore have required separate fluxing and underfilling steps.

Description

FIELD OF THE INVENTION[0001] The present invention relates to a novel flip chip design. More particularly, the present invention relates to a flip chip which incorporates solder bumps, flux and an underfill material, wherein the flux and underfill are provided by a single material capable of providing both flux and underfill properties.BACKGROUND OF THE INVENTION[0002] In the electronics industry, electrical components such as resisters, capacitors, inductors, transistors, integrated circuits, chip carriers and the like are typically mounted on circuit boards in one of two ways. In the first way, the components are mounted on one side of the board and leads from the components extend through holes in the board and are soldered on the opposite side of the board. In the second way, the components are soldered to the same side of the board upon which they are mounted. These latter devices are said to be "surface-mounted."[0003] Surface mounting of electronic components is a desirable t...

AI Technical Summary

Benefits of technology

0015] Lastly, the invention relates to a process for affixing a flip chip to a circuit board. The method involves providing a printed circuit board having a plurality of solderable contact sites on a surface, providing an integrated circuit chip of the type described above (i.e., a chip having solder bumps and a flux-integrated underfill material present on its surface), and positioning the integrated circuit chip relative to the printed circuit b

Problems solved by technology

Tolerances in devices using flip chip technology are critical, as the spacing between individual devices as well as the spacing between the chip and the board is typically very small.

One problem associated with flip chip technology is that the chips, the solder and the material forming the circuit board often have significantly different coefficients of thermal expansion.

As a result, differing expansions as the assembly heats during use can cause severe stresses, i.e., thermomechanical fatigue, at the chip connections and can lead to failures which degrade device performance or incapacitate the device entirely.

Such filled epoxy thermosets therefore reduce the likelihood of device failure resulting from thermomechanical fatigue during operation of the device.

While underfill has solved the thermal mismatch problem for flip chips on printed circuit boards, it has created significant difficulties in the manufacturing process.

Flux is used since the pads on printed circuit boards often oxidize, and since solder bumps on flip chips are always oxidized.

Unfortunately, the properties required for a good flux and those required for a good underfill are not totally compatible.

As such, a compromise of properties results.

Additionally, flux-containing underfills still require the use of special equipment including robot dispensing machines.

Also, since solder assembly and underfill application are combined into a single step, the flip chip cannot be tested until the assembly is complete.

Thus, if the chip does not operate satisfactorily, it cannot be removed because the underfill will have hardened, thereby preventing reworking.

Finally, certain problems have been found to arise when applying flux / underfill materials to bumped surfaces of flip chips.

The problems result because the rough surface geometry of the bumped surface is not readily amenable to the application of fluids, particularly those having high viscosity.

Thus, providing the flux / underfill directly onto a bumped surface raises at least the possibility of discontinuities and air bubbles forming during the flux / underfill application process.

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