Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Making plural separate devices
Reexamination Certificate
2000-07-19
2001-08-14
Graybill, David E. (Department: 2814)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Making plural separate devices
C438S122000, C438S117000
Reexamination Certificate
active
06274407
ABSTRACT:
TECHNICAL FIELD
The invention relates to methods and articles for attaching a relatively high-operating-temperature electronic component, such as a semiconductor die, to a substrate, such as a package substrate, heat spreader or electronic circuit board, to thereby mechanically secure the component to the substrate while further providing a thermally conductive path between the component and the substrate.
BACKGROUND OF THE INVENTION
In accordance with a known method for attaching an electronic component to a package substrate, a high-operating-temperature silicon semiconductor die is soldered to a copper heat spreader using a solder material whose melting or phase-transition temperature exceeds the nominal operating temperature of the resulting package. Since the coefficient of thermal expansion mismatch between the silicon die and the copper heat spreader is high, thermal stresses are induced in the solder during operation of the resulting package. Over time, thermal cycling is likely to generate cracks or voids in the solder, thereby reducing the area available for electrical and/or thermal transfer between the die and the heat spreader. The reduced heat transfer area, in turn, produces an increased junction temperature and may ultimately lead to a premature failure of the die. In some cases, the die can get separated from the heat spreader due to propagation of cracks in the solder.
In response, the prior art teaches the use of custom heat spreaders in an attempt to more closely match the thermal expansion coefficient of each heat spreader with that of the silicon die. For example, a laminated heat spreader is taught which includes a layer of material featuring a relatively low coefficient of thermal expansion, such as Invar, sandwiched between two layers of relatively higher thermal coefficient material, such as copper. However, such special copper-Invar-copper heat spreaders are costly to produce and, further, are less thermally conductive than their cheaper, monolithic copper counterparts. The die must, therefore, be operated at a lower power level to avoid undue thermal stresses and their attendant reliability problems.
Alternatively, the prior art teaches the use of relatively high temperature solder material to resist such crack propagation responsive to heat cycling. However, when such high-temperature solder materials are used, residual stresses are induced during solder reflow, which residual stresses contribute to premature failure of the assembly.
DISCLOSER OF THE INVENTION
Accordingly, it is an object of the invention to provide a method and article for attaching an electronic component to a substrate by which the resulting package is rendered less susceptible to failure due to thermal cycling.
Under the invention, a method for attaching an electronic component, such as a semiconductor die, to a substrate for use at a temperature not to exceed a maximum operating temperature includes the step of applying to the substrate a quantity of a first thermally conductive die attach material, such as a solder material in the form of either a paste or a preform of nominal thickness, which softens or fully liquifies at a temperature less than the maximum operating temperature. The method also includes the steps of placing the component atop the applied solder material such that the die overlies a portion of, and more preferably, entirely overlies the applied solder material; and applying to the substrate, proximate to the applied solder, a quantity of a second die attach material, such as a relatively compliant bonding material, in an amount sufficient to contain the applied solder material and otherwise bridge the gap between the component and the substrate upon subsequent liquefaction and collapse of the applied solder material along its thickness dimension.
The method further includes the step of heating the applied solder material to a second temperature above the maximum operating temperature, whereupon the solder material collapses along its thickness dimension. Preferably, the solder material fully liquefies at the second temperature, and the solder material, otherwise contained between the component and the substrate by the bonding material, advantageously “wets” the opposed component and substrate surfaces as it collapses. The bonding material thereafter mechanically couples the component to the substrate while otherwise advantageously serves to contain the solder material between the component and the substrate, notwithstanding subsequent softening/liquefaction of the solder material during component operation.
In accordance with a feature of the invention, in an exemplary method of practicing the invention, the solder material is applied to the substrate in the form of a paste or a preform, whereby the solder material defines a lateral periphery of nominal thickness. Further, when depositing the curable bonding material directly on the substrate proximate to at least a first portion of the periphery of the applied solder material, a lateral space or void may advantageously be defined between deposited bonding material and the periphery of the applied solder material, thereby defining, within the confines of the applied bonding material, a volume into which liquified solder material may flow during the heating step without fouling either the bonding material or the solder material.
In accordance with another feature of the invention, the applied solder material may define an interior void into which the solder material may flow during the heating step, thereby ensuring that the solder material will not foul either the opposed die/substrate surfaces or the bonding material immediately prior to the curing of the latter.
In accordance with another feature of the invention, where the solder material is applied such that the applied solder material has a smaller footprint than the underside of the component, the method may alternatively include the steps of placing the die atop the solder material so as to overlie at least a portion of the applied bonding material; and heating the solder paste and the bonding material to collapse the thickness of the solder paste through the softening or at least partial liquefaction thereof such that the bonding material engages both the die and the substrate, and to subsequently cure the bonding material. As a further benefit, the collapse of the liquified solder material during the heating step ensures that the solder material will engage and “wet” the surface of both the die and the substrate, whereby a superior thermal couple and, where desired, a superior electrical couple are achieved.
In accordance with yet another feature of the invention, the bonding material may alternatively be deposited on (and about the periphery of) the solder material preform before the preform is applied to the substrate. In this instance, the peripherally deposited bonding material has a thickness that is no greater than, and preferably less than, the nominal thickness of the periphery of the preform. The heating step will then preferably include collapsing the solder preform along its thickness dimension, whereby appropriate “wetting” of opposed die and substrate surfaces with the liquified solder material is ensured.
In accordance with the invention, a semiconductor die package includes a die, a substrate, and a first thermally conductive die attach material, such as a low-melting-temperature solder material, extending between the die and the substrate to thermally and, perhaps, electrically couple the component to the substrate while preferably further providing a partial mechanical couple between the die and the substrate when the temperature falls below the package's maximum operating temperature. A second die attach material, such as a curable bonding material, is applied to the substrate proximate to the solder material. In an exemplary embodiment, the bonding material completely encircles the solder material to thereby define, upon engagement of the bonding material with both the die and the substrate, a weir by to retain the solder ma
Baker Jay DeAvis
Jairazbhoy Vivek Amir
Paruchuri Mohan R.
Reddy Prathap Amerwai
Graybill David E.
Kajander John E.
Visteon Global Technologies Inc.
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