Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Encapsulating
Reexamination Certificate
2001-09-07
2002-06-04
Bowers, Charles (Department: 2812)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Encapsulating
C438S108000, C438S126000
Reexamination Certificate
active
06399426
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to semiconductor chips electrically and mechanically connected to a substrate, particularly to flip-chip configurations.
BACKGROUND OF THE INVENTION
Flip-chip technology is well known in the art.
FIG. 1
illustrates a semi-conductor chip having solder bumps formed on the active side of the semi-conductor chip
100
that is inverted and bonded to a substrate
101
through the solder joints
102
by reflowing solder to wet metallized pads
106
. Structural solder joints
102
are formed from solder bumps situated between the semi-conductor chip and the substrate to form the mechanical and electrical connections between the chip and substrate. A narrow gap
103
is left between the semi-conductor chip and the substrate.
One obstacle to flip-chip technology when applied to polymer printed circuit substrates (i.e., circuit boards) is the unacceptably poor reliability of the solder joints due to the mismatch of the coefficients of thermal expansion of the (i) chip, which typically has a coefficient of thermal expansion of about 3 ppm/° C., (ii) the polymer substrate, e.g. epoxy-glass, which has a coefficient of thermal expansion of about 16 to 26 ppm/° C., and (iii) the solder joint which has a coefficient of thermal expansion of about 25 ppm/° C. As shown in
FIGS. 2 and 3
, as the flip chip
100
and printed circuit substrate
101
undergo thermal excursions, the substrate expands and contracts at a greater rate than does the chip. (This occurs, for example, in electronic components that are switched on and off.) Because the chip and substrate are stiffer than the solder joints
104
, the solder joints are flexed and distorted since they are constrained only by their attachment at both ends by the substrate and chip. The air gap
103
between the chip and the substrate allows these two parts to expand and contract at relatively different rates thereby distorting the solder joints. As the chip and substrate are thermally cycled through normal use, this flexing and bending weakens the solder joints which causes them to quickly fail.
In the past, the problem of solder joint fatigue life in flip-chip/substrate interconnects was addressed by several methods. A typical approach, which is described in U.S. Pat. No. 5,801,449, involves positioning an interposer made of flexible circuitry between the chip and the solder joints. The flex circuit undergoes expansion and contraction without distorting the solder joints despite the presence of the air gap around the solder joints.
As illustrated in
FIG. 4
, another approach to fatigue life involves underfilling the air gap between the chip and the substrate completely with a solid underfill encapsulant material
104
that consists of a composite of polymer and an inorganic filler and that has a thermal expansion of 20-30 ppm/° C. and an elastic modulus of 2-20 GPa. The underfill composite material is typically dispensed around two adjacent sides of the semiconductor chip after the chip
100
has been soldered to the substrate
101
. The underfill composite material
104
slowly flows by capillary action to fill the gap between the chip and the substrate. The underfill material is then hardened by baking for an extended period. Underfilling the chip with a subsequently cured encapsulant has been shown to reduce solder joint fatigue failure caused by thermal expansion mismatch between the chip and the substrate. For the underfill encapsulant to be effective, it is important that it adheres well to the chip
100
and the substrate
101
. Unlike the previous interposer methods, there cannot be an air gap or separation between the underfill
104
and the chip
100
or the substrate
101
. The cured encapsulant reduces the fatigue cycling of the solder joints by virtue of the relative stiffness, or high modulus, of the underfill material in conjunction with the strong solid contact made between the underfill material, the semiconductor chip, the solder joints, and the underlying printed circuit.
As illustrated further in
FIGS. 5 and 6
, since the solid underfill composite
104
fills the entire gap between the chip
100
and the substrate
101
, and since it has a thermal expansion coefficient that is close to that of the solder
102
, the substrate and chip no longer expand and contract independently of each other. Instead the relatively larger expansion and contraction of the substrate relative to the chip is constrained by the underfill
104
which is rigidly adhered to both; this also causes the entire assembly to bulge upwards as the temperature is decreased or downwards as the temperature is increased. This bulging effect essentially keeps the relatively fragile solder joints encased solidly in the underfill, and prevents them from appreciably distorting. The result is a large reduction in solder joint fatigue. The hardened, gap-free encapsulant transforms the expansion and contraction forces of the substrate that are induced by temperature changes, into bulging of the entire assembly, which virtually eliminates distortion of the solder joints. The bulging reduces the fatigue of the solder joints and virtually eliminates solder fatigue failure. As a result, the underfilled flip chip assembly solder joint lifetime is greatly increased relative to that of an air gap flip chip solder joint.
The underfilling process, however, makes the assembly of encapsulated flip-chip printed wire boards a time consuming, labor intensive and expensive process with a number of uncertainties. The process involves first applying a soldering flux, generally a no-clean, low residue flux, to the solder bumps on the chip. Then the chip is placed on the substrate. The assembly is subsequently subjected to a solder reflowing thermal cycle whereby the solder melts and joins the chip to the substrate under the action of the. soldering flux. The surface tension of the solder aids to self-align the chip to the substrate terminals. After reflow, due to the close proximity of the chip to the substrate, removing any remaining flux residues from under the chip is such a difficult operation that it is generally not done. Yet these residues are known to reduce the reliability and integrity of the subsequent underfill encapsulant.
After soldering, underfill encapsulation of the chip generally follows. In the prior art, as described, for example, in U.S. Pat. No. 5,880,530, the polymers of choice for the underfill encapsulation have been epoxy resins. The thermal expansion coefficients and elastic moduli of the resins can be reduced by the addition of inorganic fillers, such as silica or alumina. To achieve optimum reliability, a coefficient of thermal expansion in the vicinity of 20-30 ppm/° C. and an elastic modulus of 2 to 20 Gpa are preferred. Since the preferred epoxies have coefficient of thermal expansion exceeding 80 ppm/° C. and elastic moduli of 0.01-2 GPa, the inorganic fillers selected generally have much lower coefficient of thermal expansions and much higher moduli so that, in the aggregate, the epoxy-inorganic mixture is within the desired range for these values. Typically, the filler to resin volume ratio is in the range of 50 to 65%, but this high filler concentration tends to make the resin mixture very viscous, which slows the rate at which it can flow into the gap between chip and substrate during underfilling. Consequently, the slow underfill process is expensive to perform in a large throughput manufacturing environment.
The underfilling encapsulation techniques of the prior art have at least five principal disadvantages:
1. The reflowing of the solder bump and subsequent underfilling and curing of the encapsulant is an inefficient multi-step process.
2. Underfilling a flip-chip assembly is time consuming because the viscous resin material must flow through the tiny gap between the chip and the substrate.
3. Air bubbles can be trapped in the underfill encapsulant during the underfilling process and these bubbles later become sites for solder joint failure.
4. The flux residues remaining in the gap redu
Burress Robert Vinson
Capote Miguel Albert
Lee Yong-Joon
Zhu Xiaoqi
Bowers Charles
Burns Doane , Swecker, Mathis LLP
Capote Miguel Albert
Zarneke David A.
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