Metal treatment – Compositions – Fluxing
Reexamination Certificate
2002-03-15
2004-07-27
Jenkins, Daniel (Department: 1742)
Metal treatment
Compositions
Fluxing
Reexamination Certificate
active
06767411
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention generally relates to solder compositions of the type used with electronic packaging, such as flip chip packaging. More particularly, this invention relates to lead-free solder alloys having reflow temperatures that are compatible with typical circuit board assembly processes and exhibit improved reliability over the commercially-available eutectic 63Sn-37Pb, Sn-3.5Ag and Sn-3.5Ag-1.0Cu solder alloys, particularly under high temperature and high current conditions.
(2) Description of the Related Art
Surface-mount (SM) semiconductor devices such as flip chips and ball grid arrays (BGA's) are attached to circuit boards with beadlike terminals formed on interconnect pads located on one surface of the device. The terminals are typically in the form of solder bumps near the edges of the chip, and serve to both secure the chip to the circuit board and electrically interconnect the flip chip circuitry to a conductor pattern on the circuit board. Due to the numerous functions typically performed by the microcircuitry of flip chips and BGA's, a relatively large number of solder bumps are required. The size of a typical flip chip is generally on the order of a few millimeters per side, resulting in the solder bumps being crowded along the edges of the chip.
Because of the narrow spacing required for the solder bumps and their conductors, soldering a flip chip or other SM component to a conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and entail precisely depositing a controlled quantity of solder on the pads of the chip using methods such as electrodeposition and printing. Once deposited, heating the solder above its melting or liquidus temperature (for eutectic and noneutectic alloys, respectively) serves to form the solder bumps on the pads. After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reheating, or reflowing, the solder so as to form solder connections that metallurgically adhere to the conductors. The temperature at which the solder bumps are reflowed to form solder connections is referred to as the reflow temperature, and is conventionally about 20° C. to about 50° C. above the melting or liquidus temperature of the particular solder alloy.
Flip chip interconnect pads are electrically interconnected with the circuitry on the flip chip through vias. Because aluminum metallization is typically used in the fabrication of integrated circuits, interconnect pads are typically aluminum or aluminum alloy, which are generally unsolderable and susceptible to corrosion if left exposed. Consequently, one or more additional metal layers are often deposited on the pads to promote wetting and metallurgical bonding with solder bump alloys. These additional metal layers, referred to as under bump metallurgy (UBM), may be, for example, sputtered nickel and copper, respectively, or an evaporated multilayer structure of chromium, a diffusion barrier layer of a chromium-copper alloy, and a solderable layer of copper. In each example, copper forms the outer layer of the UBM because it is readily solderable, i.e., can be wetted by and will metallurgically bond with solder alloys of the type used for solder bumps.
FIG. 1
represents a cross-section through a solder bump connection or joint
12
of a flip chip
10
attached to a circuit board
14
, such as an organic circuit board known in the industry as FR-4, though the chip
10
could be mounted to a flexible circuit, ceramic or silicon substrate, or another suitable material. The solder joint
12
is bonded to an aluminum runner
16
on the chip
10
and a copper trace
18
on the board
14
, thereby electrically and mechanically connecting the chip
10
to the board
14
. As shown, a portion of the runner
16
is exposed by an opening in a passivation layer
22
to define an interconnect pad on which a UBM
20
has been deposited. The solder joint
12
has a shape characteristic of a reflowed solder bump alloy, such as the eutectic 63Sn/37Pb solder alloy (melting point of 183° C.) and the eutectic Sn-3.5Ag solder alloy (melting point of 221.0° C.) widely used for flip chip assemblies. As would be expected, controlling the width of the solder joint
12
is necessary to prevent shorting with adjacent connections. Controlling the height of the solder joint
12
is also necessary to prevent the molten solder from drawing the flip chip
10
excessively close to the circuit board
14
during the reflow operation, when the molten solder bump tends to spread outward as a result of wetting the surfaces it contacts. The ability to control solder bump height and width is determined in part by the reflow characteristics of the solder alloy used, based on its melting point (for a eutectic alloy) or solidus and liquidus temperatures (for non-eutectic alloys), and the peak reflow temperatures required by the particular circuit board assembly.
There is a desire in the electronics industry to limit the use of lead-containing materials due to environmental concerns for the toxicity of lead, as well as reliability concerns due to the alpha particles emitted by lead-containing bump alloys. There are many commercially available Pb-free alloys, most notably Sn-5Ag, eutectic Sn-3.5Ag (melting point of 221° C.), and eutectic Sn-0.9Cu (melting point of 227° C.), and their derivatives, including Sn-4.0Ag-0.5Cu, Sn-3.9Ag-0.6Cu, Sn-3.8Ag-0.7Cu, Sn-4Ag-1Cu, Sn-4.7Ag-1.7Cu, Sn-2.5Ag-0.8Cu-0.5Sb, Sn-5Sb, Sn-8.5Sb, Sn-3.4Ag-1Cu-3.3Bi, Sn-3Ag-2Bi, Sn-3.4Ag-4.8Bi, Sn-9Zn, Sn-8.8In-7.6Zn, Sn-58Bi, Sn-3.5Ag-1.5In, Sn-3.2Ag-1Cu-10In, and Sn-2.8Ag-201n. To be compatible with widely-used FR-4 circuit board assembly processes, the maximum reflow temperature of a solder alloy must not be higher than about 270° C., and preferably not higher than 260° C., in order to avoid damage to the circuit board and its components through board warping, pop-corning, delamination, etc. On the other hand, reflow temperatures below about 240° C. may result in poorly formed solder joints as a result of poor wetting, cold solder joint, etc. An additional requirement for automotive applications is the ability to withstand 150° C. junction temperatures for an extended period of time (e.g., 2000 hours continuous operation at 150° C.). This requirement excludes all Pb-free alloys with solidus temperatures under about 180° C., such as Sn-52n, Bi-42Sn and Sn-2.8Ag-20In. In view of these and other demands for solder bump connections, eutectic Sn—Ag—Cu and near-eutectic Sn—Ag—Cu alloys have become widely used as a lead-free solder for reflow assembly processes.
The eutectic composition for Sn—Ag—Cu is not yet clear. Sn-4.7Ag-1.7Cu, Sn-4.0Ag-0.5Cu, Sn-3.9Ag-0.6Cu, Sn-3.8Ag-0.7Cu and Sn-3.5Ag-1.0Cu have each been treated by various researchers as the eutectic composition for SnAgCu. The Sn-4.7Ag-1.7Cu alloy, the only alloy in this group having a copper content above 1 weight percent, is disclosed in U.S. Pat. No. 5,527,628 to Anderson et al. Although described as being a eutectic composition for SnAgCu with a eutectic melting temperature of 217.0° C. and a melting range (“mushy” zone) of not more than 15° C., the differential scanning calorimetry (DSC) chart of
FIG. 1
from this patent shows this composition to actually have a very wide melting range, from 217.0° C. to about 240° C., and therefore a plastic range of about 23° C. Anderson et al. also teach that, while the relative amounts of tin, silver and copper can be varied (within a disclosed range of 3.5 to 7.7 weight percent silver, 1.0 to 4.0 weight percent copper, balance tin) to provide a melting range above the eutectic melting temperature (217.0° C.), such variations are said to result in higher melting temperatures. In any event, the wide plastic range of this a
Carter Bradley H.
Melcher Curtis W.
Yeh Shing
Chmielewski Stefan V.
Delphi Technologies Inc.
Jenkins Daniel
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