Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1999-02-16
2002-08-20
Gallagher, John J. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C148S402000, C148S908000, C029S281500, C156S311000, C156S312000, C156S325000, C156S381000, C269S903000
Reexamination Certificate
active
06436223
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a process and apparatus for assembly of semiconductor modules and, more specifically, to a process and related apparatus for clamping a cover to a substrate of a semiconductor module during a bonding process.
BACKGROUND OF THE INVENTION
Semiconductor modules, including single chip modules (SCM) and multi-chip modules (MCM), are used in a number of applications. Such modules generally comprise a substrate, a chip mounted on the substrate, and a lid or cover over the chip or chips. The cover is usually attached to the substrate using an adhesive that is heat-cured or a solder that is reflowed.
The lid or cover may have multiple purposes. It may provide mechanical protection of the device from handling and assembly tooling. It may also enhance heat transfer, especially for flip chip packages, where thermal passes are typically used to thermally connect the back side of the chip or chips to the inner or lower surface of the lid or cover.
When the lid or cover is sealed to the substrate, it also provides environmental protection of the devices from chemicals and moisture. When thermal pastes are used to cool flip chips, the seal typically prevents premature drying of the paste. Although some sealed packages must be hermetic, most only need to pass a bubble leak test. Industry competition demands low cost, high volume, and high-yield assembly of such modules.
Stamped fixtures, typically of stainless steel, such as “Auer Boats” manufactured by AUER Precision Company, Inc. of Mesa, Ariz., are prevalent in the industry as fixtures used for such assembly. Referring now to
FIG. 1
, there is shown the configuration of a typical stamped stainless steel fixture
90
of the prior art for assembly of semiconductor package or module
95
. A typical module
95
consists of a substrate
100
and cover
102
, the substrate
100
having mounted upon it an integrated circuit chip
104
and having pins
106
extending from the bottom of the substrate
100
.
To assemble module
95
, substrate
100
with one or more attached chips
104
is set in a baseplate
110
aligned by alignment features or guides
111
. An alignment plate
112
is aligned to baseplate
110
using alignment pins
114
attached to the alignment plate
112
, each alignment pin
114
comprising a spacer portion
116
and a pin portion
118
adapted to fit in hole
119
in baseplate
110
. Substrate
100
and chip
104
are typically prepared with chip underfill (not shown) applied around and wicking under chip
104
, thermally conductive paste (not shown) applied on top of chip
104
, and seal adhesive (not shown) placed on the surface of substrate
100
where cover
102
will contact the substrate
100
. Solder may also be used in place of seal adhesive.
The underfill protects the interface between the chip
104
and substrate
100
and prevents oxidation of the solder balls
103
used to attach the chip
104
to the substrate
100
. The thermally conductive paste creates a conductive pathway from the top of the chip
104
to the cover
102
, so that heat may be dissipated away from the chip
104
through the cover
102
. Finally, the seal adhesive or the solder bonds around the perimeter of the cover
102
, sealing the area inside the cover
102
to protect it from oxidation and to prevent paste drying. Cover
102
is then placed on top of substrate
100
so prepared.
Pressure is then applied to press substrate
100
against cover
102
, using a clip
120
. Clip
120
consists of a bridge
122
having tabs
124
punched through the bridge
122
, and prongs
126
attached at both ends of bridge
122
. Each prong
126
has an upper stop tab
128
, a lower stop tab
130
, and an angled end
132
. Tabs
124
are spaced to hold the ends of a leaf spring
134
between them. The compression force, usually between 2 to 10 pounds, imparted by the spring
134
“squishes” the paste layer on top of the chip
104
to conform it to the space between the chip
104
and the cover
102
, has assuring a good conductive connection and cover seal. The force of the spring
134
also seats the cover
102
on the substrate
100
, thinning the adhesive, before the cure step.
The clip
120
is inserted manually by squeezing the prongs
126
slightly toward one another and inserting them through alignment plate holes
136
and baseplate holes
138
, thus compressing spring
134
. Once the lower stop tabs
130
have completely penetrated baseplate holes
138
, the prongs
126
are allowed to spring back away from one another, and the lower stop tabs
130
hold the prongs
126
into place to prevent the force of compressed spring
134
from retracting the clip
120
. Upper stop tabs
128
prevent the prongs
126
from being inserted too far into baseplate holes
138
.
A semiconductor module
95
so assembled is then put in an oven or furnace to heat cure the seal adhesive or to reflow the solder to create a strong bond and seal between cover
102
and substrate
100
. A typical stamped stainless steel fixture
90
might accommodate anywhere from one to ten such modules
95
, and typically five modules
95
on a single baseplate
110
with a single corresponding alignment plate
112
. Other module-assembly fixtures have been developed, however, as detailed further in the description of the invention.
In any such assembly fixture, the force of the spring that compressively holds the cover against the substrate during the adhesive curing or solder reflow step is an important factor in producing an acceptable quality seal between the cover and substrate for modules produced In that fixture. Generally, the higher the spring force, within the force tolerances of the module and fixture components, the better the yield of acceptable quality modules.
Despite the yield advantage of using springs having a higher resistive force to deflection, such springs are more difficult for process operators to use. Special tooling may be required to open and close fixtures using multiple, high-force springs. In addition, certain module designs, such as modules having column grid array (CGA) input/output (I/O) connections, may be easily damaged by using springs having higher resistive forces. Thus, a need exists for fixtures incorporating springs that provide easy manipulation by operators when loading a fixture, but enable high forces during bonding for Improved product yield.
SUMMARY OF THE INVENTION
To meet this and other needs, and in view of its purposes, the present invention provides a fixture for assembly of a semiconductor module comprising a substrate and a cover on the substrate. The fixture comprises a baseplate having alignment features, adapted to accept the substrate, and a spring-loading device. The spring-loaded device is mounted over the baseplate and has a shape memory alloy spring engaging the cover.
The shape memory alloy spring may have a lesser force below a transition temperature range, and a higher force above the transition temperature range. The transition temperature range may be above room temperature and below the bonding temperature of a bonding agent, such as solder or an adhesive, that is used to attach the cover to the substrate.
The present invention further comprises a process for assembling a semiconductor module having a substrate and a cover attached with a bonding agent, the process comprising the steps of:
a) loading the semiconductor module into an assembly fixture and aligning a shape memory alloy spring over the module at room temperature;
b) placing the fixture and module into a heating chamber;
c) heating for a designated period of time the fixture and module in the heating chamber at a temperature sufficient to bond the bonding agent and that is above a transition temperature of the shape memory alloy spring so that the spring exerts an elevated force on the module; and
d) cooling the fixture and module to a lower temperature below the transition temperature so that the spring exerts a lesser force on the module, and disengaging the spring at the lower temperature.
It is to be underst
Abreu Enrique C.
Edwards David L.
Hering Ronald L.
Olson David C.
Blecker, Esq. Ira D.
Gallagher John J.
RatnerPrestia
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