Hybrid molds for molten solder screening process

Details Hybrid molds for molten solder screening process Hybrid molds for molten solder screening process

2002-04-23

2004-12-21

Mackey, James P. (Department: 1722)

Static molds

Container-type molding device

Plural article forming mold, or molds with community feature

C249S134000

Reexamination Certificate

active

06832747

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to hybrid molds for use in a molten solder screening process for molding solder bumps and methods for preparing such molds and more particularly, relates to hybrid molds for molding a multiplicity of solder balls which are constructed by a crystalline silicon face plate provided with a multiplicity of cavities formed in a front surface and a rigid backing plate which has substantially the same coefficient of thermal expansion as crystalline silicon for bonding to a back surface of the face plate and methods for preparing such hybrid molds.
BACKGROUND OF THE INVENTION
In modem semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such devices. Conventionally, a flip-chip attachment method has been used in the packaging of IC chips. In the flip-chip attachment method, instead of attaching an IC die to a lead frame in a package, an array of solder balls is formed on the surface of the die. The formation of the solder balls is normally carried out by an evaporation method of lead and tin through a mask for producing the desired solder balls. More recently, the technique of electro-deposition has been used to produce solder balls in flip-chip packaging.
Other solder ball formation techniques that are capable of solder-bumping a variety of substrates have also been proposed. These techniques work fairly well in bumping semiconductor substrates that contain solder structures over a minimum size. One of the more popularly used techniques is a solder paste screening technique which can be used to cover the entire area of an 8 inch wafer. However, with the recent trend in the miniaturization of device dimensions and the reduction in bump-to-bump spacing (or pitch), the solder paste screening technique becomes impractical. For instance, one of the problems in applying solder paste screening technique to modem IC devices is the paste composition itself. A paste is generally composed of a flux and solder alloy particles. The consistency and uniformity of the solder paste composition become more difficult to control with a decreasing solder bump volume. A possible solution for this problem is the utilization of solder pastes that contain extremely small and uniform solder particles. However, this can only be done at a high cost penalty. Another problem in using the solder paste screening technique in modern high density devices is the reduced pitch between bumps. Since there is a large reduction in volume from a screened paste to the resulting solder bump, the screen holes must be significantly larger in diameter than the final bumps. The stringent dimensional control of the bumps makes the solder paste screening technique impractical for applications in high density devices.
A more recently developed injection molded solder (IMS) technique attempted to solve these problems by dispensing molten solder instead of solder paste. However, problems have been observed when the technique is implemented to wafer-sized substrates. U.S. Pat. No. 5,244,143, discloses the injection molded solder technique and is hereby incorporated by reference in its entirety. One of the advantages of the IMS technique is that there is very little volume change between the molten solder and the resulting solder bump. The IMS technique utilizes a two inch wide head that fills boro-silicate glass molds that are wide enough to cover most single chip modules. A narrow wiper provided behind the solder slot passes the filled holes once to remove excess solder. The IMS method for solder bonding is then carried out by applying a molten solder to a substrate in a transfer process. When smaller substrates, i.e., chip scale or single chip modules (SCM's) are encountered, the transfer step is readily accomplished since the solder-filled mold and substrate are relatively small in area and thus can be easily aligned and joined in a number of configurations. For instance, the process of split-optic alignment is frequently used in joining chips to substrates. The same process may also be used to join a chip-scale IMS mold to a substrate (chip) which will be bumped.
A more recently developed method that does not have the limitations of the solder paste screening technique of significant volume reductions between the initial paste and the final solder volume is the molten solder screening (MSS) method. In the MSS method, pure molten solder is dispensed. When the MSS solder-bumping method is used on large substrates such as 8 inch or 12 inch wafers, surface tension alone is insufficient to maintain intimate contact between a mold and a substrate. In order to facilitate the required abutting contact over large surface areas, a new method and apparatus for maintaining such are therefore necessary.
For instance, in a co-pending application of Ser. No. 09/070,021 commonly assigned to the Assignee of the present application and is hereby incorporated by reference in its entirety, a method for forming solder bumps by a MSS technique that does not have the drawbacks or shortcomings of the conventional solder bumping techniques has been proposed. In the method, a flexible die member is used in combination with a pressure means to enable the die member to intimately engage a mold surface and thus filling the mold cavities and forming the solder bumps. The flexible die head also serves the function of a wiper by using a trailing edge for removing excess molten solder from the surface of the mold.
The MSS process can be carried out by first filling a multiplicity of cavities in the surface of a mold with molten solder. This is accomplished by first providing a stream of molten solder and then passing a multiplicity of cavities in the mold surface in contact with the surface of the stream while adjusting a contact force such that the molten solder exerts a pressure against the surface of the mold to fill the cavities with solder and to remove excess solder from the surface of the mold. The stream of molten solder is supplied through a die head constructed of a flexible metal sheet that is capable of flexing at least 0.0015″ per inch of the die length. The solder has a composition between about 58% tin/42% lead and about 68% tin/32% lead. The multiplicity of cavities each has a depth-to-width aspect ratio of between about 1:1 and about 1:10. The mold body is made of a material that has a coefficient of thermal expansion substantially similar to that of silicon or the final solder receiving material. The contact between the multiplicity of cavities and the surface of the molten solder stream can be adjusted by a pressure means exerted on the flexible die.
Referring now to
FIG. 1A
, wherein a flexible die head
10
for forming solder bumps by a molten solder screening process is shown. The die head
10
has a die body
12
which is made of a thin, flexible metal sheet such as stainless steel or any other suitable material that is non-wetting to solder. The die body
12
has a gate opening
14
and a slot opening
16
. The gate opening
14
has a narrow width and is generally positioned at the center of the die body
12
. The gate opening
14
provides fluid communication between a front side
18
and a back side
22
of the die body
12
. The gate opening
14
further provides a passageway for receiving a molten solder
24
stored in a solder reservoir
26
. The molten solder
24
is kept under an inert gas atmosphere at a pressure of approximately 5 psi. A suitable inert gas used is nitrogen, i.e., as shown in
FIG. 1A
, supplied by a nitrogen source
28
. The functions of the inert gas in the solder reservoir are two fold. First, it provides an inert gas blanket over the solder
24
such that any oxidation of the molten solder
24
can be avoided. Secondly, the nitrogen blanket in the reservoir
26
provides a positive pressure such that molten solder
24
flows easily through the gate opening
14
into the slot opening
16
. In continuous operation, the

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