Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-08-31
2002-04-30
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S613000, C438S614000, C228S180100
Reexamination Certificate
active
06380060
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microelectronic connection components and more specifically relates to methods of attaching solder balls to conductive pads or terminals on a microelectronic element, such as a microelectronic connection component.
BACKGROUND OF THE INVENTION
Soldered connections are typically used throughout the electronics industry to connect electronic components. Where the components to be connected include dielectric elements, such as a printed circuit board or a flexible dielectric sheet having conductive metal traces, the traces may be provided with enlarged regions, commonly referred to as “lands” or “conductive pads.” A mass of solder may be deposited on each conductive pad by exposing the assembly to a liquid solder or, more typically, by applying solder preforms commonly referred to as “solder balls” onto the pads and heating the assembly to melt or reflow the solder balls. Solder balls are typically reflowed by raising the temperature of the solder balls above a predetermined temperature, generally referred to as the melting point of the solder balls. The melting point is defined as the temperature at which the solder balls transform from a first solid or frozen condition to a second molten or at least partially liquid condition. Once the solder balls transform to the second at least partially liquid condition, the solder balls remain in the least partially liquid condition as long as the temperature of the solder material is maintained at or above its melting point. The reflowed solder typically wets to the metal of the conductive pads to form a strong bond between the pads and the solder. After the solder balls have wet to the conductive pads, the temperature of the solder balls may be reduced to a level below the melting point, whereupon the solder balls transform from the second at least partially liquid condition to the first solid condition.
After application of the solder masses, the component typically has solder masses protruding from the surface. A semiconductor chip package having an array of solder masses on a surface in a grid-like pattern is sometimes referred to as a “ball-grid array” element. The use of ball-grid arrays in packages for microelectronic devices such as semiconductor chips is described for example, in the article “TBGA Package Technology,” IEEE Transactions on Components, Packaging and Manufacturing Technology, Part B, Vol. 17, No. 4, VP 564-568 by Andros and Hammer and in “Ball Grid Array Technology,” Lau, J. H. ed, pp. 460-464.
Commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266, the disclosures of which are hereby incorporated by reference herein, describe, in certain preferred embodiments, microelectronic packages having a set of conductive pads in the form of terminals which may be mounted on a dielectric layer such as a flexible sheet. The conductive pads may be connected to contacts on a semiconductor chip by flexible leads and may be supported above the surface of the chip by a compliant layer such as an elastomer located between the terminals and the chip, typicaly between the dielectric layer and the chip. Solder masses may be attached to the pads or terminals for connecting the assembly to a circuit board or other substrate having corresponding pads.
The microelectronic packages described above can be engaged with another component having a corresponding set of contact pads. After engaging the protruding solder masses with the pads of the other component, the solder masses may be heated again to melt all or part of each solder mask and bond the solder masses to the contact pads of the other component. The resulting solder columns electrically interconnect pads on both components with one another electronically and also form a mechanical connection between the components.
The presence of oxides on solder balls adversely affects the ability of the solder balls to wet to and ultimately bond with contact pads. As is well known to those skilled in the art, if an oxide layer is not adequately removed from a contact pad, then molten solder will not wet to the pad, thereby resulting in the formation of a poor bond between the molten solder and the contact pad. In order to avoid this problem, some bonding processes include the steps of removing oxides from both the solder balls and the contact pads, such as by scrubbing the surfaces of the solder balls and contact pads. Another solution for removing oxides is to use flux during the bonding process. In addition to facilitating placement of solder balls on conductive pads, flux aids in the removal of oxides that develop when the pads and solder balls are exposed to the environment.
FIG. 1A
shows a flexible dielectric sheet
10
having a first surface
12
and a second surface
14
remote therefrom. The dielectric sheet is preferably a flexible polymeric material. The connection component
10
includes one or more conductive pads
18
formed over the first surface
12
of the dielectric sheet
10
. The dielectric sheet includes metal lined vias
20
extending between the first surface
12
and the second surface
14
of the dielectric sheet
10
. The metal lined vias
20
have first ends
22
attached to conductive pads
18
and second ends
24
attached to conductive traces
26
. Referring to
FIG. 1A-1
, the conductive traces
26
A and
26
B extend along the second surface
14
of the dielectric sheet
10
. In the embodiment shown in
FIG. 1A-1
, the first conductive trace
26
A extends in a first direction that is substantially perpendicular to the direction of the second conductive trace
26
B.
Referring to
FIG. 1B
, it may be desired to bond and/or attach one or more solder balls
28
to the conductive pads
18
. Before the solder balls
28
are placed atop the conductive pads
18
, a flux material
30
is placed atop each conductive pad
18
. The flux material facilitates placement of the solder balls
28
atop the conductive pads by temporarily holding the solder balls in place until the solder balls can be reflowed and permanently connected to the conductive pads
18
. The flux material
30
also removes oxides that may have formed atop the exterior surface of the conductive pads
18
. The use of flux for removing oxide layers is of critical importance as the reflowed solder
28
will be unable to wet to the surface of the conductive pads unless the oxide layer is removed therefrom.
FIG. 1B-1
shows a top view of the connection component
10
of FIG.
1
B. The pads of flux material
30
are provided atop conductive pads
18
. Solder balls
28
are centered over the conductive pads
18
and atop the flux
30
. As shown in FIGS.
1
B and
1
B-
1
, the flux
18
and solder balls
28
cover the via openings
20
. As will be described in more detail below, because the via openings
20
are covered by flux
18
and the solder balls
28
, air and vapors from the flux may be trapped in the vias
20
when solder balls are reflowed. As a result, the heated air and flux vapors have no room to expand.
FIG. 1C
shows the connection component
10
during a solder reflow step. The solder balls
28
may comprise a tin/lead solder composition having a melting point of 320° C. When heat is applied to the solder balls
28
, the flux
30
at least partially fills up the vias and the expanding hot air and/or vapors from the flux generally has no opportunity to escape through the via opening. As a result, as the air and/or vapors from the flux within the via
20
expand, the air and/or the vapors flow into the center of the liquid solder
28
to form one or more voids
32
in a central portion of the solder balls
28
.
FIG. 1D
shows the solder balls
28
after they have been reflowed and permanently attached to conductive pads
18
. When viewing the attached solder balls from an exterior surface, the balls
28
appear to be solid. However, the centers of the balls
28
typically have one or more voids. As mentioned above, the voids may dramatically diminish the reliability of the connection component. Although the present invention is not limited to an
Lerner David Littenberg Krumholz & Mentlik LLP
Picardat Kevin M.
Tessera Inc.
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