Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Reexamination Certificate
2001-09-18
2003-10-28
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
C428S576000, C228S246000
Reexamination Certificate
active
06638638
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates semiconductor devices having solder balls as an external termination means and, more particularly, to a semiconductor device using a Ball Grid Array (BGA) having improved reliability solder joints and a method for manufacturing the same.
2. Description of the Related Art
A common mounting and electrical-connection mechanism used in integrated circuits (ICs) is a ball grid array (BGA), wherein small solder balls are placed and retained at each one of a multitude of connection pads of the IC for solder connection to a mounting substrate or planar surface having opposing metal connecting pads. When heat is applied, the solder balls liquefy and flow via a fluxing agent included in the solder compound over any exposed metal surfaces of the connection pads, thereby forming a reliable electrical connection with each mounting pad. After cooling, the hardened solder additionally provides a rigid mounting structure for mechanically retaining the IC to the substrate or planar surface.
Typically, the size of the mounting package for the IC can be reduced to the size of a semiconductor chip, thus creating a Chip Size Package or Chip Scale Package (CSP). In the CSP, unlike conventional periphery-leaded (i.e. wire-bonded) packages, an array of external terminals and the BGA solder balls are distributed over the surface of the IC to directly interconnect the package to a printed circuit board (PCB). After processing, the resulting solder structures are generally inelastic, and provide a solid mounting mechanism for the assembly.
Disadvantageously, since the material composition of the chip and the opposing epoxy-glass material of a conventional PCB can have widely mismatched Coefficient of Thermal Expansion (CTE), any thermal cycling effects, such as those normally associated with turn-on and turn-off of related circuitry, can produce differing expansion movement of the opposing planar surfaces. This movement produces lateral shearing stresses on the solder joint, which is absorbed by the solder balls, more specifically by the junction of the solder ball and the metal connecting pads. With repeated thermal cycling, metal fatigue at this junction can cause the solder structure to crack and fail, rendering an entire circuit board inoperable. In other words, when the chip heats up during use, both the chip and the board expand, and when the heat is removed, both the chip and the substrate shrink. The problem that arises is that the chip and the substrate expand and contract at different rates according the CTE, thereby stressing the interconnections or solder balls connecting them.
FIG. 1
illustrates a cross-sectional view of a conventional ball grid array (BGA) interface structure
10
having a multitude of solid solder balls
12
. Structure
10
consists of upper planar element
14
having a first metal conductor pad
16
rigidly connected via solder ball
12
to a lower planar element
18
having a second metal conductor pad
20
so as to provide both electrical and mechanical connection between electronic circuits on each of the planar elements. Conductors are separated from lateral neighboring conductors by isolation spaces
22
appropriately located on each planar element. Each of planar elements
14
and
18
can have a different CTE. However, an excessive disparity between the CTEs can produce thermal cycling failure in the form of cracking of the rigid solder joints as previously discussed.
Occasionally, conventional out-gassing of vaporized flux is not completed due to process and/or solder compound irregularities, such as insufficient wetting of the solder compound, and results in small voids being located at the conductor-solder interface. Such voids create an area of fatigue weakness in the resulting joining structure.
FIG. 2
illustrates a cross-sectional area of a conventional BGA structure, wherein small voids
24
due to insufficient conventional wetting are shown at the junction of conductor pad
16
and solder ball
12
. Such voids
24
are formed when each of the molten solder balls
12
wets on the conductor pad
16
, and the flux in the solder paste compound flows outward from the center of the conductor pad
16
. As the temperature of the solder compound rises further, the flux is vaporized and a major portion of the flux vapor is dissipated into the atmosphere. However, minor portion of this vapor remains trapped in solder ball
12
as it is cooled and forms the small voids
24
inside the solder ball
12
.
The configurations shown in
FIGS. 1 and 2
are susceptible to cracking at each joint connecting the rigid conductor pad
16
and the rigid solder ball
12
under application of opposing lateral forces on the planar elements
14
and
18
. Such a failure of both the electrical connection and the mechanical mounting mechanism has heretofore precluded the use of epoxy-glass as a reliable substrate material for BGA applications, in favor of a more expensive ceramic material which has a CTE closer to that of the chip. A detailed joint interface of solder balls used in a conventional BGA type semiconductor device mounting construction is disclosed, for example, in U.S. Pat. No. 6,122,177 and PAJ No. 1998-209591. Detailed manufacturing assembly technical reports from International Business Machines Corporation, entitled “Doubled-Sided 4 Mb SRAM Coupled Cap PBGA Card Assembly Guide,” and from Texas Instrument Corporation, entitled “MicroStar BGA Packaging Reference Guide,” would also be beneficial to the reader.
The prior art addresses the presence of these small voids in the solder balls and a resulting joint embrittlement as significant problems. A small void in this context is defined as a gaseous volumetric displacement within the interior of a solder ball due to thermal expansion (i.e. boiling) of low-temperature solder flux solvents, since such gas material will remain trapped within a cooled solder structure. Conventional solder processing typically incorporates a warm-up period to allow time for de-gassing of such solvents, thereby minimizing such voids to yield a recommended finished total gaseous volume of less than 0.1% of the total solder structure volume. Disadvantageously, such heating can prematurely dry the solder paste included in the solder ball, leading to degraded electrical connections.
Thus, conventional BGA structures are not sufficient to prevent solder cracking or the breakage of solder ball interconnection, especially when used with a chip and an epoxy-glass substrate. Therefore, what is needed is a newly designed CSP with improved interconnection reliability, especially between the chip and the PCB, and a method of manufacturing of the same.
SUMMARY OF THE INVENTION
In a preferred embodiment of the present invention, improved reliability of solder ball connections in a ball grid array (BGA) semiconductor device can be attained by forming a large cavity in each finished solder ball structure or eliminating small voids in the board land structure of the annular metal patterns. Such cavities can be controllably formed via a seeding catalyst in the form of an annular ring in contact with a solder ball having a volatile fluxing agent. During melting of the solder ball, the vaporized fluxing agent accumulates around the non-conductive hole in the annular ring and effectively “inflates” the solder ball, such that, when cooled, a hollow solder structure having flexible thin walls results that can absorb subsequent lateral movement of the opposing planar elements without degrading the solder joint. This is particularly useful in applications where thermal stresses generated during the thermal cycling can be absorbed or dissipated efficiently without breakage or degradation of the joints (physical connection) between the hollow conductive solder balls and the underlying structure.
Accordingly, a solder structure according to preferred embodiments of the present invention provides added resiliency to the overall solder bond by inducing the creation of a large gaseous cavity w
Kim Sang-Young
Mok Seung-Kon
Moon Ho-Jeong
Shin Dong-Kil
Lam Cathy
Lee & Sterba, P.C.
Samsung Electronics Co,. Ltd.
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