Method for coupling surface mounted capacitors to semiconductor packages

A low cost method for mechanically and electrically coupling a surface mounted capacitor to a semiconductor package. In one embodiment a metal coating is deposited over the electrodes of a capacitor. Concurrently, or at some other time, solder paste is applied to the electrical contact pads of a semiconductor package. The connection between the capacitor and package is made by positioning the metal coated regions of the capacitor over the electrical contact pads of the package and running the unit through a reflow furnace where the solder paste is wetted onto the metal coating of the capacitor and onto the electrical contact pads of the package.

FIELD OF THE INVENTION 
The present invention relates to the field of semiconductor packaging. More 
specifically, the invention relates to a method for establishing a low 
cost electrical and mechanical connection between a surface mounted 
capacitor and a semiconductor package. 
BACKGROUND OF THE INVENTION 
Integrated circuits are typically housed within a package that is mounted 
to a printed circuit board (PCB). The package is designed to protect the 
integrated circuit (IC) device from damage, to provide adequate heat 
dissipation during operation, and to provide electrical connection between 
the IC device and a PCB (e.g., a peripheral card, a motherboard and the 
like). In addition, the semiconductor package often serves as a platform 
for mounting other electronic or electrical devices necessary for the 
proper operation of the IC device. For example, in order to control 
ringing caused by noise and a host of other electrical problems, 
decoupling capacitors are often mounted onto the package substrate where 
they are electrically coupled to the IC device. The present invention is 
directed at establishing a low cost electrical/mechanical connection 
between a surface mounted capacitor, such as a Low Inductance Capacitor 
Array (LICA), and a semiconductor package. 
Among other features, a semiconductor package typically consist of a 
substrate having one or more sets of electrical contact pads positioned 
about one or more surfaces of the substrate for electrically coupling 
other electronic or electrical devices to the package. These electrical 
contact pads are commonly referred to as "lands." The lands typically 
comprise an electrically conductive material coated with a solder-wettable 
metallurgy. Using existing techniques, surface mounted capacitors are 
electrically and mechanically connected to the package by first forming a 
chromium layer over the electrodes of the capacitor. The chromium layer is 
generally referred to as a "ribbon." The ribbon is produced by forming a 
knife edge metal mask over the capacitor surface and then evaporating 
chromium onto the exposed portions of the capacitor surface. Once the 
chromium ribbon is formed under-bump-metallization (UBM) layers, such as 
chromium/copper/gold, are sequentially deposited around the electrodes of 
the capacitor through the holes of a metal mask using another evaporator. 
The UBM chromium layer acts as an adhesion promoter and a diffusion 
barrier metal layer which makes contact with the chromium ribbon of the 
capacitor. The UBM copper layer is deposited over the UBM chromium layer 
to promote the eventual wetting of solder onto the capacitor. In order to 
prevent the copper from oxidizing, a layer of gold is deposited over the 
copper layer. Finally, a Pb layer and a Sn layer is deposited over the UBM 
layers through the same mask using another evaporator. The capacitor is 
then placed in a hydrogen ambient furnace for reflow. During reflow the Pb 
and Sn layers mix to form solder bumps and are wetted onto the chromium 
ribbon of the capacitor. Eventually, the connection between the capacitor 
and semiconductor package is made by positioning the solder bumps of the 
capacitor over a corresponding set of lands on the package and running the 
unit through another reflow furnace. 
Although the evaporation process is well established, costs associated with 
the process are high. The process requires at least three evaporators and 
involves the use of at least two metal masks. Furthermore, the process 
demands the use of a hydrogen ambient furnace to facilitate the melting of 
the Pb and Sn layers during reflow. 
What is needed then is a low cost method for forming reliable connections 
between surface mounted capacitors and semiconductor packages. As will be 
seen, the present invention provides an improved method for establishing a 
low cost electrical and mechanical connection between a surface mounted 
capacitor and a semiconductor package. 
SUMMARY OF THE INVENTION 
An Improved method for electrically and mechanically coupling capacitors to 
semiconductor packages is disclosed. 
The improved method for connecting surface mounted capacitors is 
accomplished primarily through the following steps: First, chromium, 
nickel, and gold layers are sequentially deposited onto the surface of the 
capacitor using a sputter deposition process. The metal layers are applied 
in such a way that the electrodes of the capacitor are fully covered by 
the resultant Cr/Ni/Au film layer. Either concurrently or at some other 
time Pb/Sn solder paste is applied over the semiconductor package 
electrical contact pads using a screen mesh apparatus and squeegee. The 
connection between the capacitor and the package is then made by 
positioning the Cr/Ni/Au film region of the capacitor over the lands of 
the package and running the unit through a nitrogen ambient reflow 
furnace. Thus, the present invention provides a method for electrically 
and mechanically coupling a capacitor to a semiconductor package while 
eliminating the high costs associated with the evaporation method.

DETAILED DESCRIPTION 
A method for electrically and mechanically coupling an electrical device to 
a semiconductor package is described. In the following description, 
numerous specific details are set forth such as material types, 
dimensions, processing steps, etc., in order to provide a thorough 
understanding of the present invention. However, it will be apparent to 
one of skill in the art that the invention may be practiced without these 
specific details. In other instances, well known elements and processing 
techniques have not been shown in particular detail in order to avoid 
unnecessarily obscuring the present invention. 
FIG. 1 illustrates a capacitor 10 having electrodes 12 disposed about one 
of its surfaces. Capacitor 10 typically consist of a combination of metal 
and ceramic layers with all metal layers being electrically isolated by 
the ceramic layers. Electrodes 12 are provided at the ceramic surface 11 
of capacitor 10 for electrically coupling the capacitor to other 
electrical or electronic devices. 
As previously discussed, decoupling capacitors are often mounted onto the 
substrate of a semiconductor package where they are electrically coupled 
to an IC device. FIG. 2 illustrates a side view of a typical semiconductor 
package 24 having capacitors 10 mounted on package substrate 26. Package 
substrate 26 contains electrical interconnects (not shown) that 
electrically couple the IC device 30 to electrical contact pads located on 
the surface of the substrate. Package substrate 26 is made of any 
temperature resistant material, such as alumina oxide, high temperature 
PCB, etc. in order to prevent the substrate from melting during normal 
reflow conditions. The electrical contact pads are comprised of an 
electrically conductive material, such as, for example, gold plated 
nickel. 
As mentioned earlier, practitioners have used a process that depends 
predominately upon the use of evaporators to form the metallurgy necessary 
for electrically and mechanically coupling a surface mounted capacitor to 
a package. However, the costs associated with the evaporation process are 
high. To reduce manufacturing costs the present invention uses a multiple 
target, sputter deposition chamber to form a metal coating over the 
electrodes of the capacitor. Further cost savings are realized by 
eliminating the existing method of evaporating separate Pb and Sn layers 
onto the UBM layers of the capacitor. In lieu of using the evaporation 
process of prior art methods, the present invention utilizes a method 
wherein solder is screen printed directly onto the electrical contact pads 
of the package substrate. The coupling of the capacitor and the package 
substrate is made by positioning the metal coated regions of the capacitor 
over the electrical contact pads of the package substrate and running the 
unit through a reflow furnace. During reflow the solder is wetted onto the 
electrical contact pads of the package substrate and onto the adjoining 
metal layers of the capacitor. 
FIG. 3 shows a side view of capacitor 10 before metal layers are deposited 
over electrodes 12. As depicted in FIG. 4 and 5, metal layers 16, 17 and 
18 are formed over the surface 11 of capacitor 10 such that all the 
electrodes in a given row are electrically connected. In one embodiment of 
the invention metal layers 16, 17 and 18 comprise chromium, nickel and 
gold, respectively. The metal layers are formed by placing the capacitor 
within a multiple target, sputter deposition chamber wherein metal layers 
16, 17 and 18 are sequential deposited over surface 11 and electrode 12 
through the holes of a mask. In one embodiment chromium layer 16, nickel 
layer 17 and gold layer 18 have thicknesses of approximately 0.43, 0.5, 
0.05 microns, respectively. 
Either concurrent with the formation of metal layers 16, 17 and 18, or at 
some other time, solder paste is applied over electrical contact pads 28 
of package substrate 26. (See FIG. 6.) Pb/Sn solder paste is applied to 
contact pads 28 by first positioning a screen mesh apparatus 30 over 
package substrate 26 such that mesh openings 32 are aligned with 
electrical contact pads 28. When the screen mesh apparatus is in place, 
solder paste is dispensed onto the top surface 31 of apparatus 30 and then 
forced through mesh openings 32 onto contact pads 28 by the use of a 
squeegee or some other device. FIG. 7 shows a cross-sectional view of the 
substrate 26, after solder 36 has been applied to electrical contact pads 
28. 
The connection between capacitor 10 and package substrate 26 is 
accomplished by positioning the metal coated regions of capacitor 10 over 
electrical contact pads 28 of package substrate 26, as illustrated in FIG. 
8, and running the unit through a nitrogen ambient reflow furnace. The 
reflow furnace temperature is adjusted above the melting temperature of 
solder 36 such that during reflow solder 36 is wetted onto electrical 
contact pads 28 and the Cr/Ni/Au metal layers 19 of capacitor 10. 
FIG. 9 illustrates a flow chart of the coupling process in one embodiment 
of the present invention. 
Solder 36 may comprise any solder material composition whose properties are 
conducive to the manufacturing process just described. In one embodiment 
of the present invention solder 36 comprises a 97/3 Pb/Sn solder paste 
composition that contains a resin flux. In this embodiment the reflow 
furnace temperature is set at approximately 350 degrees Celsius. The 
diameter and thickness of solder 36 will vary depending upon the 
particular application. Generally, the diameter of solder 36 will be 
approximately equal to the diameter of electrical contact pad 28. The 
thickness of solder 36 will typically vary between 0.005 to 0.006 inches. 
Note also that a solder paste not containing a flux may be used, however, 
in this instance it may be necessary to coat electrical contact pads 28 
with a flux before applying solder 36 to the pads. It is further 
understood that any of a variety of methods may be used to apply solder 36 
to electrical contact pads 28. For example, in lieu of using the screen 
mesh apparatus described, an extrusion method of applying solder paste 36 
may be utilized. 
In the foregoing description an embodiment of the present invention is 
disclosed having metal layers 16, 17 and 18 comprising chromium, nickel 
and gold, respectively. It should be understood, however, that this 
combination of metal layers is not essential to the implementation of the 
present invention, nor is the invention limited to three metal layers. The 
implementation of the present invention requires only the use of an 
electrically conductive metal layer or metal stack that bonds to 
electrodes 12 and surface 11 of capacitor 10 and is wettable with solder 
36. As an example, metal layer 16 may comprise titanium or titanium 
tungsten and metal layer 17 may comprise copper. Note, also that in 
another embodiment metal layers 16, 17 and 18 are deposited over the 
surface and electrodes of capacitor 10 using an evaporation process rather 
than a sputter deposition process. 
It is appreciated that the methods and apparatus of the present invention 
may be used in other technologies to form electrical and/or mechanical 
connections between other types of electrical devices. It is further 
understood that the relative dimensions, geometric shapes, materials and 
process parameters set forth within the specification are exemplary of the 
disclosed embodiments only. Other embodiments may utilize different 
dimensions, shapes, materials, and process settings, etc., to achieve 
substantially the same results.