Method for forming solder bumps on bond pads

Solder paste (16) is dispensed onto a platen (12) that includes depressions (14) that are preferably conical or in an inverted pyramid shape. The solder paste (16) is formed of a plurality of particles composed of a solder alloy. Excess solder paste (16) is removed, and a predetermined solder paste volume fills the depressions (14). A substrate (18) is superposed onto platen (12) such that solder-wettable bond pads (20) on the substrate (18) register with the depressions (14). The platen (12) is heated to melt the solder alloy, and the solder alloy coalesces to form molten solder droplets (22). The solder droplets (22) are transferred onto the solder-wettable bond pads (20) to form solder bumps (24) bonded to the bond pads (20).

FIELD OF THE INVENTION 
This invention relates generally to a method for forming solder bumps on 
bond pads. More particularly, this invention relates to such a method 
wherein a solder paste is dispensed onto a platen and heated to coalesce 
the solder alloy in the solder paste into a molten solder droplet that is 
transferred to a solder-wettable bond pad. 
BACKGROUND OF THE INVENTION 
Solder bumps are formed on bond pads located on circuit boards and 
integrated circuit components, such as ball grid array and chip scale 
packages, to facilitate electrical connection. As the size of electronic 
packages has decreased, so has the size of the solder bumps used to 
connect them. One method of attaching solder balls to bond pads is 
engagement of individual solder balls, with an end effector or the like, 
and placement of the solder balls onto bond pads. However, due to the 
small size of the solder balls, typically between about 10 and 30 mils, 
problems arose from either having multiple solder balls on one bond pad, 
or missing a solder ball on a bond pad. This led to inadequate and 
unreliable solder bump interconnections. 
Another limitation on prior art solder bumping methods is that forming 
small solder bumps, such as those having a diameter of less than about 10 
mils, is difficult to achieve due to the handling problems associated with 
solder balls of this size. 
Therefore, a need exists for a method for forming solder bumps on bond pads 
on an integrated circuit component or substrate that is reliable and able 
to form small solder bumps. 
Further, a need exists for a method to attach a single solder bump onto 
each desired bond pad.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The present invention provides a method for forming a substrate having 
solder bumps bonded to solder-wettable bond pads formed on the substrate. 
For purposes of this invention, a substrate can be a rigid printed circuit 
board, a flexible printed circuit board, or an integrated circuit 
component, such as an integrated circuit die, a semiconductor wafer, or an 
integrated circuit package such as a ball grid array package. In 
accordance with the present invention, a solder paste is dispensed onto a 
platen that includes depressions defined by surfaces that are non-wettable 
by the solder paste. The solder paste, which includes a plurality of 
particles composed of a solder alloy, fills the depressions. The platen is 
heated to melt the solder alloy, whereupon the solder alloy coalesces to 
form molten solder droplets in the depressions. The molten solder droplets 
are transferred onto the solder-wettable bond pads on the substrate to 
form solder bumps bonded to the bond pads. 
A preferred embodiment of the present invention can be better understood 
with reference to FIGS. 1-3. In accordance with a preferred embodiment of 
this invention, FIG. 1 depicts a platen 12 that includes a face 26 and a 
plurality of depressions 14 that are sized and shaped to receive a 
predetermined solder paste volume. Platen 12 is formed of a 
solder-nonwettable composition, such as silicon or aluminum, such that 
solder does not bond to platen 12. In a preferred embodiment, depressions 
14 are conical. Depressions 14 can also be formed in an inverted, 
four-sided pyramid shape. The conical or inverted pyramid shape of 
depressions 14 facilitates proper formation of solder droplets upon 
heating of the solder paste filling depressions 14. 
A solder paste 16 is dispensed onto platen 12, preferably onto face 26. 
Solder paste 16 is formed of solder particles dispersed in a liquid 
vehicle. In a preferred embodiment, the liquid vehicle comprises a fluxing 
agent to facilitate formation of the solder bumps. A predetermined amount 
of solder paste is then distributed into depressions 14 with a doctor 
blade, squeegee, or the like. Excess solder paste not filling any of the 
depressions 14 is removed from platen 12. In this manner, depressions 14 
contain a predetermined solder paste volume effective to form a solder 
bump of a predetermined size. 
In a preferred embodiment, a spacer 28 is superposed onto face 26. A 
substrate 18 is superposed onto platen 12 by placing substrate 18 onto 
spacer 28. Substrate 18 is spaced apart from face 26 by a predetermined 
distance determined by the height of spacer 28. In an alternate 
embodiment, substrate 18 is superposed directly onto face 26. Substrate 18 
is preferably an integrated circuit component, such as a ball grid array 
package, that includes a plurality of solder-wettable bond pads 20, 
preferably formed of copper. Substrate 18 can also be an integrated 
circuit die, a printed circuit board, or a semiconductor wafer. Substrate 
18 is superposed onto platen 12 such that each bond pad 20 registers with 
a corresponding depression 14. In this manner, solder droplets formed from 
the solder paste in the depressions 14 will align with and contact bond 
pads 20. 
Platen 12 is then heated and the fluxing agent in solder paste 16 
facilitates coalescence of the solder alloy dispersed in solder paste 16. 
The solder particles in the solder paste coalesce to form solder droplets 
22, as shown in FIG. 2. In the preferred embodiment, solder droplets 22 
protrude from depressions 14 by a distance greater than or equal to the 
predetermined distance. In this manner, solder droplets 22 contact 
solder-wettable bond pads 20 and wet the surface to form a reliable 
interconnection. 
The solder droplets are then cooled, which causes solder droplets 22 to 
harden to form solder bumps 24, as depicted in FIG. 3. Solder bumps 24 
attach to bond pads 20. In a preferred embodiment, substrate 18 is then 
removed from platen 12 and spacer 28 and is prepared to be attached to 
another substrate or integrated circuit component. 
Thus, the present invention provides a method for forming solder bumps on 
bond pads located on a substrate or an integrated circuit component. By 
dispensing a solder paste on a platen and distributing a predetermined 
amount of solder paste into depressions in the platen, uniform solder 
droplets are formed that in a preferred embodiment protrude from the face 
of the platen. The solder droplets are then transferred to bond pads on a 
circuit board, integrated circuit component, or semiconductor wafer, which 
is facilitated by the platen being formed of a solder-nonwettable 
material. The present invention forms a more reliable microelectronic 
assembly by removing problems associated with prior art methods that pick 
and placed small solder balls and placed them onto bond pads. Further, 
smaller solder bumps can be formed, having a diameter of less than 10 
mils. Smaller solder bumps allow the size of substrates to decrease, and 
further allows components to be mounted in smaller areas on circuit 
boards, thereby making the circuit boards and the resulting assemblies 
smaller and lighter. 
While this invention has been described in terms of certain examples 
thereof, it is not intended that it be limited to the above description, 
but rather only to the extent set forth in the claims that follow.