Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-12-26
2003-02-11
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C257S738000
Reexamination Certificate
active
06518163
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming bumps on electrode pads provided on a substrate, to an electronic component on which bumps are formed, and to a solder paste.
2. Description of the Related Art
There has been a growing need for higher mounting density with electronic components in recent years, and bare chip mounting methods have been attracting attention. There are two types of bare chip mounting method: a face-up method involving wire bonding, and a face-down method featuring metal bumps. Face-down mounting is becoming more and more prevalent today. A benefit of connecting with metal bumps by face-down method is the lower resistance of the connection. On the other hand, numerous demands are imposed on this method, such as lower cost, ensuring a precise bump height in order to achieve stable connection reliability, and forming bumps at a fine pitch corresponding to the electrode pads of a semiconductor chip.
Plating and vapor deposition are just two conventional bump formation methods. These bump formation methods require a tremendous equipment investment, and make it difficult to control bump height and metal composition, among other problems. In view of this, engineers have been taking a closer look at printing, which allows a metal paste to be supplied at low cost.
One type of printing method makes use of a metal mask. In addition, as disclosed in JP-A-7-273439 and JP-A-11-340270 and elsewhere, there is also a method that utilizes a resin mask. When a metal mask is used, one in which openings have been formed corresponding to the locations where the electrode pads are formed is placed over a substrate. When a resin mask is used, a resin layer is formed over a substrate, after which the portions corresponding to the electrode pads are removed to form openings. The two methods are similar in that after this, a squeegee is used to push a solder paste applied over the mask into the openings and thereby form bumps. When a metal mask is used, it is removed after the openings have been filled with the solder paste, but when a resin mask is used, it is removed as needed after the bumps have been formed.
However, if a large proportion of the solder powder that makes up the solder paste has a large particle diameter (such as an average particle diameter of 30 to 40 &mgr;m), there tends to be variance in the size of the bumps that are formed. Causes of this include the fact that some of the solder powder that has filled the openings is wiped away when the squeegee is moved back and forth over the mask, and that when the metal mask is removed after the openings have been filled with the solder paste, the solder paste clinging to the inner walls of the openings ends up being taken away with the mask.
To avoid this problem it is necessary to use a solder powder with a small proportion of particles whose diameter is large. For instance, it is good to use a solder powder with a large proportion of particles whose diameter is no more than ⅓ the thickness of the mask (when the thickness of masks commonly in use is considered, this is substantially a particle diameter of 15 &mgr;m or less).
Meanwhile, methods for producing a solder powder include disc atomizing and gas atomizing. With these methods it is difficult to stably produce a powder with a small particle diameter. Accordingly, the current approach is to produce a powder having a particle size distribution within a certain range, and then separate and collect the fines. However, not only does separating out the fines require considerable labor, it is also difficult to collect a large quantity of fines. For instance, with existing technology a solder powder of 20 &mgr;m or less only accounts for about 20% of the total powder, which is also disadvantageous in terms of cost. Also, because a fine powder with a small particle size has a larger specific surface area and is therefore oxidized more readily, the solder paste made up of this solder powder has a shorter life.
SUMMARY OF THE INVENTION
The bump formation method provided by the first aspect of the present invention is a method for forming bumps on a substrate provided with a plurality of electrode pads, comprising the steps of providing a mask having a plurality of openings corresponding to the plurality of electrode pads, filling each of the openings with a solder paste, and heat treating the solder paste, wherein the solder paste contains solder powder and a flux vehicle, and the solder powder contains no more than 10 wt % particles whose diameter is greater than the thickness of the mask and no more than 1.5 times this thickness.
Unless otherwise specified, the term “substrate” as used in the present invention includes all substrates on which electrode pads are formed, which of course includes circuit substrates and silicon wafers, but also includes semiconductor chips and so forth. When an opening is not circular, “open diameter” refers to the diameter of a circle having a surface area equivalent to the surface area of the opening.
The solder paste used in this bump formation method must have a small proportion of solder powder with a relatively large particle diameter as compared to the thickness of the mask. This reduces the danger that the solder paste filling the openings will be wiped away when the mask is coated with the solder paste and a squeegee is then moved back and forth over the mask in an effort to pack the insides of the openings with the solder paste. Also, when a metal mask is used, there will be less danger that the solder paste clinging to the inner walls of the openings will be taken away with the metal mask when the mask is removed after the openings have been filled with the solder paste. Accordingly, there will be less variance in the bumps if they are formed by the above method.
The smaller is the quantity of solder powder within the above-mentioned particle diameter range, the more pronounced this effect will be, and the ideal proportion for such solder powder is therefore 0 wt %. For the above effect to be realized even better, it is preferable to use no more than 10 wt % solder powder having a particle diameter of 40% or more of the open diameter of the openings.
In a preferred embodiment, the solder powder contains at least 30 wt %, and preferably at least 50 wt %, particles whose diameter is 40 to 100% of the mask thickness.
This solder paste has a larger proportion of solder powder of suitable particle size as compared to the mask thickness, and a smaller proportion of solder powder of relatively small particle size. If the thickness of the mask is about 50 to 100 &mgr;m, for example, then the proportion of solder powder having a particle diameter of 20 &mgr;m or less is small. As discussed above, it used to be that preparing a solder powder having a particle diameter of 20 &mgr;m or less not only was labor intensive, but also produced a low yield and was expensive, but if the proportion of solder powder with such a particle diameter is reduced, then these drawbacks are automatically ameliorated. Also, if the proportion of solder powder with a small particle diameter is small, the solder powder as a whole is not as susceptible to oxidation, so another advantage is a longer life for the solder paste.
The average particle diameter of the solder powder as a whole should be suitably determined as dictated by the thickness of the mask, the diameter of the openings formed therein, and so on, but is 5 to 20 &mgr;m, for example.
One or more elements selected from the group consisting of tin, lead, silver, antimony, bismuth, copper, indium, and zinc can be used favorably as the solder component that makes up the solder powder, for example. More specifically, 63%Sn-Pb (melting point: 183° C.), Sn-3.5%Ag (melting point: 221° C.), 5% Sn-Pb (melting point: 315° C.), and the like can be used to advantage.
Meanwhile, the flux vehicle can contain rosin, an activator, and a solvent.
The primary role of the rosin is to increase the adhesion of the solder paste. A variety of known rosin
Mizukoshi Masataka
Sakuyama Seiki
Yamagishi Yasuo
Fujitsu Limited
Hoang Quoc
Nelms David
LandOfFree
Method for forming bumps, semiconductor device, and solder... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for forming bumps, semiconductor device, and solder..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for forming bumps, semiconductor device, and solder... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3183160