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What is IMS?
Why IMS?
The Process

Injection Molded Soldering
What is IMS?
IMS (Injection Molded Soldering) is a flip chip bumping technology developed at IBM research. The objective of this technology is to reduce wafer bumping costs by reducing process steps. At the same time, the process must track semiconductor industry objectives, which include wafer sizes up to 300 mm and chip footprints with decreasing bump sizes and pitches, while maintaining excellent yields and bump uniformity. The process of IMS melts bulk solder and dispenses same into a wafer-sized mold which is CTE matched to the device silicon wafer. The mold is scanned with molten solder and thereafter cooled so that the solder solidifies. It may be inspected and is subsequently aligned to the wafer. The aligned assembly is passed through a furnace for bump transfer. The molds are continually reusable, thus reducing wafer bumping cost  (Click on figure 1 to see a VIDEO of the IMS head).
Why IMS?
Despite the ever-increasing complexity of integrated circuits (IC), competitive pressures also demand a more efficient and inexpensive method of package interconnection. Evaporation of bump metallurgy has been practiced for many years in IBM, and more recently processes such as plating and solder paste screening have also been used. IMS technology is being developed to exploit the following advantages:
Versatility - Due to process simplicity, various solder alloys are readily processed. These include lead and lead-free (ternary and quarterary) alloys. Also, unlike solder paste screening, there is essentially no volume change between molten and solidified solder, thus allowing a wide range of feature sizes to be processed. These include solder balls as large as BGA's or larger, down to aggresively small sizes and pitches for wafer bumps.

Economical & Environmental - IMS uses no hazardous gases or chemicals and is able to process newer lead-free solders. Since it uses only the solder volume required for each part, there is no solder waste which is especially important for costlier alloys. Processing steps are also reduced by using bulk alloys. Thus it is economical and environmentally friendly.

Quality - Initial inspections of IMS bumped wafers show good bump uniformity and yields. Once diced and packaged, these IC's exhibit reliability comparable to those processed with standard bumping methods.

The Process
IMS is usually a transfer technology. Mold plates containing cavities are filled with solder and aligned to substrates that receive the solder from the cavities. This two step approach allows for inspection of the mold plates after the solder fill and before the final transfer. As a result, low wafer cycle times and high final bump yields are achieved   (Click on figure 2 to view the step by step process) .

 The head of the IMS apparatus is filled with molten solder and moves in relation to a cavity containing mold plate, both of which are usually above solder liquidus temperature. As the head scans across the mold plate, the solder from the reservoir, under constant positive pressure, passes through a dispensing slot and into the cavity volumes. After the scanning process, the mold plate is cooled to solidify the solder. It is then inspected, which can be done by various automated optical techniques. After inspection, mold plates may be either immediately sent for transfer or stored in a non-oxidizing environment.

Mold Plates
Mold plates can be made from a variety of materials. Since transfer of the solder from the mold plate to the final solder receiving substrate occurs at elevated solder reflow temperatures, matching the coefficient of thermal expansion of mold plate and substrate is important. This is especially true as the area to be bumped increases. Thus, for single chip, or even a 4" diameter wafer, the smaller distance to neutral point is such that the mold and substrate can tolerate some CTE mismatch and yet still work successfully.  However, for larger areas such as full 8" and 12" diameter wafers, it is important that the mold material closely match the CTE of silicon (click on figure 3 for a close up of solder bumps).

 The ability to fabricate the material to a high degree of planarity is important to avoid solder leakage during scan. Cavities in the mold plates are in a pattern that is the mirror image of the solder receiving pads on the final substrate or wafer. Cavities can be produced in the mold plate by any one of a number of techniques, the selection of which is dependent upon the cavity size and pitch as well as the mold plate material. Cavity volume uniformity is essential since these directly determine the solder bump volume on the wafer.

Figure 4                   (click on figure 4 for a VIDEO of cavity filling)

Wafer Bump Transfer

Alignment of the mold and wafer is critical to the success of wafer bump transfer. When transparent mold plates are used, it is easy to align filled cavities to wafer pads. When non-transparent material is used, alignment using split optics is necessary. Depending on the environment of the transfer, flux may or may not be used. If used, it is applied in a thin even coat onto the filled mold plate or wafer before proceeding to the transfer fixture. If not used, oxide reducing methods such as pressure variation or hydrogen reflow can help to facilitate the transfer. After proper alignment, the assemly is placed in a solder reflow furnace. When the solder is in a liquidus state and the wafer pads are oxide free, the solder wetting forces exceed the surface tension forces that maintain the molten solder in their cavities. After cooling to solidify the solder, the solder bumps are released from the mold plate as the latter is lifted from the wafer. The shape of the bumps is the same as the mold cavity shape. Depending on the actual cavity shape used, along with the needs of the subsequent flip chip assembly operation, the wafer can now be independently subjected to a final solder reflow excursion to obtain spherically shaped bumps.

   The IMS development tool in Yorktown is highly flexible to permit varied bumping applications. Yorktown Research also works in conjunction with other IBM locations both within the U.S. as well as Canada and Japan to facilitate bump transfer as well as developing other uses of IMS. To the right is Peter A. Gruber who works with IMS as part of  the Interconnect technology dept.  Feel free to contact him for further information on IMS.
BLM - Ball Limiting Metallurgy
CTE - Coefficient of Thermal Expansion
IC - Integrated Circuits
IMS - Injection Molded Soldering
BGA - Ball Grid Array
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