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 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.

 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.