Page 53 - Chip Scale Review_May June_2022-digital

Figure 2: Clip sintering assembly process.
is because of the coefficient of thermal with the most commonly used being paste After printing, the next assembly step is
expansion (CTE) mismatch. To understand and film—based on our observation with nitrogen drying (see Figure 2, (C)).
this importance, we can look at the CTE HPE package developer end users. The purpose of drying is to remove
between the semiconductor die and metal Figure 2 shows a typical clip pressure the solvent within the printed paste. A
clip. For example, if the semiconductor die sintering assembly process that starts with properly dried printed paste increases
is made from silicon carbide with a CTE the application of paste onto a substrate via the paste evenness and firmness to
range of 3-4ppm per °C, and the CTE for a custom-designed and fabricated stencil. avoid paste spluttering or roll-up during
clip copper with a few microns of plating on The challenges faced during printing are to clip placement. The last step of the clip
its surface ranges from 16-18ppm per °C, the avoid overprinting and smearing, as well as sintering assembly process is pressure
order of thermal expansion would be about having to deal with the need for a two-step sintering, where the two connection
4 to 5 times the stress differential. If this printing process. Printing is performed in points – namely the source pad area and
stress differential for the clip sintering is not two separate steps because the clip attached the top of the semiconductor chip – are
designed with sufficient stress relief features, has a two-ended location – namely the pressure sintered with metal stamps.
such stresses during thermal cycle testing DBC, where the source pad area is located Optimal stamp design is critical within
may lead to breakage of the HPE. Therefore, and the top of the semiconductor chip – to the sintering tool to ensure sintering
designs of stress relief features play a complete the flow of electrical current. The pressure is evenly applied onto the
crucial role in reducing the interfacial stress printed area where the clip will be attached interconnection of the clip and its contact
between the copper clip and semiconductor is a fraction of the area of the total die size. point, i.e., the source pad and the top
chip metallization. Figure 1 shows There exists a height difference between of the die. After the pressure sintering,
some potential ways to reduce the stress the source pad area and the substrate, the quality of the sintered clip bond is
because of CTE mismatch. therefore, merging a two-step printing into characterized by automated shearing or
a single step would lead to insufficient peel testing. The testing method employed
Clip interconnect pressure sintering printing pressure on at least one of the is dependent on the design and size of
assembly processes connecting points. Furthermore, fabricating the clip. If the thickness of the clip is
The clip assembly for HPE begins with a 3D stencil would be a challenge for a sizable for the shear tool to contact during
printing of the sintered material onto the single-print process. The first printing is shearing, it will be used to evaluate the
substrate. The example shown in Figure 1 is performed on the DBC, which we term clip bond force, otherwise peeling of the
a direct-bonded copper (DBC) substrate. It here as the first connection on the source clip would be the alternative—which is
requires the application of pressure sintering pad area (A), and the second printing (B) is most commonly used. Scanning acoustic
paste to form a bond with the metal clip. performed on top of the semiconductor top microscopy will be employed to check for
The sintering paste comes in many formats, as shown in Figure 2. voiding, delamination, and the uniformity
of the pressure of the completed
Exploring optimally-printed results sintered bond.
To obtain optimally-printed sintering
paste, the following factors must be Using local facilities for a
optimized: 1) paste viscosity at room compatible clip sintering process
temperature, 2) paste mixing and To cope with the rapid growth of
rollability, 3) printing speed, 4) printing HPE applications, the manufacturing
force, 5) printing direction, 6) stencil requirements of the whole production line
frame spring force, and 7) squeegee should be considered. The clip sintering
release speed and distance. Figure 3 process for HPE needs to be compatible with
demonstrates the difference between all the upstream and downstream processes.
a poor printing outcome versus a good Therefore, many manufacturers today are
printing outcome for clip sintering, if looking for local facilities to speed up the
printing parameters are not optimized. development processes and time to market.
Figure 3: Difference between poor and good printing.

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