There is a joke often shared among semiconductor design engineers. “The more chips you stack, the better, but the problem is that stacking them is far too difficult.” In real packaging processes where dozens of chip layers are stacked, the first thing to fail is not the design drawing, but flatness. In a world where a 1 micron difference can disrupt overall yield, the meaning of “grinding well” carries far more weight than expected.
Why TTV Matters in Chip Stacking Structures
In structures where chips are stacked in multiple layers, thickness variation in each layer is not just a dimensional issue. As layers accumulate, microscopic errors build up and eventually lead to electrical bonding defects or concentrated structural stress. The metric used to express this numerically is total thickness variation, or TTV.
The TTV required in chip stacking structures is at the level of 1 micron or less, demanding precision far below one hundredth of the thickness of a human hair. If this standard is not met, bonding surfaces can lift during subsequent bonding processes or heat dissipation can become uneven, undermining the reliability of the device itself. For products such as AI semiconductors or high performance memory that vertically connect dozens of chips, the allowable variation range becomes even narrower.
One reason TTV control is difficult in the field is the structural variables of the grinding equipment itself. The wear pattern of the grinding wheel, microscopic vibration generated during spindle rotation, and changes in the flatness of the chuck that fixes the wafer act together to make the thickness profile uneven. Uniform flatness cannot be achieved simply by turning on the equipment. How precisely these variables are controlled is the key factor that determines the yield of the entire packaging process.
Ultra Precision Grinding Design of the PKG Grinder
There is a way to address this issue from the process equipment design stage. The PKG Grinder from Huvics combines a high precision stage with a diamond wheel to process various package types such as FCBGA, WLCSP, and QFN/DFN at micron level accuracy. The key is a control structure that keeps TTV variation within a defined range even during high speed processing. The TTV requirement for ultra thin wafer stacking processes is approximately 2.0 microns or less, and to meet this value reliably in a high speed grinding environment, spindle runout compensation and chuck flatness maintenance must be built into the equipment from the design stage.
Another practical challenge in production sites is line flexibility. In many cases, the same equipment must handle both low volume high mix processes and high volume continuous production. The option to choose Full Auto A·B types or a Manual type is a design choice made to respond to this requirement, and it directly contributes to reducing setup loss during process changeovers. Speed and uniformity are usually in conflict, but in grinding equipment, they are implemented in a way that satisfies both conditions at the same time.
The Role of Cleaning and Drying After Grinding
Flatness control does not end just because the grinding process is complete. Immediately after machining, grinding residue and fine particles remain on the wafer surface, and the way they are removed determines the final surface quality. This is where the choice of cleaning and drying methods becomes important.
Comparison of Air Knife and IPA Vapor Drying
The two drying methods mainly used in the industry are Air Knife and IPA vapor drying. Air Knife mechanically pushes liquid off the surface using a high speed gas jet, and according to research published on ResearchGate, drying within 2 seconds is possible under a speed condition of 150mm/s. In contrast, IPA vapor drying uses ultrapure isopropyl alcohol vapor to lower surface tension and guide moisture to flow away naturally without leaving marks.
Because the two methods control processing speed and the risk of surface residue in different ways, the selection criteria vary depending on the surface characteristics of the target device and the process requirements.
Multi Step Cleaning for Components Sensitive to Surface Contamination
For components sensitive to surface contamination, such as camera modules, sensors, and VCM motors, a multi step cleaning approach that combines micro bubbles and water jets is becoming established.
More than how many chips are stacked, how uniformly they are stacked determines the actual yield. From grinding precision to the selection of cleaning and drying methods, the quality of the packaging process is completed on a structure where each stage supports the next. The joke mentioned at the beginning, “stacking them is far too difficult,” turns out to describe the essence of process design quite accurately.
Huvics is an advanced technology company that develops and manufactures automation equipment and
production systems across the semiconductor, LED, Mobile, and Cosmetic industries.
With top tier talent and continuous technological innovation, we support improvements in customer productivity and
help secure quality competitiveness.
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Phone: 031-374-8285
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