Stacked-Die Wire Bonding

Advanced Bonding Application

The demands for high electrical performance and pin count have resulted in significant advances in integrated circuit fabrication and microelectronics packaging. As the popularity of mobile phones, digital cameras, personal digital assistant, etc drives the move towards smaller geometry chips, new packaging method needs to be developed to integrate highly complex chips into the smallest possible space.

The miniaturization of such products was made possible by the application of stacked-die Chip Scale Package (CSP) packaging technology. Indeed, the demand for small form factor packages, increased functionality in the same area, and low cost are the product requirements for stacked-die CSP’s. Inside the stacked-die CSP’s, it contains 2 or more dice, stacked over each other. Wire bonding is most commonly used for interconnection.

Pyramid Stacking Overhang Stacking

Stacked die packaging is the most versatile wire bonded package among all other packages. It came in different design configurations - from pyramid stacking to overhang stacking, from standard bonding to low loop and reverse bonding. Given the complexity of the chip configurations, capillary design needs to be optimized for each particular bonding configuration.

  • Pyramid stacking is a conventional chip stacking. Dimensional selection as per standard capillary selection guide. However, as more and more chips were stacked, the wire length for the upper chip increases. Wire sway could be a major issue. PI Capillary design has been used extensively to minimize wire sway issue, especially for long loop application.

  • Overhang and same die size stacking. Die deflection due to the impact force during bonding, especially at the corners of the die can result in NSOP. Normally parameter optimization or re-grouping of the corner wires is necessary. Reverse bond using ball stitch on ball technique is normally used to achieve the low loop height requirement.

  • It should be noted that stacked die bonding represents one of the most complex bonding in the wire bond process. Within one package, different bonding techniques using forward bonding, reverse bonding, multi-tier bonding together with the new loop trajectory can be performed. Again, each type of bonding has its own uniqueness and need to be considered separately when selecting the optimum capillary design.


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