Wafer Prep Key To Thinning SiP

In its ongoing push to create smaller, thinner, and denser chip packages, the semiconductor industry has intensified its focus on integrating separately manufactured components with different functionalities into systems-in-package (SIPs). Known as heterogeneous integration (HI), this approach now drives the industry’s roadmap for advancement. SiPs enable power-efficient, high-bandwidth connections between components within a compact form factor, and deliver enhanced functionality and improved operating characteristics. This makes them ideal for such applications as consumer and communications devices.

Optimizing SiPs for use in these high-demand end products that need to be smaller and faster and consume less power means using backgrinding to make them as thin as possible. Quik-Pak has a full range of capabilities in this area. We can perform automated, volume backgrinding of a cassette containing up to 25 whole wafers (between 100mm and 200mm in diameter), as well as partial wafers, 300mm quartered wafers and even individual die. We can thin 300mm wafers down to 75 microns, and 200mm and smaller wafers down to 25 microns.

Package types that we can address include ball grid arrays (BGAs) with flip-chip and ball attach. Flip-chip can be thinned to 4 mils, or 100 microns. Going thinner than that with bumped wafers requires using special tape to ensure the bumps aren’t damaged. We can also thin and polish such materials as gallium nitride (GaN), glass, quartz and ceramic.

DAF assures material integrity

Thinning wafers requires addressing and mitigating integrity issues. Commercially available since 2000, die-attach film (DAF) has become essential in the HI era for use in wafer prep and singulation of wire-bonded single and multi-chip stacked solutions. Applied to the backside of the wafer, highly adhesive DAF serves as the adherent material for the die and as the support film. Once wafer singulation is complete, the DAF-protected die are picked off the support tape and placed on the substrate. Multiple die are then stacked vertically in a slanted/offset pattern, and the die are wire-bonded on the edges.

Figure 1 shows an example cross section of a stacked-die device with seven die layers created using a diamond-filled DAF. The silicon wafers were thinned to 200 microns before laminating onto the DAF, and the die on the DAF-treated device wafers were singulated using a dual-spindle dicing saw, as shown in figure 2.

Fig. 1: 7-die stack with highly thermal conductive DAF.

Fig. 2: Dual-spindle saw for wafer dicing.

Whereas paste can outgas, void and bleed into other areas, DAF is easily controlled. It allows not only the sharp edges and shape of the die to be maintained, but also the die footprint since the die and film are cut simultaneously. DAFs can also be applied to non-die-attach processes such as lid sealing and wafer bonding.

Expertise in DAF is a key aspect of wafer prep capabilities. Working with our suppliers, who provide DAF in sheets for non-automated lamination, we can offer a variety of conductive and non-conductive DAF for customers to choose from. While the cost per wafer using DAF may be slightly higher, the savings in process steps and yield improvement make the overall cost comparable to using paste for wafer prep, which creates a significant benefit for our customers.

Ensuring that the films are always in stock requires a high degree of commitment, as they last about 6 to 12 months when stored at temperatures between 0-10°C. Beyond this timeframe, their physical properties change, and the films begin to deteriorate. They can be stored at room temperature, but this cuts their shelf life in half. Given these challenges, few stateside suppliers make DAFs available.

Over the past year, our access to DAF for wafer thinning has proven even more valuable, as the industry has had to cope with interruptions to the global supply chain created by the pandemic. As the current push to bring more semiconductor manufacturing onshore continues to grow, we anticipate that having established SiP assembly and thinning capacity in place will prove even more beneficial for Quik-Pak and our customers.