Automating Solder Bump Inspection

By Katherine Derbyshire

In packaging and assembly, as in front-end semiconductor manufacturing, increasing device densities requires new processing steps. For example, increasing the I/O density of the IC requires new package-to-chip wiring methods. Ball grid arrays, which place an array of solder bumps on the surface of the chip, are a promising alternative to conventional wire-bonded leadframe designs (See related article).

The new solder bumping process introduces new sources of defects, though. Bumps can be misaligned, shorted, or missing altogether. A missing bump means a missing bond to an otherwise good chip. A shorted bump can short out the probe card, forcing an expensive replacement. Post-bump inspection is mandatory.

Unfortunately, according to Electroglas Inspection Products' (Albany, OR) VP and general manager Joe LaChapelle, rigorous process control is almost unknown in back-end processing. Even as packaging and assembly costs reach 40% of the total IC cost — the die-level back-end processes do not realize the same economies of scale as the mostly wafer-level front-end processes — most manufacturers do not have any data on back-end yield loss. For instance, LaChapelle told Semiconductor Online, solder bump inspection usually relies on human technicians using microscopes. Manual inspection is slow and tedious. Only about 2% of wafers are screened, and even those few inspections are only about 70% accurate.

Front-end process control relies on highly automated inspection review tools to screen wafers quickly and accurately (See related article). These tools can identify killer defects on sub-0.25 micron features, about 10× the resolution needed for the back end, and are priced accordingly.

Electroglas' new Quicksilver (See related article) inspection systems are targeted to post-fab applications. With appropriate software, the tools can measure solder bump dimensions and placement, inspect wafers for defects, and provide critical dimension measurements for print heads, MEMS, and other unique devices. The platform uses time delay integration (TDI) to achieve throughput of up to 40 wafers/hour. Click here for platform diagram

The TDI sensor is a linear array of columns, coupled together internally. As the wafer moves past the sensor array, the charge developed in each element of the TDI column is transferred to the next column. That is, the sensor sums the signals collected at each die position. An entire line of image data is then transferred to the signal processing system. This architecture improves both sensitivity and speed relative to conventional charge-coupled devices (CCDs).

The Quicksilver systems represent the latest move by Electroglas, a leading wafer prober supplier, into post-fab process control. As LaChapelle explained, the company's strategy combines data collection tools like wafer probers and the new inspection systems with data analysis software. Recent acquisitions of Knights Technology (Sunnyvale, CA), a yield management software company, and Techné Systems (Albany, OR, renamed Electroglas Inspection Products) have helped broaden the company's offerings.

For more information: Electroglas, Inc., 225 SW Washington, P.O. Box 1065, Albany, OR 97321. Tel: 541-967-1211; Fax: 541-967-7992.