SAN FRANCISCO -- IBM's fabled C4 flip chip concept has been updated and was a popular attraction at a major semiconductor manufacturing expo last week.

IBM's Peter Gruber has conceived a new solder bumping method that builds on the controlled collapse chip connect invention yet eliminates several steps -- including all mold cleaning. Moreover, it is fluxless and can be used with any solder alloy.

In concert with Süss MicroTec, the Munich-based semiconductor equipment supplier, IBM has developed and commercialized C4NP (NP stands for "new process"), a solder transfer method in which molten solder is injected into pre-fabricated (and reusable) glass-molded etch cavities that correspond to solder bumps. The mold and wafer are aligned and the bumps transferred onto the entire wafer. It's an elegant, one-step solution: no lithography, no plating, no flux, complete alloy flexibility.

The companies rolled out both the invention and the wafer bonding equipment that makes it possible during SemiCon West last week. (The concept was also detailed in a paper presented at IMAPS in late June.)

According to Klaus Ruhmer, director of global marketing and sales for C4NP at Süss, the process is capable of 25 micron balls and 50 micron pitches. It can be used on 300 mm or smaller wafers.

The companies are moving quickly to meet anticipated interest. External customer evaluations are scheduled to begin in September, with high volume production ramped in the March to April 2006 timeframe, Ruhmer told Circuits Assembly.

The arrangement came about like this: Süss, which supplied lithography tools to IBM's Fishkill, NY, semiconductor manufacturing site, mentioned to IBM a new wafer bonder that aligns mold plates, then applies temperature and force. That caught the attention of Big Blue, which was looking for a company to partner with on C4NP. A couple weeks later, IBM invited Süss engineers to Fishkill to present their ideas. A joint development agreement was signed last September.

Traditional wafer bumping involves screen printing, electroplating or evaporation - plus several subsequent cleaning steps. In C4NP, on the other hand, the wafer is exposed to a single process step during solder transfer. Thermally matched mold plates made from Pyrex glass are covered with an array of etched cavities representing the unique bump process of the wafer. The trick lies in controlling the melt of the solder and the volume of the cavities, which must be precise to ensure uniform bumps. The molds are built using an IBM wet-etching process. In short, C4NP shortens wafer bumping to four steps, from at least eight.

IBM has been working on C4NP for years. The company described a version of the process using eutectic solder as early as September 2000.

But it's the push to lead-free alloys that will provide a boost to C4NP, its inventors say. Conventional bumping uses high lead content alloys, which evaporate quickly. Lead-free alloys, however, tend to be high in tin content and evaporate much more slowly due to the low vapor pressure of tin. Printing has its warts, too. It requires parts to be reflowed, a process step that reduces solder volume and can cause voids. Also, problems can stem from the dimensional stability of the stencil, especially when 300 mm wafers are used. Plating baths are notoriously hard to control.

"Lead-free wafer bumping is a painful process," Ruhmer says. "But everyone has to do it." (IBM demonstrated C4NP in the Süss booth and Ruhmer was tabbed as media spokesman since IBM's engineers, including Gruber, were asked not to speak directly to reporters.)

Testing conducted at IBM showed remarkable uniformity in bump size and shape, with results for 200 mm wafers comparing "very favorably" to eutectic solder. Chip yield was 99.2% and bump yield was greater than 99.995% in trials on 36 processed wafers, IBM reported. The company says similar results are expected from 300 mm tests. Reliability data are currently being collected for the lead-free bumps.

C4 was invented by IBM researcher Paul Totta in 1964.

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