Functional but Unusable? It is indeed possible to have a functional die that won't work in an application. For example, if the bumps are not on the correct pitches, or are uneven heights on an otherwise functional die, they most likely will be problematic. Larry Gilg thinks it's a problem with a viable solution. He says the DPC is trying partnering, an approach it has used before. "Automated optical inspection can easily detect these physical defects," says Gilg. "That's why the DPC is working with August Technology [augusttech.com] and MCT [mct.com] to demonstrate a post-singulation test, AOI and sort solution for KGD processing." The goal is to create new ways of implementing inspection, test and die sorting capabilities to increase quality and lower the cost of die products in final manufacturing. "With advancements in die inspection, sawn wafer testing, and die sorting capability, the technology exists to change traditional thinking about the final manufacturing process for die products, including WLCSP," explains Dan Nelson, Corporate Development and Strategic Marketing, August Technology. Nelson says an exploding demand for increasing functionality in smaller form factors at the lowest cost is straining the current back-end manufacturing processes, requiring new manufacturing paradigms. Test Die and Package Instead of performing wafer test immediately after IC fabrication and before post-processing operations such as bumping, pad redistribution, sawing and visual inspection, the proposed process flow positions the wafer test and visual inspection after all die manufacturing operations including wafer sawing are complete. Fully tested and inspected die need only to be sorted and shipped to the customer for assembly. IEC Meets Euro Good-Die Yes, there is solid international cooperation for setting KGD practices. Good-Die is the European Initiative to Promote KGD Technologies, essentially a European counterpart of the DPC. [gooddie.net] The Network of Excellence, GOOD-DIE III, began last year. The working members of the Network are Alcatel Microelectronics, Eltek Semiconductor Ltd., IMEC, Infineon Technologies, Philips Semiconductor, SAAT Technology Ltd. (previously Rood Technology) and the DPC in the USA. Its newest partners are Bosch and S-T Micro-electronics (STM). Exciting Changes One expert we contacted was Jerry Secrest, Principal Consultant at Secrest Research [secrest@ix.netcom.com]. "KGD are growing in importance because there are more and more MCMs being manufactured using KGD and die in CSPs" says Secrest.2 He cited some classic application examples: stacked die combing flash and RAM for cell phones; microprocessor modules (Intel) that include RAM in the module; and the die going into Bluetooth modules. Faster than the Speediest Tester Secrest added that conventional KGD manufacturing is becoming increasingly difficult since IC speed and complexity are outpacing tester performance (see the International Technology Roadmap for Semiconductors).3 The problem for die going into packages and die being shipped as KGD is how to guarantee the performance when the tester is not fast enough. Secrest's workaround is to predict the speed of the die from fab process data gathered from the PCM patterns. Another significant issue, according to Secrest, is how to gather information on the reliability of a product being sold as KGD. Packaging a sample for burn-in could change the reliability, and burn-in in die form requires significant handling care. One way to remove at-risk die is to find them using "suspect by association" or by "deviation from typical" strategies. There are technologies, such as Self-Organizing-Maps, that can be used to find "maverick behavior" in a die population. Figure 3 is a full-wafer contact burn-in cartridge that accommodates contactors for parallel test electronics.
Some Known Good Conclusions Until now, standards have not been something users could easily influence. But that has changed with the IEC's efforts, which resulted in the draft standard seeking solicited comments from designers, vendors and users. Second, WLP, which already generates functional packaged die, will help fill the KGD pipeline with probed and tested die in many forms. KGD is more a process issue than a product issue so WLP will be very useful with its inherent batch manufacturing and probe advantages. Packaging challenges for discrete and IC devices will only increase as the transition to more powerful and integrated products gains momentum, especially with portable products. KGD will make overcoming the OEM packaging hurdles much easier. References 1. IEC 62258: Semiconductor Die Products, (draft proposal). 2. J. Secrest, "Known-Good Die for Stacked CSPs: It's Not Your Father's KGD Anymore!" Chip Scale Review, July 2001, p. 63. 3. International Technology Roadmap for Semiconduc-tors, see Semiconductor Industry Association (SIA) [semichips.org]
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