By Terrence E. Thompson, Senior Editor
The critical demands of today's larger, higher I/O die, thinner packages and reduced solder ball sizes and/or pitches are forcing new roles on dispensing equipment for encapsulants and underfills. It's no longer enough to buy a gantry dispensing system that can handle most package types, particularly when the cost is measured in the mid-six figures. It took a while, but total cost of ownership (COO) considerations have moved from the wafer fab to the dispensing line. Tool Flexibility Not surprisingly, in recessionary times customers are willing to forego some flexibility-but not too much-for tools that do almost everything they need. A common mantra today that users deliver to machine suppliers is, "Send us a machine that does x quickly, but don't add cost to it," says Joe Battaglia, product marketing manager for Cookson Elec-tronics Equipment*.
Product segments are driving tools sets in different directions, according to Steven J. Adamson, Asymtek's manager of semiconductor packaging and assembly products. "Very high count I/O-type devices pull you in one direction. Low-end, globtop cell phone applications pull you another way." A significant trend, says Dan Crowley, director of sales, Newport/MRSI, is for high-volume manufacturing to move to contract firms. "They, of course, must run very economically while being able to switch from lot-to-lot or part-to-part as production needs change. This certainly adds more pressure to the COO." The result, according to the experts, is that more equipment is being designed for specialized applications, than in the past. Still, many users, buckling under spending constraints, want systems that are lean but flexible. The purchase decision, says Adamson, boils down to cost. "Users have one job today, another job tomorrow. They only want to pay minimum dollars. Yet they want the ultimate flexibility." Adamson equates need for flexibility to being "a lot like buying a Hummer to go to the grocery store while having a Honda Civic budget." The trick for tool makers, says Adamson, is to make the machine's changeover as quick and painless as possible.
Meeting the Price Point Cookson's Battaglia observes that sometimes customers do wind up with the equivalent of a Hummer to use for the supermarket-or a small Honda for the Indianapolis 500. The challenge for both dispensing equipment makers and their customers is the price point. "Where is the maximum value for an unpredictable market with many variables?" asks Battaglia. While Dr. Tom Di Stefano, president of Decision Track, agrees that equipment is becoming more specialized, the key is still standardization-at least within the broad segments.
"Users don't want different equipment for each package," says Dr. Di Stefano. The result is that standardization and compatibility with acceptable pulse rates are key. "As requirements escalate," he adds, "it becomes more difficult to maintain standards." There is now a potpourri of challenges for users and vendors. The issue with both thinner ICs and packages is material flow into more constricted spaces. "As users reduce solder ball sizes, or pitches, underfilling becomes more difficult," Dr. Di Stefano notes. With the growth of multichip packages, he says, the combination of flow rate and flow time with shorter solder ball heights will become even more important. While it seems like a moving target, today's Holy Grail of dispensing revolves around several items, according to the experts.
Among them is the ability to meet or exceed dispense pulse rates with ever-tighter (and smaller) staggered pads and rows on devices and boards, according to Cookson's Battaglia.
Adamson agrees, "Machine cost and unit cost to dispense encapsulants or underfills is now very much an issue. "What's it really going to cost at a given uph? In addition, tool makers are facing technical issues, he observes. We're looking at large silicon chips now," adds Adamson. For example, "just getting fluid underneath a 2x2-inch chip design is tricky." Each specific market segment also continues to drive the dispensing tool market. Cookson's Battaglia points out that adding large die underfill capability for very low gaps requires materials and processes that ordinary capillary applications don't need, something that can impact the entire dispensing process. Technology Challenges The stacked die package is representative of new, major challenges in the dispensing area. "Not everyone understands what stacked die designs imply," notes Asymtek's Adamson.
"People understand the concept from a perspective of silicon functionality. But with very constricted spaces, you may have to slow down the dispensing machine for access." One possible answer to the limited space problem is "jetting." Several tool makers are involved in extensive research programs to validate jetting's ability to dispense in constricted spaces on a consistent basis. In traditional dispensing, the needle occupies needed space. A jetting dispenser, however, flies above the die and jets material where needed. Adamson says Asymtek views this use of jetting as "the future." The list of dispensing challenges appears to grow longer by the day. Cookson's Battaglia says high-volume capability coupled with scalable systems "is one thing we must address for incremental uph improvements." Another challenging area, he adds, is dispensing for tiny devices like 0201s and other active devices that are difficult to handle and place.
Chip Systems Perhaps the biggest challenges, according to Dr. Di Stefano, are heterogeneous chip systems where standard chips and wafer-level packaged parts are encapsulated on the same substrate with different spacings under the chip. "This trend is going to present real challenges for encapsulating dissimilar parts in constrained spaces," he says.
Be aware that material properties, such as viscosity, change over time. The result may be inconsistent dispensing, since encapsulants can be quite abrasive, reminds Christian Vega, business development manager for GPD-Global. "As vendors, we try to cope with new ways to moderate material flows using different pumps or nozzles."
Cycle time, adds Vega, is a major issue, too. "We see multiple sets of fiducials and different sizes of fiducials as a way to speed up some encapsulation. "With smaller dispense areas and smaller fiducials, this challenges the machine's vision systems, which then have to compensate for smaller sizes and different substrate colors," says Vega. Ironically, the conundrum that some users confront is that the features they need from their dispensing tools will change from day to day and month to month. One day they want their tools to dispense encapsulants, the next day adhesives and the next week underfills.
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