By Ron Iscoff, Editor
Solder ball placement is a key process in area array package assembly, which includes the rapidly growing BGA family and its offspring. As consultant Vern Solberg explains, "The attachment of the solder balls is a process step performed at the end of the manufacturing cycle. The ball attachment process requires accurate placement of very small solder alloy spheres and reliable reflow soldering."* Solberg wrote those lines in July 1998. While four years is not a long time in the history of humankind, it is eons in semiconductor time. At the time, solder ball diameters were running 0.30mm. Changes We have visited solder ball placement machines once a year for the past several years. This year we noted several former vendors have left the market, at least for the time being. These include Cookson Electronics (Speedline), Kulicke & Soffa and MECO Engineering. Motorola's foray into solder ball placement equipment, through its Manufac-turing Solutions division in Arizona, appears to have been unsuccessful. "The solder ball placement market is a maturing one," says Seth Alavi, president of SunSil Inc., Alamo, Calif. SunSil is the manufacturer's rep for Shibuya Kogyo. "The strip-level market is and will remain the larger market, but wafer-level bumping will display a faster growth rate than strip-level placement in the future, Alavi adds. Shibuya is the market leader in both market areas, Alavi maintains. Today, current equipment is still delivered with 0.30mm solder balls for volume production, but with the next generation of machine, 0.20mm is likely to become the standard. The equipment photos shown are representative of current solder ball placement tools.
Preparing for Smaller Spheres "Some equipment vendors have already completed trials on 0.20mm ball placement with positive results, but the evaluation data is limited to smaller sample sizes," according to Yoon Young Sun, vice president of assembly operations at STATS, Singapore. Vision inspection, says Yoon, continues to be a challenge for 0.20mm ball sizes and smaller. "The key item here is to be able to inspect and distinguish between different ball sizes." There is a growing list of demands currently facing solder ball placement suppliers, says James Carrigan, worldwide sales director for RVSI Ball Attach Products, Tucson, Ariz. These demands include yield improvements, increased throughput and a reduction of tool set pricing, Carrigan adds. Adjusting for Process Inconsistencies "Of strategic importance is the capability to adjust for upstream process inconsistencies-the condition of the substrate when presented to the sphere placement machine. "Packages with excessive warpage, mold flash extending beyond the intended boundaries, smaller geometries and an unusual variance in array styles (JEDEC trays, boats/carriers/strips) all result in the need for a more sophisticated system to meet EOL expectations," Carrigan says. The RVSI executive says that the key to remaining successful is to offer a flexible platform that can handle devices in a variety of input formats and to be able to place either solder flux or solder paste. In addition, Carrigan sees the market expanding to encompass non-semiconductors such as sockets and connectors.
"New markets are constantly emerging based on the capability of placing a spherical object onto a relatively flat surface to create a connection, such as those found in wafer-level and flip-chip devices. The Customer's Point of View From a customer's point of view, says STATS' Yoon, the basics remain unchanged, and are focused in the areas of high first-pass yield, especially when moving to finer-pitch ball placement, lower cost of ownership and quick changeover time. At User Unitive Electronics Inc., Triangle Research Park, N.C., Dan Mis says input and output wafer cassettes should be compatible with standard automated wafer-handling equipment (pitch and form). Mis, Unitive's vice president of engineering, notes that output cassettes populated with balls in tacky flux pose a handling risk. Wafers, he says, "should be kept somewhat horizontal during transport to the next operation and the cassette should ensure that a wafer cannot slide out if jostled." Mis also feels that placement and inspection should be able to handle any ball pattern, including unique patterns for test structures. A defect map file should be generated to guide later rework for any detected defect. "Placement accuracy of flux or ball to pad," says Mis, "should be within 20µm. Mounting head loading/correction routines (missing ball or excess ball) should perform with minimal errors."
Mis observes that mount head errors take place more often with smaller ball diameters and with soft, easily deformed balls, such as those containing 95 percent lead. Pattern Recognition As our vendor table shows, virtually all manufacturers employ AOI based on 2D pattern recognition. "An AOI scheme based on an angled reflection will vary as the ball surface texture varies," says Mis. "AOI that is capable of distinguishing the outline (circumference) of the ball would be less affected by texture and might improve position detection." Most solder ball placement machines follow a fairly standardized process, although there are subtle and not-so-subtle differences in the way they operate. Malaysian assembler Carsem has been evaluating a system that employs screen printing, allowing the placement of thousands of balls onto a BGA substrate in a single stroke, says Lily Khor, R&D advanced packaging man-ager, Ipoh, Malaysia. This system, says Khor, houses solder balls in a cartridge in the screen printer head. Stencils with openings are placed at the intended ball location and used as a guide to place the balls." This method, adds Khor, "is deemed to be more flexible as every change of design only requires two stencil changes-one for the solder balls and one for the flux." Conclusion For solder ball diameter, how small is small? RVSI's Carrigan put that question in better perspective: "During my tenure in the sphere placement industry, sphere sizes have plummeted from a 0.76mm diameter to a recent request to place 0.10mm spheres." * Application Note-0798, "Automated Solutions for Package Level Solder Ball Attachments," Tessera, San Jose.
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