IMAPS meeting Report

12/01/2001

Innovation alive and well at IMAPS 2001

The exhibit floor at the IMAPS Symposium was not exactly packed, but most conference attendees thought that the turnout and content made it worthwhile. There were a few no-show exhibitors and some papers were withdrawn because of travel restrictions, mainly among international presenters. While walking the floor, though, you would not necessarily guess that the industry was in a major slump and in an environment of worldwide turmoil.

Optoelectronics Still a Key Focus
In spite of the current plight of the optoelectronics market, there were several innovative new products targeting those applications. Lambda Technologies (Morrisville, N.C.) displayed its latest microwave curing technology, in which the microwave energy can be focused even better than before, to heat just the area needed. This is a particularly useful capability in optoelectronic devices, which are likely to have components that are more susceptible to damage from temperature excursions and have tighter tolerance requirements.

Benchmark International (Goffstown, N.H.) had a new piece of equipment for sealing packages, with optoelectronic devices being one of the main target markets. The new equipment included better power delivery and linear encoders to make the sealing process faster and cooler; this helps to keep optical fibers in a package below the Corning temperature specification, according to Ron Hemond of Benchmark. It also has a laser system for sensing the location of the device to be sealed, and it has better force accuracy for a higher yielding process.

Creative Automation (Sun Valley, Calif.) showed a digital dispensing system, and one advantage discussed by VP Gary Helmers was better control of an underfill process. By programming a variable density of dispensed dots along the edge of a device that needs underfill, it is possible to control the flow of material very accurately under the device to get a uniform layer of underfill.

Sonix (Springfield, Va.), a manufacturer of acoustic imaging products, was happy to report on a production application of its products, which are more typically used in failure analysis labs. According to John Goings of Sonix, wafer bonding processes still need full screening, so its equipment has been purchased for production uses in which the quality of the bond in bonded wafer pairs is evaluated with acoustic imaging.

New Interconnect Processes
In the interconnect area, Potomac Photonics (Lanham, Md.) and Superior Micropowders (Albuquerque, N.M.) had an impressive video demonstration of their laser direct deposition process. The laser helps to deliver powderized metal to precise, programmable locations on a substrate. It is conceptually similar to spray painting, and it offers advantages of avoiding masks for interconnect deposition.

Tecan Components Ltd. (Weymouth, UK) was on hand to discuss its precision metal processing capabilities. An obvious application of its precision etching process is etched leadframes, but it also makes masks for wafer bumping. The tolerances, even on 300-mm wafers, are easily within the current requirements, according to Noel Cherowbrier of Tecan. A more intriguing capability from Tecan is found in photoelectroforming. This is similar to electroplating, and one application is the creation of a male tool for imprinted interconnect. The tool makes very fine dimension imprints in a dielectric material, and then a conductive material can be put into those imprints, creating very fine pitch interconnect traces. This imprinted interconnect is a fundamentally different method of manufacturing - similar to stamping CDs - and it could result in cost structures that move fine pitch interconnect into very cost-sensitive applications.

Indium Bumps for Cyrogenics
In a presentation entitled "Indium Bump Bonding for Cryogenic Applications," Allen C. Keeney of Johns Hopkins

University Applied Physics Laboratory (Laurel, Md.) reported the development of a unique indium bump plating process for creating the pad geometries needed for flip chip assembly techniques. This process is prompted by the use of advanced imaging detectors and high-speed computing systems that sometimes require operation at cryogenic temperatures.

Indium bump bonding was chosen because of the high ductility, high malleability and conductive properties of indium at cryogenic temperatures. For both the detector and the ASIC wafers under testing, the base metallization of 500 Å of titanium-tungsten and 1 or 2 µm of gold was sputtered onto oxidized wafers and patterned using a liquid positive-working photoresist. Indium bumps as large as 23 microns high were successfully created using the electroplating technique developed at the Applied Physics Laboratory. The resulting parts were successfully flip-chip bonded but experienced some significant alignment errors. Ongoing work will aim to address repeatability and the improvement of alignment.

Developments in Lead-free Solder
The successful reduction of lead use in electronics is always a hot topic, and that was surely the case at IMAPS. In a presentation entitled "Wetting Characteristics of PWB Finishes with Pb-free Solder Alloys," R. Wayne Johnson of the NSF Center for Advanced Vehicle Electronics at Auburn University (Auburn, Ala.) evaluated the compatibility of lead-free printed wiring board surface finishes with lead-free solder alloys. The alloys used were Sn37Pb, Sn3.4Ag4.8Bi, Sn4.0Ag0.5Cu, Sn3.5Ag and Sn0.7Cu; the finishes tested were NiAu, Sn, Ag and OSP. Extensive testing indicated that Ag is a rather stable finish. It was notable that the SnAgCu components used (1206 chip resistors) performed worse than tin-lead, and that Ag and NiAu are the best finishes when multiple reflows are required.

Yasushi Moriwaka of Advanced Products Company (Tokyo) presented the development of super ultra low alpha (SULA) tin. These lead-free, tin-based solders have ultra low alpha ray levels at the detection limit (approximately 0.001 cph/cm2) and no time variation of alpha ray flux. This development is significant in that when flip chip mounting, alpha rays are apt to cause soft errors. SULA materials are available as solder balls, solder paste and liquid plating solution.

Advances in Flip Chip Technology
The session on flip chip technology, which was perhaps the best attended session at the entire symposium, was opened by E. Jan Vardaman of TechSearch International Inc. (Austin, Texas), who provided an overview of flip chip market trends. The potential expansion of flip chip into the mid-range pin counts will require technology improvements in the infrastructure and a maturing of the industry. Improvements that have been achieved in the past four years include the decline in wafer bumping costs (due mostly to an increase in the number of bumping foundries), and the introduction of new underfill materials with shorter cure times and improved properties. Vardaman noted that consumer products, such as watch modules, still represent some of the highest volumes for flip chip applications, and that the major remaining challenge is the widespread commercialization of cost-effective testing at the wafer level.

A subsequent presentation by Scott Popelar of IC Interconnect (Colorado Springs, Colo.) addressed the advantages of incorporating electroless nickel/ immersion gold under bump metallurgy (UBM) and solder paste stencil printing into flip chip wafer bump technology. Tests indicated compatibility of electroless nickel UBM with 63Sn/37Pb, 90Pb/10Sn and 95.5Sn/3.8Ag/0.7Cu.

Yet another presentation that coupled the hot topics of flip chip technology and lead-free solders was "Lead-free Solder Bump Technologies for Flip-chip Packaging Applications" by Jim Martin of Shipley LLC (Marlborough, Mass.). Martin's group demonstrated the successful fabrication of five different types of lead-free solder bumps: pure tin, tin-bismuth, tin-copper, tin-silver and tin-silver-copper. The method of fabrication for all alloys was by electroplating using a plated copper under-bump metal. Results indicated that the application of a suitable barrier diffusion layer, such as nickel, is necessary to limit the excessive formation of tin-copper intermetallic compounds.

AP