Multiple Area-Array Devices Challenge Wafer Probing

The total cost of test is the overriding consideration driving changes in probing technology. Memories are not the only devices being tested; but because their pricing is so competitive, test costs must be controlled closely. Dataquest’s recent DRAM cost model estimates that 16-Mb DRAM manufacturers spend about $1 per good device in test and packaging, but that 64-Mb DRAM manufacturers spend nearly $5.1

As memories get larger, they require only a few more connections, but their test time goes up linearly with density. According to Mark Brandemuehl, director of probe card marketing at FormFactor, “Probe test times for DRAMs have increased to minutes per device per wafer touchdown with the 64-Mb generation and will exceed 10 minutes for early 256-Mb devices. The only way to counter this trend is to test a higher number of devices per touchdown.”

Similarly, the need for simultaneous, multiple-device testing is the answer if you consider die shrinks that are routinely performed to increase die per wafer. “The 64-Mb DRAMs shipped early in 1997 were 10 mm × 19 mm, and only about 150 fit on a wafer,” continued Mr. Brandemuehl. “By early 1999, a 64-Mb die will be less than 8 mm × 5 mm, and more than 1,000 will fit on a wafer.” To take advantage of increased die per wafer efficiencies, more testers must be bought, or more die have to be tested in parallel.

Contacting up to 64 devices simultaneously also means that the usual few grams force per contact will add up to tens of pounds. Peter Andrews, senior product manager at Cascade Microtech, said, “A high force option for our 11000 and 12000 Series Probe Stations supports a total loading of 44 lb with minimal deflection at the edge of the wafer. A high-density probe card with up to 2,048 contacts typically exerts a loading of 27 lb at 6 gm per contact.”

In addition, the thousands of connections that must be made between the probe card and the prober interface board will strain the present spring-contact pin (SCP) approach. These contacts are arranged in concentric rows around the periphery of the two circular boards. More than 4,000 connections through SCPs could exert 600 lb force on the probe card.


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What Technologies Are Available?



The state of the wafer probe needle business is aptly described by Mark Twain: “Reports of my death have been greatly exaggerated.” Even 10 years ago, reports that conventional needle probes were obsolete, dying, or about to be replaced by another technology were common. Yet today, they still are used by the millions.

“Advanced Probing Systems has been manufacturing probe needles for almost nine years,” said Michelle Gesse, president. “During that period, people have frequently intoned the death knell for cantilevered probe cards. Originally, their demise was supposed to result from the introduction of membrane probe cards.”

But needle technology has continued to evolve through increased taper consistency, smaller diameter probe wire, and material enhancements. For example, some silver-plated needles reduce the skin effect at 1 GHz by about 50% compared to standard nickel plating. The 0.2-mil thickness of the plating also increases the DC current-carrying capacity of the needles by about 20%.

Probe needles are manufactured from 5-mil- to 15-mil-diameter tungsten, tungsten-rhenium, Paliney®, and beryllium-copper wire in lengths of 0.75″ to 3.0″. Tips either are sharp or radiused (Figure 1). A polymeric coating increases needle-to-needle isolation and improves adhesion to the probe card epoxy ring.

In addition to improvements in the needles themselves, probe-card innovations have led to tighter pitches and larger numbers of contacts. Karen Lynch, vice president of marketing at Cerprobe, said, “Products with approximately 2,000 needles are being shipped to customers testing 32 DRAMs in parallel. This proven technology is very cost-effective.”

Contact spacing down to 2.6 mils is available for in-line applications and down to 1.6 mils for staggered pads. High-temperature probing also is possible using new composite adhesives that maintain dimensional stability at 125°C.

And, it’s simply not the case that cantilevered probe cards are limited to 100-MHz applications. The average bandwidth of Cerprobe’s present ECHO™ series of epoxy ring probe cards is 1.5 GHz. High-frequency performance has been improved by better card layout and ground planes. Stringent trace and ground routing requirements can result in probe cards with more than 32 layers.

“Cantilevered probe cards remain a viable, cost-effective, and proven method of test,” noted Ms. Gesse of Advanced Probing Systems. “Continued development has contributed to the capability of cantilevered probe cards to be used in fine-pitch situations unimaginable years ago.”

With the manufacturing variables well under control, epoxy-ring cantilever probe cards have become a mature technology. Suppliers now are improving their capability to address the special needs of their customers. Many are diversifying into other types of probing solutions as well.

Flex Vertically or Horizontally?

IBM has been using area-array packaging internally for more than 30 years. Because complex ICs require many contacts, and fast, high-power devices also have multiple power and ground connections, area-array connection schemes now are used in the semiconductor industry. The arrangement of contacts, such as flip-chip solder bumps, is not constrained to having pads only around the periphery of the chip. Instead, pads are placed in a grid pattern within the chip area.

In 1977, IBM patented the COBRA Vertical Probing System to test its own devices.


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The technique uses a number of parallel spring wires mounted in guide plates at either end. In operation, the wires are compressed between the probe card and the wafer-under-test. With sufficient force, the wires will buckle slightly, creating a spring action and a small scrubbing motion.

Conventional cantilevered probes develop contact force by flexing the relatively long, horizontal shank supporting the probe tip. With an overtravel on the order of 3 mils, the spring constant of the material and the length of the shank combine to produce a force of a few grams. Probe geometry and overtravel determine the scrub distance.

In spite of the proven benefits of cantilevered probes, area-array packaging is a barrier to their continued use. It is almost impossible for cantilevered probes to contact all of the pads reliably on multiple area-array chips. Multiple-device cantilevered probe cards are multitier assemblies that have high test and repair costs because of their mechanical complexity.

Cerprobe’s version of vertical probing, shown in Figure 2, provides >2,000 probe capacity and contacts 2.4-mil pads on 5- to 6-mil spacing. “Our vertical probe technology has checked densely populated ASICs with flip-chip bumps,” noted Ms. Lynch. “It also has tested 32 microcontrollers in parallel as well as several memory devices in parallel.”

Vertical probes are relatively robust because of the guide plates located near each end of the wires. However, in their present form, they are limited in bandwidth much as are cantilevered probes. In addition, a vertical probe system does not reduce contact density immediately as does an epoxy ring probe card because the vertical wires are parallel.

A multilayer ceramic space transformer at the end opposite the DUT forms the interface between dense contact spacing and more generous PCB track spacing. Currently, the ceramic assemblies are custom-designed for each application. Their cost and long delivery times are major drawbacks to vertical probing.

Are There Other Options?

Membrane probes were developed as a higher bandwidth, self-aligning alternative to cantilevered needles. “Pyramid™ Probes, Cascade’s patented version of membrane technology, provide frequency performance as high as 20 GHz and inductance down to 0.2 nH, explained Dean Gahagan, probe card design manager at Cascade Microtech (Figure 3). “Bypass capacitors can be placed within about 0.25″ of the DUT.”

Against these advantages, present Pyramid Probes are limited to 512 contacts and are not well suited to probing multiple DUTs. However, a recent U.S. government Advanced Technology Program (ATP) award may change this situation.

According to Mr. Gahagan, “The award will help us design and demonstrate large-area membrane probes for parallel testing of ICs, arrays of chip-scale packages, and high-density interconnect substrates. Then, customers can simultaneously probe up to 6,000 contacts and 4,000 I/O lines.”

Another type of probe that resembles cantilevered needles but uses multilayer flexible circuits is the P4™ probe (Figure 4). P4 stands for photolithographic pattern plated probe, a technology being developed jointly by Cerprobe and Mitsubishi Materials. Each quadrant of a conventional epoxy-ring card is replaced by a single flexible circuit comprised of ground-plane, insulating, and nickel contact layers.

A paper presented at 1998’s ITC documented the performance of the P4 after >1M touchdowns. As with membrane probes, the flexible, multilayer construction supports microstrip 50-W lines right out to the probe tips. A bandwidth of 4.23 GHz was achieved even though this assembly was optimized for high pin-count rather than high bandwidth.


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No Technology Is Perfect


A Cerprobe application note compares each of these technologies and concludes that all fall short of the ideal.


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Epoxy-ring cantilevered needles are well established and competitively priced and accommodate single or multiple medium-speed memory DUTs having peripheral contacts. They don’t handle area-array chips well, nor is the bandwidth as high as other technologies.



References

1. Khandros, Dr. I. et al., “New Methods for Reducing Costs in Semiconductor Back-end,” Semicon, Japan 1998, p. 2.

2. Anderson, J., “Integrated Probe Card/Interface Solutions for Specific Test Applications,” 1998 IEEE International Test Conference Proceedings, pp. 282-283.

3. Taber, F.L. Jr., “An Introduction To Area Array Probing,” 1998 IEEE International Test Conference Proceedings, pp. 277-281.

4. Ishii, T. and Yoshida, H., “Fine Pitch (45 Micron) P4 Probing,” 1998 IEEE International Test Conference Proceedings, pp. 272-276.

5. “Technologies Compared,” Cerprobe Application Note MFN 216, November 1998.

Wafer Test Products

Thin-Wafer Prober

A thin-wafer handling capability now is available on the Horizon 4090m Prober. The enhanced prober uses a patented Smart Mapping wafer map system. Fiber-optic through-beam sensing detects and compensates for thin-wafer irregularities such as bowing, warping, and sagging. Wafer-transfer entry and exit points on the cassette are dynamically adjusted based on wafer position. The complete system handles both standard and thin wafers automatically and accommodates standard wafer carriers, eliminating the need for special cassettes or extra transfer steps. Call company for price. Electroglas, (408) 727-6500.

Flexible Probe Station

CheckMate CM-112 Series Probe Stations can be configured as basic manual or fully automated systems. Options include cassette-to-cassette wafer handling, wafer prealignment, and auto loading. The probe stations can be upgraded in the field from 200 mm to 300 mm wafer stages and from manual to automatic. Stage movement upgrades to automatic can be done incrementally, from manual to motorized, to joystick-driven, to computer-software controlled, to semi- or fully automatic. From $26,000. Signatone, (408) 848-2851, www.signatone.com.

High-Density Probing Solution

ViProbe® (Vertical Integrated Probe) wafer probing provides contact to area-array or flip-chip solder-bump devices. The technology is licensed by Feinmetall GmbH and supports more than 2,000 probes in multiple-DUT configurations. A buckling-beam design ensures that contact pressure on every pad is independent of overdrive. This minimizes the effect of planarity variations, specified as <1.0 mil. Capabilities include a minimum pad size of 2.4 mils, a 5-mil pitch for aluminum pads and 6 mils for solder bumps, a ±0.6-mil alignment accuracy, and overdrive from 3 mils typical to 6 or 8 mils maximum. Operation to 125°C is optional. From $10,000. Cerprobe, (602) 333-1500.

Membrane Probe Card

The Pyramid Probe Card installed in a Hewlett-Packard HP 84000 Tester performs repeatable on-wafer RF measurements of a wireless device. The combination of card and tester supports RF measurements up to 18 GHz in a production environment. On-wafer RF testing reduces costs of chip on-board and multichip modules by ensuring that only known good die are used. The two-metal layer, thin-film technology at the heart of the Pyramid Probe provides controlled impedances to 20 GHz. Power bypass and other critical components can be mounted within 0.25″ of the DUT. Call company for price. Cascade Microtech, (800) 550 3279.

Coated Cantilevered Needles

Epoxy ring-probe needles are available with a solvent-resistant, insulative coating. TIP-M™ (thermoset insulating polymeric material) applied directly onto the surface of an etched needle probe eliminates wicking of liquids such as solder flux between the probe and a sleeve. Previously, plastic sleeves were applied manually, but these are considerably larger in diameter than coated probes and cost much more to apply. In spite of the coating’s durability and adhesion, it can be removed easily with a hot soldering iron. $0.15 to $0.20 per probe. Advanced Probing Systems, (303) 939-9384.

Spring-Contact Probe Cards

Custom, multiple-device probe cards are compatible with aluminum and gold pads and with both high-lead and low-lead solder balls. Up to 64 C4 (controlled-collapse chip connection) devices may be probed simultaneously. More than 3,000 I/Os and operating frequencies >500 MHz are supported. The probe cards provide a bandwidth >1 GHz, controlled scrub length, and reliable operation for 100,000+ touchdowns. Each contact is formed on the end of a small MicroSpring™ spring. From $20,000. FormFactor, (925) 294-4300.

Copyright 1999 Nelson Publishing Inc.

March 1999


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