High-Performance Socket Requirements for Chip-Scale Packages

Increasingly complex package assembly and test tooling, the market entry of the Rambus memory interface and the finer pitches demanded by today's CSPs are creating new challenges for suppliers of IC test and burn-in sockets.

By Dr. Rao Mahidhara, Technical Editor

Not surprisingly, the chip-scale electronics market has generated new and demanding challenges for advanced IC contactor technologies. The growing cadre of conventional test and burn-in socket suppliers has been scrambling to keep up with the lightning fast growth of CSPs. Now, as chip makers line up to offer the Rambus memory device interface, socket designers are rushing to market Rambus-compatible sockets. Small, smaller, smallest—is the battle cry from CSP users—and that means sockets that "fit" the package—not only in physical size but in sizzling performance, too. Against this background, there are more than 20 companies developing high performance contactors for CSPs.

Innovative socket design requires careful attention to thermal, mechanical and electrical issues to minimize socket cost. Reducing the real estate occupied by the socket is also an imperative so that burn-in board density is maximized.

PC Board Requirements

The increase in the number of I/Os for CSPs, from 48 balls to 240 balls in CSPs, is driving future test socket technologies to tighter pitches. Look for pitch to decrease from 0.5 mm to 0.4 mm and cost less.

For higher I/O devices, particularly, the trend towards finer pitch arrays is likely to accommodate die shrinks without changing the footprint of the I/O. In addition, the body of the test socket is also being minimized in size to compete for board space, according to Frank Dudinski of Yamaichi Electronics.

To reduce socket body size, socket suppliers are currently trying to shrink their current test technology from a contact pitch of 0.5 mm to 0.4 mm, through the development of a new interconnect concept involving sequential laminations of multilayers.

Competition for board space is bound to increase the interest in CSPs. Doug Kocher of AQL suggests that high I/O count CSPs are also creating challenges for the DUT board manufacturers, because mechanical drilling aspect ratios of 10:1 limit DUT board thickness. The concentration of the mechanical force, meanwhile, will flex the DUT around the test socket.

Pre-planning for this eventuality is critical. While adding a daughter board to the DUT board solves some of the manufacturing problems, the added board also degrades the electrical signals. New concepts for integrating from the test socket to the test machine must emerge to eliminate the number of connections required.

While conceptualizing future test sockets, the industry must also address the physical size, manufacture ability and cost of the DUT board.

Cost—a Major Concern

Semiconductor manufacturers will have to reduce the price of CSPs to make them more attractive, since additional capital investments are required by assembly houses to handle this format. Recent observations by Loranger International, a socket supplier, suggest that CSP socket demand grew by 282% last year, while BGA socket demand only doubled from 1997.

Because CSPs offer an improved electrical environment for ICs, device testing will become more exacting, and contactor performance will have to improve to keep up, suggests James J. Brandes of Prime Yield Systems. Impedance characteristics—capacitance, inductance and contact resistance—as well as path length—will have to be reduced. Overall electrical performance will have to become more transparent to avoid filtering out test signals.

Several fundamental issues which challenge socketing are apparent in CSP testing, according to Bruce Rogers of OzTek. These include die shrink, RF and high-speed devices and long test times for memory.

Mechanical Improvements

Just a few years ago, virtually every test socket used some form of bent metal. However, due to current electrical requirements and pitch, this technology is probably no longer useful, says AQL's Douglas Kocher. Soon, OEMs and users testing CSPs will look for significant mechanical improvements requiring a consistent force adequate to penetrate oxides—yet light enough to leave the solder balls undamaged. Newer contactor technologies, such as elastomers, spring probes and interposers have emerged as possible solutions.

Figure 1. Liberty Research Socket

The trend toward fine pitch BGAs, with pitches of 0.75 mm or higher, is also driving low-cost, open-top, zero insertion force designs for high volume production. Nick Langston of Liberty Research says test and burn-in sockets utilizing Pogo Pins are barely meeting the needs of high-performance testing. The limitations of Pogo Pin strip sockets include their inability to test at operational speeds. The pins also leave "witness" marks as they leave the solder balls.

The Rambus revolution has created an urgent need to test Rambus 64Mb and 128Mb RDRAMS at data rates equivalent to 1.6Gb/sec. ATE manufacturers have re-sponded by developing equipment that can test and handle 32/64 devices in parallel.

Because of the enormous ATE test costs, long test times and the high volume of parts, CSPs are an ideal format to test in strip form; for now, however, they are tested largely in singulated form.

As single packages, CSPs are rather susceptible to ball deformation at temperatures above 100žC, when forces above 15-20 grams are used. Therefore, new technologies are being developed that can make electrical contact with the balls using as little force as 10 grams.

Two promising contact technologies, according to Liberty Research, are isotropic conductive polymers (ICP) and pressure conductive silicon (PCS). The ICPs offer compatibility with very fine pitch packages (~0.15 mm pitch) as well as excellent thermal characteristics. The PCS, on the other hand, employs very low force for contact (~10 grams)

Synergetix, according to Joe Bunch, is introducing a chip-scale probe that provides a signal path length of .090" and a diameter of .013"—for devices such as Rambus—to push the test envelope over 1 GHz, where the need exists for ultra-small contact technology. This probe can be utilized for area array packages and peripheral packages requiring high performance testing, low contact resistance, inductance, capacitance and contact forces.

High-performance contactors for volume production test differ dramatically from general-purpose sockets that might be used for burn-in. AQL's Kocher envisions technologies customized for different applications. By separating test and burn-in, the attributes of the elastomers and/or spring probes can be used without the negative aspects.

Due to unique electrical requirements during testing, the leading socket technology today is based on a spring probe that offers good compliance, operates at the highest bandwidth and can be purchased in quantity for 0.5 mm pitch. The small, compliant probe used by AQL has been tested at 15 gigahertz in a GSG configuration. In addition, the probe offers 0.040 of compliance with less than 30 grams of pressure.

Handling Systems

Figure 2. Loranger Socket

Test handling systems are quickly moving toward testing 32 sites simultaneously, according to Synergetix' Bunch. Multiple site testing consists of individual sockets mounted side-by-side with several device sites designed into a single socket. For strip testing, contactors can test multiple die on a strip. ESD materials for test socket housings are available where static dissipation is a concern.

Socket Survey

Here's a review of several of the socket choices now available, alphabetically: AQL

The company offers two different socket technologies in its product line. AQL concentrates its design effort on test sockets for BGA packages. For most BGA-type applications, a compliant probe, which, according to AQL's Kocher, "dramatically different" in design from a double-ended spring probe is used.

The compliant probe's two-piece construction keeps the spring away from the electrical path. According to Kocher, the compliant probe has been tested by an independent lab to more than 15 gigahertz, with a bandwidth under 3dB. Life expectancy of the compliant probe exceeds 1,000,000 compressions and the part can be replaced individually.

JSR

The company's socket technology includes an easy-to-align, metal-framed pressure sensitive conductive rubber targeted for 0.5 mm pitch CSPs, according to Mary Burkhart. Gold plated particles are aligned in specific pitch columns to "cup" solder balls softly and form a Z-axis conductive path for testing.

Liberty Research

Liberty Research has focused primarily on test rather than burn-in sockets due to the tolerance of QFPs and BGAs to higher forces for short test times. For the last 14 years Liberty Research has been using Shinetsu Polymer MT Matrix elastomer with the wires embedded in the elastomer for BGA sockets (Figure 1).

Liberty also makes mainstream BGA sockets which use low force (<30 grams) and low inductance (1.5 nH) Pogo Pins. The variety of the company's Pogo Pin sockets makes them useful for testing flip chips to 0.3 mm pitch as well as strip-level testing.

For CSP devices, Liberty Research is employing pressure conductive silicone (PCS) because of the low force (<12 grams) needed for contact and the very low inductance of the contact (<0.4nH). PCS is very useful for burn-in sockets where the pitch is greater than 0.3 mm, ball size is greater than 0.2 mm, temperature is below 150žC and contact force is light.

The company also offers sockets and board adapters to enable board designers to convert high density, fine pitch sockets to easier-to-handle 1 mm connections.

Loranger International

Ariane Loranger of Loranger Inter-national Corporation (LIC) says that her company's sockets (Figure 2) are characterized by a relatively direct connection that fits within the shadow of the package ball. Other sockets, she claims, require more room due to their connection to the side of the solder ball.

Loranger claims several unique features associated with LIC's socket design. These include:

• The socket does not promote solder ball deformation and cracking.
• The force within the socket is applied perpendicular to the XY plane.
• Van Der Wall forces hold the solder ball.
• The socket has a higher contact tolerance of 0.2 mm for the package.
• The socket exhibits surface mount contact to the printed circuit board
• The socket displaces flux during contact, and
• The socket does not touch the underside of the package's solder mask.

Figure 3. OzTek HP 9500 auto-test contactor

OzTek offers a proprietary system for using existing socket hardware by employing interchangeable, package-specific inserts in a universal socket base (Figure 3). This system is unique, and can save customers thousands of dollars over the life of the socket base, claims OzTek's Rogers. OzTek, adds Rogers, is the "only company offering customers a single, integrated test interface unit." OzTek will take a customer's design requirements for a specific package and tester from CAD to CAM, to socket and board production, to assembly through continuity testing. This provides the customer with the specified socket, load board and test fixture in an integrated, tested and assembled test interface unit. The long test times associated with memory devices significantly hinder throughput in final test, Rogers notes. OzTek is providing panel test solutions that enable the customer to test CSPs in strip form, greatly increasing output. The company, adds Rogers, is also developing several wafer-level test strategies that should increase test functionality at wafer level, while increasing throughput.

Plastronics

The company offers 0.80 mm, 1.00 mm, 1.27 mm and 1.50 mm pitch sockets. Plastronics has eliminated the lid on BGA sockets which enables them to accommodate virtually any BGA package with low contact resistance.

By employing a cam handle or rocker (for automated loading and unloading) Plastronics' open-top BGA sockets offer zero insertion force loading. The ZIF open-top socket can handle more than 1,000 solder balls. All contact forces, regardless of solder ball count, are contained inside the socket and are not transferred to the burn-in board. This eliminates bowing and the resultant difficulty in contacting solder balls. (Figure 4).

Prime Yield Systems

Figure 4. Plastronics high performance socket.

Brandes says the company's contactors use anisotropic conductive elastomers to provide a short-path length, low-impedance path between the device and the interface (DUT) board. In the case of BGA contactors, including chip-scale BGA contactors, the elastomeric quality is used to provide the resiliency needed to compensate for co-planarity deviations in the device, and to push back on the device to break through any oxides on the surface of the contact. To prevent solder accumulation on the elastomer, all production designs use a contact element to isolate the elastomer from the solder ball. Prime Yield Systems is focusing on alignment techniques that are independent of package outside dimensions—a need that is becoming more imperative as the targets on the device (the solder balls) shrink without a corresponding improvement in the accuracy of the ball array with respect to the package.

Synergetix

Synergetix employs a spring-loaded contact technology which provides true vertical probing (Figure 5). Spring-loaded contacts, according to Synergetix' Bunch, offer the advantage of high reliability through several hundred thousand cycles. These contacts consist of an outer barrel and an inner spring and plunger.

The probes are gold-plated metal components and are virtually unaffected by temperature or environment, Bunch claims. A spring probe which has been designed for a test socket offers a .015" to .030" range of stroke, versus the .008" to .015" stroke of an elastomer based socket.

Tecknit

Tecknit utilizes its Fuzz Button*#153; technology as the electrical interconnect between the CSP and the PC board. Fuzz Buttons are single lengths of very fine wire, die compressed to form tiny cylindrical spring contacts. The Fuzz Buttons are then combined with miniature "Hardhat™" contact pins and placed into sockets with arrays that mirror the pads on the BGA or LGA package. The advantage of Fuzz Buttons, according to Richard G. Mellert, general manager, is their short signal path length which offers low inductance and high spring characteristics for high levels of insertion and repeatability.

Texas Instruments

TI's sockets are open-top, zero insertion, which need only a low actuation force (Figure 6). This feature, according to the company, makes them ideal for high-speed, automated load and unload.

Figure 5. Synergetix Socket

The open-top design allows air circulation around the package which reduces thermal build-up and the possibility of ball damage during burn-in due to excessive temperatures. The small socket outline maximizes burn-in-board density and process throughput. To solve the interface challenge between the tight pitch of the socket I/O and the burn-in board, TI provides a fan-out interposer. This small PC board reduces the complexity of burn-in board design and fabrication by converting the 0.75 mm socket I/O pitch to a larger diameter pin at the more conventional 1.27 mm pitch.

Sockets are delivered with the fan-out interposer soldered to the I/O of the socket to permit one-step assembly to the burn-in board. TI's burn-in sockets for chip-scale packages use individual, mechanical dual-beam "pinch" contacts that touch each solder ball independently. Protrusions on the tip of the contact pierce the oxide on the solder ball ensuring reliable contact. The contacts touch the solder balls close to the IC package eliminating ball deformation or damage in the re-flow zone. Once the package is seated in the socket, latches clamp it in position to prevent movement caused by vibration.

Yamaichi Electronics

Figure 6. Texas Instruments Socket

Yamaichi Elec-tronics expenses considerable effort to re-engineer and upgrade the design of its sockets for ease-of-use. This is visible in Yamaichi's special features, such as parallel clamp to reduce lead bending during insertion, and the use of multi-row board footprints to enable better board tracing, according to Dudinski.

Conclusion

According to a forecast published in Electronic News2 CSPs will post increases from 105 million units in 1997 to 415 million last year, and to 5.5 billion in 2001. This tremendous market growth is acting like a beacon for socket suppliers to improve existing products and develop new ones.

References

R. Crowley, "Socket Technology: Meeting the Needs of Fine Pitch CSPs," Advancing Microelectronics, Vol. 24, No.6, November/December 1997, p. 21.

B. Levine, "The Year of the Chip Scale Package," Electronic News, January 5, 1998, pp. 62.