Spring Contact Probe Simplifies IC Device Testing

The spring contact probe—a technology originally designed for PC board testing--provides extremely linear and reliable contact force and electrical performance, even when used in temperature extremes from -50°C to 150°C.

By Tim Dowdle and Kevin Koontz, Synergetix, Kansas City, Kansas

With the number of test socket companies growing rapidly, the test engineer has an increasingly difficult task to sort through the clutter of information. Current test socket manufacturers utilizes a half dozen or more core technologies, with several variations of each technology. One technology, spring contact probes, has moved to the forefront for test sockets (Figure 1).

Disproving the Myth

For many years, conventional wisdom has argued that spring contact probes did not make optimum test socket contact due to their long signal path length and high inductance.

Figure 1. General test socket.

Yet, with the lack of available standard test sockets for the wide range of new devices being introduced, test engineers have been forced to build their own sockets by borrowing a readily available technology—the spring contact probes originally designed for PC board testing. The PC board spring probe has demonstrated high reliability over decades of use in contacting test points on bare and loaded boards in dirty environments. This probe has also proven to be successful for testing IC devices when the lead pitch was .050"-.100" and test speeds were below 40 MHz.



The PC board probe, however, was close to 1" in length and would not support the testing requirements for the new generation of fine-pitch, high-speed devices, such as BGA and CSPs.

The miniaturization of today's devices with lead pitches as small as .25mm (.010") and test speeds well above 1GHz, has challenged the expertise of the spring probe manufacturer to develop a series of contact designs to meet the industry's ultra-fine pitch requirements.

By reducing the size and length of the spring contact probe, it becomes essentially transparent in the electrical test path and maintains low and consistent contact resistance.

Spring Contact Probe Features

Spring contact probes utilize a true vertical probing motion. The probe can be placed directly below the lead of the device, allowing the device-under-test board design to be a true reflection of the device lead pattern, which greatly simplifies board layout. Spring probes are retained in test sockets with simple drilled holes and offer unlimited capacity to customize the socket for specific applications.

The traditional double-ended spring contact probe consists of an outer barrel, an inner spring, and two plungers, all of which are gold plated. To create a self-contained contact assembly, the outer barrel is crimped around the plungers. The spring is designed in such a way that it creates a "biasing" effect inside the barrel, forcing the plungers to stay in intimate contact with the inner surface of the barrel.

Using this biasing feature, current flow follows a plunger-barrel-plunger path and keeps the spring out of the conductive path. Hundreds of thousands of flawless cycles can be achieved with this style of contact, when manufactured to exacting standards.

Reduced Cost of Ownership

The use of individual self-contained contacts simplifies field replacement of a single contact or an entire array when refurbishment is required. High cycle life and easy refurbishing capability reduces the overall cost of ownership.

The spring contact probe offers a full range of features, permitting the same test socket to be used in automated test, engineering characterization, and custom burn-in.

Design Flexibility and Quality Management

Key elements involved in the development of a new contact technology include designing the contact for the specific IC test application while maintaining a cost effective manufacturing process.

Figure 2. Probe comparison Most socket manufacturers are dependent upon purchasing spring probes from an outside supplier. Synergetix designs and manufactures spring probes in-house. This enables us to take advantage of two decades of experience in designing and mass producing spring contact probes to develop sockets for specific high performance uses.

Device manufacturers are developing new CSP, BGA, LGA, and peripheral packages at a dizzying pace. Due to the variables involved in each package design and test methodology, a single spring contact probe will not be effective in every application (Figure 2).

For instance, a BGA device with a .5mm diameter solder ball on .8mm pitch, may use a spring contact probe which differs from a BGA device with a .75 mm diameter solder ball on 1.27 mm pitch.

Obtaining proper ball fit when the apex of the ball is not to be contacted, the .75 mm diameter ball must use a probe tip that is approximately .75 mm in diameter. The probe tip will utilize a four-point crown (or other tip style) to push the contact area away from the center of the ball. This will allow the probe tip to contact only the outer periphery of the ball, avoiding all contact with the central keep-out area (Figure 3). The .8 mm pitch device would, consequently, use a probe optimized for its specific ball diameter.

Optimal Device Test Conditions

The principle reason for the different spring probe designs in this example is the lead pitch and ratio of solder ball diameter to the diameter of the contact tip.

The use of varying probe tip diameters and spring forces to achieve proper ball fit and an optimal testing condition for each device under test is imperative. A probe tip that is too large may overwhelm a smaller solder ball and possibly strike the bottom of the device without correctly seating on the ball.

Factors involved in the selection of the contact are lead pitch, lead type, solder ball diameter, number of leads, available insertion force of the handling system and rigidity of the DUT board. These variables must be analyzed before a proper contact can be developed.

High contact forces are becoming a critical concern for BGA devices which are being developed with as many as 4000 leads, and with handling systems testing a higher number of devices during a single actuation.

Figure 3. Witness mark

The contact force of the spring probe can easily be redesigned to avoid extreme forces in the handler and minimize the pressure against the devices and the DUT board, while maintaining the electrical and mechanical integrity of the socket. Many device manufacturers are now requesting a single point on the probe tip, recognizing that the light spring forces of a spring contact probe create little deformation in the center of the solder ball, and do not affect the reflow process on the PC board.

The spring contact probe also offers many variable design factors that can be customized to improve the performance of the test socket. These include spring force, tip geometry, plating type and thickness, diameter of the contact surface and overall length of the probe.

Electrical Performance

Spring contact probes are proving to be electrically superior to many other technologies. Today, test sockets utilizing this technology are characterized with less than one nanohenry of self-inductance.

Spring contact probes with a self-inductance of 0.5 nH and a signal path length of approximately .090" will be used in sockets early this year.

These diminutive contacts excel in the areas of low contact resistance and electrical repeatability. A standard design threshold of less than 70mW has been observed for most contacts of this style.

Designs under development for new spring contact technologies will drive the contact resistance even lower.

Long Life by Design

The mechanics of a spring probe are largely contained inside the probe's barrel, essentially creating a closed contact architecture.

This architecture has made spring probes an ideal choice for use in contaminated environments. Spring probes used in today's test sockets can last more than half a million cycles when manufactured to very tight specifications and properly cleaned and maintained in the handler.

Missing Ball Detection vs. Failure Analysis

In most production applications, the missing ball detection type test socket should indicate when a solder ball is missing by showing the test point as an "open" and failing the device.

This is easily achieved with spring probes by controlling the "Z" axis travel and position of the probe within the socket.

The probe will not strike the empty pad on the device if a ball is missing.In some cases, the test engineer wants to test the device for functionality, whether a ball is in position or not. Additional travel is designed into the socket to always allow the probe to strike the empty pad, partial ball, or full ball. This type of failure analysis testing is easily accommodated with the long travel of a spring probe.

Device Nests

With the introduction of many vertical-plunge, device-handling systems, test sockets with floating alignment nests have become more popular.

These device nests provide an elevated platform which allows the balls of the device to physically align in the socket before actuation against the probe tips. This helps to align the solder ball to the probe prior to making contact and can reduce off-center witness marks on the solder ball.

Floating device nests are of particular importance when the solder ball array is poorly positioned compared to the outside of the package.

Future Trends

Multiple-site handling systems testing up to 32 devices simultaneously are quickly showing up on test floors.

Test sockets are shrinking in size to accommodate the reduced footprints. Many individual sockets may be used side-by-side, or several device sites can be machined into a single socket body.

New high-speed memory devices, such as Rambus, will drive test frequencies above 1GHz, requiring the new generation of high frequency chip-scale probes to utilize a signal path shorter than .100" (2.54 mm).

Conclusion

Spring contact probes have long been the test engineer's faithful friend because of the probes' availability and versatile design.

Today, spring probes are designed with specific test socket applications in mind and offer the advantages of great electrical performance, extremely long life and flexibility in design. These attributes provide the test engineer with a reliable and cost effective solution that improves profitability.

Mr. Dowdle is an engineering manager for Synergetix and supervises a staff of engineers dedicated to test socket and VLSI interface design. He previously spent five years as engineering manager at a packaging equipment manufacturer. Readers may contact him at tim@mail.idinet.com or phone 913.342.0404.

Mr. Koontz is a product manager for Synergetix, a division of Interconnect Devices Inc. He joined the company as a design engineer developing interface and connector systems. He earlier spent 17 years in design and manufacturing engineering of aerospace and medical devices, imaging systems and thermoelectric cooling systems. Contact him at kevink@mail.idinet.com, or phone 913.342.0404.