Figure 1. The Flip-Top™ BGA Test Socket from Advanced Interconnections offers a compact testing solution for 1.0 and 1.27mm BGA or LGA devices without external hold-downs.
Burn-in Test Socket Challenges
By Gail Flower, Advanced Packaging
This article provides a broad review of the issues affecting socket usage: lead-free challenges, finer pitch adjustments, cost control, standardization, practical customer concerns, and improvements needed for 3D packages and other innovations on the horizon. Through conversations with industry experts, we explore a few common themes from this year's Burn-in and Test Socket Workshop (March 8 -11, 2009) in Mesa, AZ.
The life of a socket begins when designer sits in a workstation modeling a finite element analysis for the force required to make stable electrical contact between the socket used in test and the device under test (DUT). The designer takes into consideration the information at hand: pad pitch, pad composition, and I/O count. The designer then refers to past data, and, depending on this, the hit-or-miss approach continues until the right results are obtained. The design represents the first step in a series of challenges to modern socket production.
Lead-free Changes
When lead-free was added into the mix, the designer's job became more complex. RoHS mandates became effective on July 1, 2006, and from that point on sockets needed to be able to test lead-free components made with harder, more forceful contacts. Unlike tin/lead 63:37 solders, the historical data was not applicable to new contacts. Variation in probe tip geometries, spring parameters, DUT metallization, and design differences had to be taken into consideration.
Electrical, mechanical, and cost of ownership remain the constant concerns of test hardware engineers, but the hardness of lead-free materials such as nickel-palladium-gold can wear contact pins quickly and transfer excessive force to load boards, adding to maintenance and replacement concerns. One solution mentioned is to use a crown tip on a spring pin to balance off the hardened stress of lead-free materials. As more force is required to make solid contact, wear issues become more critical.
A lead-free oxide-rich matt tin solder causes increased and variable resistance. Matt tin solder builds up on contact pins, causing a fall in yields. Frequent cleaning solves this problem, but causes a drop in throughput.One manufacturer designs and builds spring probes for lead-free testing using new alloys, hardening procedures, and surface finishes, as well as different test socket engineering design sets and mechanical properties (geometries, force, bias, internal pin capture). Next, they validated changes through testing using modified IC test handlers and pin cycling machines with test coupons plated with different lead-free finishes. They test with customer applications using lead-free devices.
Though some problems with lead-free have been solved, questions still remain. For example, some customers are still experimenting with different lead-free BGA formulations with varying ratios of tin, silver, and copper. However, the quality of the interconnect is still in question. It is clear that there is not "one" solution to lead-free. For instance, an optimized contacting solution for a pure-tin IC device lead is not the same for a nickel-palladium-plated device. Some probe styles, as mentioned earlier, prove to be suited to hardened connections, depending on the device metallurgy and the way contact is applied in a test environment. Research continues as lead-free contacting solutions are still being developed and optimized.
At 2008 BiTS workshop, Nick Langston Jr. presented "An Examination of the Causes of Cres Degradation Which Affect the Life of a Test Socket." He looked at the SAC alloys 105, 305, and 405, the lead-free formulations most often associated with BGAs. SAC 105 is easy to use because it requires little transition from SnPb. However, Au-Sn intermetallic compounds form at Au/snAgCu interfaces, and these cause Cres degradation, which in turn affect the life of a test socket. Other problems that lead-free introduces is the need to use a stab-like contact with a "crown" headed probe. The need to clean contact pins repeatedly is another necessary issue with lead-free.
Each package type and terminal geometry (solder ball and pad size) introduces its own set of variables to be taken into account. The number of solutions matches the number of lead-free solders (NiPdAu, NiAgCu, matte Sn, etc.). Solutions must consider all variables: package type, test application, terminal type, terminal geometry, device type (digital, RF, high-speed digital, high-power).
The Standards Issue
What standards are needed for sockets? In a user's eyes, the industry has done a poor job establishing standards. Measurement methods for electrical signal integrity and insertion loss should be standardized. Suppliers should know how customers will use their sockets. If a supplier says that one million cycles represent socket life, what does that mean exactly? Both supplier and user should use the same approach to determine a number for socket life.
Pin-life specifications need to state resistance level at insertion count and test conditions. In many testing cases, once pins exceed 100 mΩ or so of internal resistance, they are worn out. In others, the test can tolerate even 1 Ω of pin resistance, so a specification that states a statistically reasonable resistance level at X number of insertions would be much more illustrative of the useable pin life.
Traditional IC packages have been quite standardized already to some degree. Leaded packages (2- or 4-sided) already have very standardized pitches: 1.27mm, 1mm, 0.8mm, 0.65mm, 0.5mm, 0.4mm, for instance."Each leaded package generation pretty much is just a scaled-down version of its predecessor," said Valts Treibergs, R&D engineering manager, Everett Charles Technologies – STG, a St. Paul, MN-based company. "Socket suppliers have had the option to use traditional contact designs and manufacturing processes to build test and burn-in sockets. Sockets suppliers tried to squeeze every penny out of the design to compete on cost, and squeeze the socket outline to maximize density on burn-in boards (in the case of memory burn-in). All-in-all, sockets for leaded packages are basically all the same, and this results in a commodity socket battle."
IC packages for area arrays also started out in a very standardized format. "BGAs, and LGAs, and PGAs (to some extent) followed the same pitch progression as leaded packages; however, due to the array format, had a much higher I/O density," said Treibergs. Socket designers responded with a series of contact solutions that used complex pinching contacts, buckling beams, or expensive vertical contact solutions such as pogo pins. A handful of socket designs for these packages became the standard, and each design has found its application niche in either test or burn-in. Again, sockets for area-arrays have become commodity items, with a few basic designs testing the majority of packages.
Semiconductor manufacturers are starting to get out of the paradigm of standardized IC packages, and are exploring wafer-scale packages. Wafer-scale packages are less costly, smaller, and have better electrical performance. Wafer-scale interconnects are not, and should not, be restricted to standardized pitches, grids, or sizes. These interconnects are placed where they need to be to minimize package cost and maximize package function, Treibergs adds. This is where socket standardization breaks down. Socket suppliers need to be as flexible as chip designers. A portfolio of interconnects that can be placed at the same random pitches as the wafer scale package needs to be deployed as needed. Socket suppliers need to work together with the package designer, as well as the PCB manufacturers to provide an interconnect solution, fully integrated with the manufacturing equipment. Socket suppliers need to understand the test application: whether it it requires Kelvin testing, or high speed. Socket suppliers need to deploy solutions that use the same level of advanced manufacturing technology as the package suppliers to keep pace.
Low volumes, high-technology, high-mix, and short lead-times are the issues being faced by socket suppliers of today's advanced IC packages. Standardization in IC packages and socket outlines should no longer even be considered because this ends up being an exercise in futility.The only standardization that socket suppliers and socket users really need, are standardized test methods and characterization techniques. Unfortunately, socket suppliers may not have the bandwidth to tackle these issues. Chip makers themselves have little resources to help.
At times, there are new applications that require more that standardization allows. "Applications can be simplified by communicating a clear and complete requirements list and working with test socket suppliers to provide a customized solution." said Ann Cibelli, director of communications at Advanced Interconnections Inc. in West Warwick, RI. Advanced doesn't produce burn-in, only test sockets (Figure 1).
Reducing the board space, allowing device test on production level boards, eliminating the need for screws through the PC board, or reducing the overall cost of test by providing a "pluggable" socket can be anything but standard.
Conclusion
Fine pitch forces some of the needed, original packages. In January of 2009, Munich-based Yamaichi Electronics introduced a QFN socket series NP506 with ultra fine pitch 0.4mm and 0.5mm in open-top design for the test and burn-in of small IC packages (Figure 2).
The NP506 socket series is distinguished by the active centering of the IC module and twin-beam contacts to ensure bonding and signal integrity. As packages get smaller or stacked higher, socket suppliers will be ready to meet these needs as well. The active centering of the IC module enables balancing of larger module tolerances for sawed as well as punched QFNs, the company claims.
Though challenges remain for burn-in socket providers, the issues of making sound electrical/mechanical connections, the need for standards, the requirements of lead-free, the cleaning requirements caused by new materials, and meeting the needs of the latest packaging designs are the issues that will be discussed at BiTs in 2009.
Figure 2. Yamaichi’s 0.4 mm QFN NP506 – 30grb burn-in socket matches newer, smaller package types
