Challenges & Next-Generation Pogo Solutions For Contacting Pb-Free Devices In High-Volume Production Test
Semiconductor manufacturers are in full swing, converting a majority of their IC packages away from lead (Pb) bearing solder to contact finishes that are more ecologically friendly, and compliant with the Restriction of Hazardous Substances (RoHS) legislation.
Traditionally, tin-lead (SnPb) solder-plated leads and solder balls have been contacted with hard gold electroplated interconnects. The gold-plated contactor interconnects have always had a tendency to accumulate solder from the IC leads, and have had issues with the formation of intermetallic layers and oxides that degrade the electrical performance of the connection. With proper application of tip geometries that provide a wiping surface, as well as adequate normal forces and cleaning techniques, the gold interconnects have proven to be reliable and economical for production testing of SnPb-plated device leads.
Leadframe-Based Package Contacting Challenges
Many leadframe-based packages (QFP, SO, and QFN, etc.) have now been redesigned to have either matte tin (Sn), tin-bismuth (SnBi) or palladium-nickel (PdNi) surface finishes. Each of these new surface finishes have presented contactor manufacturers with unique challenges in providing the reliable temporary interconnects needed for production testing.
It has been particularly challenging to make a reliable test connection to matte Sn and SnBi. Quite often, the matte Sn device leads have a relatively thick oxide layer on the surface that is hard, rough, and difficult to pierce. Contact geometries that easily displaced SnPb finishes, exposing fresh surfaces, no longer adequately work against a harder oxidized matte Sn finish. Laboratory studies and field testing have shown that a normal force of 30 grams or more is required to achieve good test yields, compared with lower forces acceptably used when probing SnPb parts.
One drawback of the higher force required is the tendency for the interface asperities to cold-weld, erode and fracture, resulting in adhesive wear (Figure 1). The increased stress on the surface of the plated contact layers also can induce fractures that expose base material on the contacts and result in fretting corrosion that over time increases contact resistance (Rc) and, just as important, increases Rc variability. The solution for overcoming this type of wear and corrosion is to engage the IC lead with a metallurgy that has a lower affinity for Sn and SnBi. The PrimeGuard-II™ contact finish available on ECT probes has a metallurgy that resists adhering to Sn and SnBi surfaces during IC test conditions, and provides a stable Rc.
Leadframe IC’s with PdNi-plated leads present a very different interconnect challenge. Oxide layers and intermetallic compounds at the interface are greatly reduced, however the very high surface hardness of these leads (185 HKnoop and above) results in extreme abrasive wear on contacts plated with gold, such that the base material is quickly exposed, and fretting occurs. (Figure 2)
The best way to overcome the extreme hardness and abrasive wear of the contacts is to encapsulate the surface of the probes with a material that is substantially harder than the NiPd. ECT’s PrimeGuard-I™ proprietary interconnect surface finish has been production proven to hold up against PdNi device leads to hundreds of thousands of insertions and have low and stable Rc. The PrimeGuard-I finish can be selectively applied to different contact types on the DUT side of the interconnect, while maintaining an optimized gold-plated contact surface against similarly plated gold loadboard pads.
Ball Grid Array Contacting Solutions
Prior to the switch to Pb-free interconnects for Ball Grid Array (BGA) packages, contactor manufacturers had been providing a variety of solutions to contact the relatively soft, low melt temperature (183° C) alloys of eutectic SnPb solder. Contacting eutectic SnPb solder balls has always been a challenge, as the traditional interconnect designer’s toolkit of normal force and wiping action has tended to transfer large quantities of SnPb to the contact surface, especially at elevated test temperatures. When solder gets transferred, intermetallic layers form with gold contacts and oxide films form when the contact is exposed to moist ambient air. The other constraint faced by the interconnect designer is that the contact technology may physically distort the solder balls to the point that the packages could be out of specification or create down-stream soldering reliability issues. Contact manufacturers have devised many unique, proven shapes that minimize solder transfer, while making reliable contact with SnPb BGA’s.
The Pb-free solder ball alloy adopted for BGA’s is predominantly a tin-silver-copper (SnAgCu or SAC) alloy that is mostly Sn (>95%). The melting temperature of SAC alloys is higher (217° C), and the balls are harder. The harder ball allows for a more aggressive normal force and wipe for a probe contacting the ball, thus improving the Rc consistency in test. It is interesting to note that the SAC balls exposed to ambient environments can appear very dull, and electron microscope inspection reveals that the dull finish is a result of a nearly pure Sn dendrite growth on the ball surface. The dendrites are very rough, but can be easily displaced with proper contact design. (Figure 4)
The ultimate contact solution for BGA’s is a very hard finish that is chemically inert to the alloys in SnPb and SAC balls. A number of alternative contact finishes have been proposed and implemented, with some success in minimizing the material transfer from the balls to the contacts. One solution for all BGA’s, including SAC balls, is again the PrimeGuard-II finish available from ECT. The PrimeGuard-II finish contacting the BGA results in only a slightly higher overall probe Rc, but with increased stability over thousands of DUT insertions. The PrimeGuard-II finish is also much harder than gold, so contactors can be cleaned with more aggressive methods vs. gold-plated probes. As with the PrimeGuard-I solution for NiPd leadframe parts, the PrimeGuard-II finish is selectively applied, retaining the loadboard-side gold finish with optimized radius tip geometry for overall contact integrity.
Caution: One Solution May Not be Right for All
Various surface finishes and contact shapes have been developed by ECT and other contactor manufacturers to address solder migration, surface wear, and other contact corrosion issues commonly dealt with in production test. A solution for a particular application can not be assumed to work in all cases. Beyond the first tier of issues of interface metallurgy and geometry, come other parameters that can have a significant influence on contact reliability.
Semiconductor package leadframe base material has a measurable influence on Rc. Leadframes with copper substrate material tend to have a far greater susceptibility to Rc variation vs. iron-based leadframes (Alloy42 or FeNi42). The Alloy42 base material is much harder than copper, and socket contacts with alternative surface finishes have less of a Rc variability reduction vs. standard hard gold finishes against these leads. Careful analysis and understanding of the device test parameters needs to be made – perhaps the pass/fail threshold is broad enough to allow more traditional finishes to perform well.
Other factors that come into play in making the proper probe surface finish choices are: thickness of leadframe finishes, surface roughness, cleanliness of the test
environment, chamber temperatures, and acceptable DUT witness mark levels on packages.
In production environments that use common hardware to test both Pb-free and Pb-bearing packages, special care must be taken not to use the same contact sets for both package types. Contactor setups must be identified and labeled for which type of packages are to be tested. Cross contamination of Pb and Sn on probes can lead to very unpredictable test results and the potential for contaminating Pb-free packages violate RoHS guidelines.
Interface Challenges are Increasing
Compounding all of the interconnect challenges with Pb-free devices is the fact that the devices themselves often have more demanding performance requirements. Low power, resistance-sensitive applications such as measuring MOSFET RDSon resistances of less than 100 milliohms in production test require absolutely low and stable Rc. Even the most exotic contact finishes may not be enough to meet the test requirements imposed by the device specifications, so a true Kelvin solution may need to be deployed in the contactor, loadboard, and test program.
Automotive applications that force high current through contact interfaces often cause an accelerated intermetallic formation from the galvanic effects of current flow, exacerbated by elevated temperatures due to interfacial Joule heating or extreme test chamber environments. To minimize the effects of the thermal extremes, a very low Rc is required, so low-resistance materials and finishes must be used. Similar high-current issues exist for power hungry CPU’s and GPU’s. Alternative contact finishes and innovative housing solutions allowing for parallel probe use combine to overcome these high-power issues.
Overall Effect of Pb-free Packaging on Contactor Technologies
The level of research and development, scientific analysis, and industry partnership has greatly increased over the last 5 years, and will continue to climb. The change to Pb-free packages has increased the number of interconnect options used in production test contactors, and choosing the right solution for your application requires careful analysis.
In addition to understanding the package outline and handler specifications that typically drive a contactor outline, the contactor user must have a better understanding of the package construction, metallurgy, power requirements, chip performance, test program requirements, and loadboard signal launch limitations. All of these considerations have an impact on the contactor reliability in a high volume, production test environment.
ECT is up to the challenge, and has invested in the technical resources needed to understand and overcome the difficulties in making reliable contact for production semiconductor test. Innovative surface finish technologies, such as the PrimeGuard family of coatings, as well as advanced probe designs, such as the Bantam® and DuraPak™ families of contactors enable the test engineer to effectively develop test systems for next-generation semiconductors. Technologies and processes developed for solving contacting issues for Pb-free applications today are being utilized in the development of future products for high frequency, ultra-fine pitch test contactor applications of tomorrow.
Written by Valts Treibergs, R&D Engineering Manager for ECT's Semiconductor Test Group |