By Ron Iscoff, Editor
"Lead, follow or get out of the way" urges the writing seen on a popular bumper sticker. The more experienced users of test sockets (contactors) must sometimes wish this slogan applied to a few of three dozen plus suppliers of sockets. To the uninitiated, socket selection-the how, what, why and how much-must indeed seem puzzling. At the end of the day, though, selection is also critical. Test, and this part of electrical test, still remains a leading roadblock on the assembly and packaging front. We surveyed some socket makers-as well as a few socket users. We asked the users what they wanted in the future. We asked the makers what they planned to give their users. (See figures 1-7). Let's review. Doug Kocher of socket maker AQL Manufacturing observes that socket costs continue to rise. Meanwhile pitch is getting tighter and the I/O count is rising. "Being able to customize the test socket continues to be vital for our customers," he notes. AQL, he adds, is exploring ways to simulate soldering the device to the PWB, without increasing the detectable electrical properties. Strip testing, too, is finally gaining widespread interest. Mark Godfrey of ECT points out that strip testing is accelerating the industry's move toward the efficient integration of assembly and test. Strip Testing Godfrey, operations director for ECT's Semiconductor Test Group, emphasizes that test contacting solutions must address the needs of massively parallel test of pre-singulated ICs. "DUT registration over a broad temperature range, integration with the handler's thermal management system, planarity control and ultra-high reliability are a few of the critical considerations," he says. To date, Kulicke & Soffa's Package Test Products Div. has produced strip test contactors that handle about 32 ICs with over 784 I/O for parallel test prior to singulation. The company is now developing contacts that will allow 100 ICs with over 1500 I/O to be tested, according to Guy Delisle, vice president-marketing for the division. With this socket, the longterm test cpacity will be lmited primarily by the capabilities of the handler, says Delisle. Compared to single device testing, strip testing increases throughput to the number of devices tested on a single strip. "This," adds Delisle, "leads to comparable reductions in test time and cost." New Sockets Shrinking package sizes demand that new contacting systems be developed, says Larry Furman, sales and marketing manager for Texas Instrument's Interconnection Business. "One predominant feature needed will be sockets that can support 0.5 mm pitch, or lower, honoring the "No Damage Zone" of the contact. Furman believes an open-top, dual-contact socket that makes contact above the equator of the contact ball will meet these requirements.
At this year's BITs Workshop, recalls Robert Million, Yamaichi Electronics U.S.A., a recurring theme was the problem of heat dissipation for ICs. "Test and burn-in socket makers were careful to take note of this ongoing problem, as it often entails the integral design of a heatsink into a burn-in socket," Million notes. "When you must dissipate something like 12W off the body of an IC with very little air flow provided in the burn-in chamber, the socket really becomes the accessory, while the heatsink design takes center stage." Million emphasizes that "it's becoming more and more prevalent that test and burn-in socket makers must help solve their clients' heat dissipation problems." Because of thermal demands, adds Steve Durrett, sales manager, Plastronics Socket Co., sockets for QFN and MLF devices and thermally enhanced TQFPs need center contact pins to act as ground and/or as a heat sink." Edge rates and bandwidth for testing communications ICs loom large with many socket makers. "Increasing edge speeds are a major trend that will impact socket designs, according to Nick Langston Sr., DCI division of Credence Systems. "To handle expanding memory bandwidth and increased network traffic, new generation devices must be designed at the 'edge' of the network. Bandwidth, Langston notes, is increasing to 10 Gbps. Improperly designing sockets for communications devices, adds Langston, will result in ringing, crosstalk and excessive delays-not to mention excessive error rates.
What Users Expect Packaging foundry AIT, Batam, Indonesia, offers three major specs for its socket selection: longevity, good electrical contact and precision. "We serve a considerable number of customers worldwide, and we're always looking for a reliable socket with an excellent life span," according to Chris Hamilton, test engineering manager. Hamilton expects this socket to last "for as many cycles as possible while still maintaining quality performance." In terms of precision, says Hamilton, AIT is interested in sockets that perfectly match the company's specific packaging needs for lead width, lead spacing, footprint and form factor. "Additionally, a wide range of products and cost are added factors" that go into AIT's selection. Carsem, a Malaysian-based assembler, employ specially developed sockets for certain packages, such as its MLP-M, a QFN-type package, according to H.N. Koh, final test manager. Developed by Simeca, a test handler maker in the country, the socket offers Kelvin capability and replaceable contact pins (Figure 8). The latter enable Carsem to replace broken pins instead of the complete socket. The socket also offers direct plug-in, which enables the socket to be plugged into an external connector directly, without the need to mount the socket on a PWB. Finally, it offers a precise contact guide.
High-Frequency Test Needs Earlier we spoke about the need to test high-frequency ICs, as an area that socket makers are exploring. Siliconware, a Taiwanese assembler, says it wants sockets that will handle high frequency test requirements. For example, graphic processors (GPUs) may need up to 3.2 Gbps TMDS (transition-minimized differential signaling) to support various network bus protocols, such as 3GIO, according to T.F. Lee vice president of test engineering for SPIL. Another SPIL requirement, adds Lee, is for sockets capable of handling shrinking solder ball pitches, from the current 0.8 mm down to 0.5 mm and beyond. However, he cautions, "handler technology cannot be forgotten relative to sockets, especially if it cannot make maximum socket contact." This will require users to re-screen again and again-posing a large problem for socket and handler suppliers in the future. Silicon Turnkey Solutions is a specialty assembler whose management possesses an extensive test background. Haitham Hamed, STS' director-strategic business development, observes that moisture coupled with high temperatures can affect the reliability of mass produced sockets for productions lines. "A reliability engineer tasked with socket selection should pay close attention to the amount of gold plating on socket contacts," Hamed says. "From our experience, we found that 50µ" of gold plating with nickel over copper (BeCu) is a very safe and reliable plating thickness." Hamed complains that many socket makers are switching to 30µ" of gold plating. "They are supporting this change with reliability data showing contact integrity at 1000+ hours of burn-in, but not providing any data for the contact integrity for HAST and 85/85 tests." On the plus side, Hamed notes that socket suppliers "are very keen on providing such information as plating thickness, typical ball deformation, durability, contact method and intial contact force. "Most suppliers are aware of the stringent socketing demands for FPBGA and CSPs-and used their data as a major selling point." Conclusion Socket suppliers are moving to new levels of sophistication in meeting shrinking pitch and I/O demands. They are also poised to meet demands of higher frequency and higher speed related to network devices. |
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