Testing Analog Chips

The world of analog components is broad and diverse, and while testing analog chips may not take as long as running tests on complex SoCs, there are different requirements for analog devices.

One type of chip that’s seeing more application these days is analog microelectromechanical system devices. Automotive electronics call for a number of analog chips, along with MEMS.

Mixed-signal ICs, which combine analog and digital elements, are a related field.

Risto Puhakka, president of VLSI Research, estimates the total analog tester market to be worth about $100 million a year, largely for testing linear chips and discrete devices. “This is kind of a quiet, sleepy segment,” he adds. “The testers live forever.”

In contrast, the SoC tester market is more than $2 billion a year in sales, and mixed-signal IC test, chiefly for radio-frequency and wireless chips, is also larger than analog test.

Xcerra is the market leader for analog test, while Teradyne leads in mixed-signal test, closely followed by Advantest, according to Puhakka.

Analog chips enjoyed sales growth of 10.2% last year, according to WSTS, which sees that growth moderating to 6.1% in 2018.

“We haven’t seen anything significantly happening in analog test for a while,” Puhakka says. High-power devices for automotive applications are a growth area.

Derek Floyd, director of business development for Advantest, says there are diverse test requirements for analog. Those requirements include shrinking supply voltages, including saving energy for battery-powered devices such as mobile PMICs, low-leakage measurement capability, precise trimming and measurements capability, and a low-noise test environment. There are also high-voltage/high-current designs, with the system capability to provide these — high-voltage sources and floating high-power sources.

“Precise measurements require averaging of measurement samples to remove noise, often over a powerline cycle, 50 Hz or 60 Hz,” Floyd says. “High-current devices require pulse testing to prevent devices from thermal damage and heat up, which causes measurement deviation. Testing a device with 10A of current at 10V is 100 watts, which will cause the device to heat up rapidly. So the current pulse is only applied for a millisecond or so. Device threshold tests require voltage/current ramp generation capability of the system. Binary search algorithms are also used as they can be much faster than using ramps.”

Fig. 1:  Analog signal. Source: Ram Electronics

Fig. 2: Digital signal. Source: Ram Electronics

Testing analog vs. other chips
A key difference is that testing analog reuires mostly parametric tests reporting measurement results.

“Digital-focused devices mainly report pattern pass/fail information,” says Floyd. “Depending on chip complexity, between 3,000 and 8,000 test results are measured, which is more test results compared with other devices. But fewer design-for-test techniques are implemented than for digital. It is very complex to achieve DFT for analog designs. There are several DFT methods around, such as using an internal A-to-D converter via a test bus or using loopback. However, it is often just as fast to use ATE resources in parallel compared with using DFT.”

Many analog devices also depend on a digital state machine, based on protocols such as SPI, I2C, or JTAG, which controls the signal routing within the device. “Once the setup has been done, the analog measurement usually requires some wait times to account for settling components such as capacitors in order to make repeatable analog parametric measurements,” he says. “This leads to a much more complex ‘stop/start’ test sequence compared with digital tests, which can run continuously.”

There are some similarities between analog and digital chip testing, as well. There is a similar production environment, including testing of multiple temperatures—hot, cold and ambient. Some very low-cost analog devices are sample-tested at the wafer, as well, and because the processes are so mature the yield typically is very high. On top of that, final test is done to weed out potential packaging defects.

“We have seen strong growth in the analog market, especially driven by automotive and power management devices, he notes. “The market indicates continuing growth moving forward.”

Joey Tun, principal market development manager for Semiconductor Test at National Instruments, also notes the diversity and variety of analog chips, ranging from a basic op amp to an RF front-end module.

“The fundamental reason one would test a chip, any chip, is to insure the quality. Most ICs, most chips will have some sort of a specification that comes with it that a designer would use to integrate into whatever that chip will be designed into,” he says. “For the analog chips, even the operational amplifiers, these requirements will be analog in nature. Rather than a simple pass/fail, you may be looking at things like input bias current.

“The measurements are a broader category than just pass/fail. You’re having to provide some quantitative data based on what the specification on the chip is.”

He adds, “What is special about analog test at a high level is that the parameters of analog test are going to be in some sort of analog form, such as the voltage level and the current level. That’s on the simple side. On an RF power amplifier, you may have to measure things like noise floor, you may have to measure things like gain, you may have to measure that every channel power of that particular RF standard on that analog chip. Things can get quite a bit more sophisticated, more difficult actually, as you go into these other types of analog ICs. The diversity of the type of measurement is pretty high. It really depends on what the chip is. Some of them are non-trivial types of measurements, things like measuring a noise figure on an RF power amplifier. These are not easy measurements any more. You require pretty specialized test equipment.”

Customers use NI’s Semiconductor Test Systems in validation labs and final production test, according to Tun.

“Efficiency does matter,” Tun says. “A lot of the IC vendors are always fighting the cost, or time pressures, fitting into a design cycle, fitting into a market window. We are working with a lot of customers very closely to help them accelerate their time-to-market.”

Xcerra sees a similar trend. And because it’s analog, equipment tends to last a long time.

“There is a lot of development in op amps,” says Christopher Lemoine, product marketing director in Xcerra’s ATE group. “There are always higher-performance op amps, lower bias currents, getting closer to the rails, you now have zero-headroom op amps where you can operate right down to the ground rail. At the same time, a lot of the performance hasn’t changed a whole lot.”

Brian Bogie, Xcerra’s senior director of product marketing for the Xcerra line aimed at lower-pin count analog and power management devices, notes that is one aspect of the analog world that is very different from other types of devices. “Once these parts are in production or adopted by users, they have a relatively good longevity in the field compared with more consumer-oriented digital devices, which change yearly, or even more frequently than that. That’s why customers demand a really long lifetime in the analog world.”

Lemoine notes, “These are what we call catalog parts—signal conditioning, glue parts that are bringing together different parts of the system. They’re a little bit more stable in terms of the generations changing so rapidly. For us, we also bucket discretes in analog. If you get down into discretes, some of the other requirements are very high voltages, so you get 1,000 volts, very high currents, 100 amps or more of pulsed current. There are some special requirements that we have specialized instrumentation to handle.”

Analog components can range from microvolts to thousands of volts, and from picoamps to hundreds of amperes, according to Bogie. The key is to weed out the noise from the signal.

“We have special front-end instrumentation that is airwired to handle picoamp currents,” Lemoine says. “If you do a laminated PCB, the leakage of the PCB itself would mask the performance of the op amp, so we have an airwired front-end interface. That’s an important area to have some flexibility, where you can have area for a family board or for an interface board that can handle some of those specialized requirements, and then put that in front of your more general-purpose instrumentation. Your general-purpose instrumentation becomes more of an engine behind the front-end signal processing.”

Increasing interest in MEMS devices, particularly for automotive and IoT applications, is starting to boost demand across this sector, as well.

“We’re starting to see some high-demand products, like, analog MEMS driving demand for very-high-parallel solutions,” Lemoine says. “That’s what’s driving some of these parts onto the Diamondx. The Diamondx platform is really an SoC platform. We’ve introduced a five-slot version of Diamondx that where it helps us scale down to some of the lower-pincounts required for these devices where you don’t have to have a big system, you can have a really small system, but it still has some of the higher performance capabilities required for MEMS testing.”

Analog parts often have shorter test times than more complicated devices, but there is a wide variety of analog components and test times can vary widely. Analog testing usually doesn’t allow for the test patterns that electronic design automation tools can generate for digital logic, using simulation outputs. But handcrafted analog devices sometimes need handcrafted DFT technology.

“It sounds like analog is in some ways simpler than SoC or digital, but at the same time, you’re really closer to the intrinsic transistor performance,” says Lemoine. Because it’s a pure analog part, and sometimes it’s a single transistor that you’re dealing with, you’re really getting back to basics, fundamentals as in, ‘How does this transistor work?’ You have more visibility into the process variation.”

Conclusion
Analog devices are a different semiconductor species than the microprocessors, microcontrollers, application processors, memories, and other common chips that most people are familiar with. These devices can deal with very low voltages and very high voltages, including voltages that would fry most other chips.

The physical world is analog, and being able to sense that world requires analog technology. But it also requires specialized equipment to test those devices, and that isn’t going to change anytime in the foreseeable future.