Semiconductor Validation System for Fingerprint Identification Chip Used in Mobile Devices

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"The NI system delivers full coverage for both control and measurement of all necessary inputs and outputs needed when testing the sensors. It is very reliable and can easily be scaled up in the future."

- Hans Thörnblom, Fingerprint Cards AB

The Challenge:
Creating a fast and reliable test system for both manual characterization and automatic regression testing to ensure that the first samples of a new sensor meets all the specifications necessary and can continue to mass production as quickly as possible.

The Solution:
Developing a PXI-based solution centered on a digital waveform generator/analyzer with PPMU for a flexible and easy-to-use system that can test and characterize all required parameters, resulting in full test coverage.

Author(s):
Hans Thörnblom - Fingerprint Cards AB
Sebastian Bengtsson - WireFlow AB

Fingerprint Cards AB (FPC) markets, develops, and produces biometric components and technologies that can verify a person’s identity by analyzing and matching an individual’s unique fingerprint. The technology uses biometric sensors, processors, algorithms, and modules separately or in combination with each other. FPC’s technology delivers competitive advantages including unique image quality, extreme robustness, low power consumption, and complete biometric systems. These advantages and the ability to achieve extremely low manufacturing costs make it possible to implement the technology in volume products such as smart cards and mobile phones, which face extremely rigorous demands. The company’s technology can also be used in IT and Internet security, access control, and more. One example of an implementation is in the Huawei Ascend Mate7 smart phone.

To ensure quality, FPC tests the functionality of the first samples of each version of a new sensor that come from our contractors before delivering them to customers. By design, we do not create dedicated prototypes. Once we design a new version, we send it to a manufacturer who produces first samples. If these samples are satisfactory, they are then the first in the new series. On top of the high-quality standards that FPC’s technology should have, the accelerated development rate in the last decade makes it important to also deliver the product fast. Thus, we use the test system for manual characterization testing and automatic regression testing to be sure that the first samples of a new sensor meet all the specifications necessary and that mass production can continue as quickly as possible.

About the Test Application

When a new sensor goes into production, FPC gets 10–100 of the first produced samples from our contractor. Out of these, we test a handful in our lab to verify the correct functionality as well as to characterize power consumption and to assure ourselves that we are using optimal settings for the highest possible image quality. We measure the quality of the AD converters; test all modes, parameters, and functionalities; and characterize the communication timing parameters. Since we use external contractors, we must quickly assure ourselves of proper functionality and assess the contractors’ production process as well as comparing measurements to their production data.

We can do this quickly and reliably because of a solution based on NI PXI products we implemented. Our system consists of an NI PXIe-1073 chassis, connected to a stationary computer through an MXI-Express link. The chassis contains an NI PXIe-6556 digital waveform generator/analyzer with PPMU, an NI PXIe-6361 16-bit high-speed analog data acquisition device, and a PXI-5152 2-channel oscilloscope. The built-in signal conditioning of the PXI modules is a big benefit since it reduces the amount of custom design necessary. The only custom electronics we used is an adapter board that connects all the PXI instruments to the sensor and provides high-speed current sensing signal conditioning. This generic interface facilitates when we want to change between different types of sensors. The change happens in a manner of seconds, and the LabVIEW software is identical.

LabVIEW controls the entire application, which can perform several different tests automatically but also takes more investigative measurements. Common tests include input and output voltage thresholds, power consumption, leakage currents, continuity tests, and Iddq testing as well as characterization of the timing parameters of a serial communication bus, for example, the setup, hold, and valid times. The three modules complete each other very well. The NI PXIe-6556 digital waveform generator/analyzer with PPMU handles the high-speed digital communication and analysis and sourcing, and also accurately measures small currents. The PXI-5152 oscilloscope gives characteristics of the signals, such as amplitude, frequency, and rise time. The NI PXIe-6361 high-speed data acquisition card has excellent timing and synchronization and can measure fast power consumption transients well.

Flexible and Future-Proof Solution

Before moving to PXI, we used a mixture of USB-based data acquisition cards from NI and Cheetah, but the mixture of different USB units was a cumbersome solution and inefficient. The old instruments could not meet the performance requirements for testing coming versions of our sensors. After some investigation, we decided that LabVIEW and PXI provided an ideal platform for implementing our lab solution. Having all instruments in one system instead of multiple systems makes it easier to work with and saves desk space. It also offered advantages when we wanted to duplicate the system. We started off with a single system, but soon invested in two more identical systems. Thanks to the integrated hardware and software, duplicating the system was a very simple process. Measurement & Automation Explorer (MAX) gives a useful overview of the measurement hardware. To duplicate it we only needed to export the hardware settings from MAX to the duplicated systems and start using them. Different versions of the LabVIEW code can be easily maintained through Git version control system.

We can also create a custom user interface, and the intuitive graphical code of LabVIEW is also very convenient when communicating the test code to the sensor developers who also use the test systems for exploring new sensor solutions. The sensor developers often discuss their ideas for future functionality with the test developers who then create necessary test cases, which uses the competencies of each department, but the sensor developers can then make modifications in the programs as their work evolves to achieve fast development iterations.

We also use some plug-in tools in the LabVIEW ecosystem. For example, to create waveforms we use the NI Digital Waveform Creator to import value change dump files of real use cases, manipulate them to create test cases, and save them to HWS format that we can import in LabVIEW. The powerful hardware compares functionalities of the NI PXIe-6556 device that is then used to evaluate production test vectors.

Apart from the software and hardware specifications, the NI service offerings were an important factor when selecting the platform. The NI Alliance Partner Network helped us find our integration partner, WireFlow, which we continuously use for software consulting services and for designing the socket and adapter boards. The wide selection of courses, both online and classroom based, helped us build up our own in-house competence. The phone support is useful when running into obstacles during development, and we regularly use the calibration services to confirm that the systems still work according to their specifications.

Future Development

The NI system delivers full coverage for both control and measurement of all necessary inputs and outputs needed when testing the sensors. It is very reliable and can easily be scaled up in the future. The present system has met all our needs so far with wide margins, but we plan to upgrade it soon by adding an NI PXIe-4140 SMU because we see a coming need for a better voltage source. It is very convenient to be able to upgrade subparts of the system without having to replace all hardware, but still have a single, compact system rather than four to five separate instruments. The combination of LabVIEW and NI hardware empowered us to quickly get started with our solution, and then let it evolve with needs with maximum reuse of our previous investments in both hardware and code development, and only a minimum cost for future upgrades. The great success that the validation department has seen with a PXI-based solution has created an impact at other departments of the company that are now looking into implementing their own solutions on the same platform.

Author Information:
Hans Thörnblom
Fingerprint Cards AB
Kungsportsplatsen 2
Gothenburg SE-41110
Sweden
hans.thornblom@fingerprints.com

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