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Advances in MCUs and test equipment

Posted: 20 May 2014     Print Version  Bookmark and Share

Keywords:microcontrollers  MCUs  parallel testing  multi-site efficiency  Advantest 

Until quite recently, microcontrollers (MCUs) were used purely for digital devices. While their internal functionality became more and more powerful, they typically only sent and received digital signals. The environment for testing such MCUs was set up to meet these basic needs. All that was required was to test the digital connections, and any analogue signals that were encountered were few and far between and not very complex.

Today, MCUs are fitted with a variety of interfaces and cater to the full array of electronics. To meet mobile applications requirements, such as battery consumption, their own electrical power needs are continually decreasing. The accuracy of existing test solutions can no longer cover their capabilities. To communicate directly with analogue realities of the world today, more and more high-resolution analogue-to-digital converters are being integrated in MCUs.

As we live in the age of the ever expanding internet with its devices and a new smart world, MCUs have to communicate more often using wireless channels. Remote control functionality is taken for granted where car key rings are concerned, but what if you want to network the washing machine to your other household appliances so that it only starts running when the photovoltaic system is providing sufficient electrical power? Or how about using your smartphone to remotely operate the shutters and dim the lights when you're on holiday?

Integration with sensors is a growing trend. An MCU needs not only to passively receive electrical signals, but also has to immediately detect what is actually happening via its sensors. One simple application is the wireless pressure sensor in car tyres, which continuously reports the pressure to the vehicle.

In the MCU market, it is not just the technology that is making advances. The integration of different functions is paving the way for new players in the market. Former specialists in wireless communication currently integrate MCUs because manufacturers are also incorporating their RF transmitters and receivers. That is also why costing pressures are increasing. The demand is for testing a greater number of increasingly complex MCUs in less time than before, using fewer test devices, all the while satisfying low cost demands. That's no easy task even if more memory has been integrated in the MCU, which also requires testing.

Below is a summary of challenges being faced by MCUs today and from which new testing requirements arise:

 • Lower power consumption, smaller tolerances for leakage current, diminished performance in the applied signals
 • Faster data rates, faster interfaces
 • Increasing use of analogue interfaces, AD converters
 • Wireless RF connections such as Near Field Communication (NFC), Zigbee, Bluetooth
 • Integration of sensors
 • Cost pressure.
A simple digital test was all that used to be required to test MCUs. These were installed in large numbers and tested MCUs during the production process, while still in the wafer, and then packaged in final test. At one time, this was completely adequate for testing the relatively generic requirements, such as measuring low data rates and robust, high power, high voltage designs. But when it comes down to achieving a high throughput involving parallel testing a large numbers of MCUs, the trend now is to replace these with other devices. Occasionally, memory module testers are even deployed as these systems suffice for pure digital testing. Memory module testers are not capable of testing analogue interfaces, let alone RF interfaces, because their throughput capacity is not always an economical option.

Essentially, there are two ways of increasing the throughput. The very first approach is to shorten the test time required for a single chip. The second key variable is to test multiple MCUs in parallel. Just a few years ago, parallel testing of 32 chips was not common practice, which was reflected in the pace of tester development. There was also a lack of handling equipment that was capable of moving more than 8 or 16 chips at a time. However, things have moved on considerably since then. Today, it is now possible to run parallel tests on 32 or more chips, even during final tests. It is becoming increasingly more commonplace to run parallel tests of 64 to 128 chips at the wafer test stage. Additionally, the costs for direct interfacing systems (probe cards), though achievable, is financially unfeasible to maintain.

An important indicator for parallel testing is multi-site efficiency (MSE). At an MSE of 100%, testing 64 chips, for instance, would take just as long as testing one chip. That's why the test time for one single chip is so crucial. If this is too long, then parallel tests on multiple chips can't be shortened. These figures explain further:

If the test time for one chip is 10 seconds, at an MSE of 98% a test cell that tests 32 chips simultaneously will achieve a throughput of 7,111 units per hour. An MSE of 98% is already quite high for most chips that have long been installed. For many MCUs, however, this is far too low if they are to achieve their potential.

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