Reducing Switching Costs in Automated Test System with FETs and SSRs

Table of Contents

  1. Introduction
  2. What Is a Solid-State Relay (SSR)?
  3. What Is a Field-Effect Transistor (FET) Switch?
  4. Advances in FET and SSR Technology
  5. PXI FET and SSR Switch Modules Reduce Test Costs
  6. Conclusion
  7. Related Links

Introduction

Found in products from automotive engines to thermocouples, electromechanical relays are readily available, low-cost devices that you can use to route signals originating from almost anywhere. It is no wonder that these devices have become ubiquitous in the automated test market. Despite their industry adoption, electromechanical relays are relatively slow (hundreds of channels per second) and have limited lifetime, which makes them unsuitable for use in applications such as semiconductor validation test.

Semiconductors are mass-produced devices that have evolved considerably over the years. As these devices have grown in complexity, so has the overhead associated with testing them. Today, some semiconductor chips are so sophisticated that the cost to test them is greater than the cost to produce them. To keep chips affordable in the long run, semiconductor manufacturers must drive down test costs by reducing the time to test each device and minimizing instrumentation expenses. For switch systems, this means using parts that have fast scan rates and longer lifetimes, features lacking in electromechanical relays but prevalent in devices such as solid-state relays (SSRs) and field-effect transistor (FET) switches. With switching speeds as high as 50,000 channels per second and unlimited mechanical lifetime, FETs and SSRs reduce overhead in many applications by reducing test times and eliminating the need for frequent switching component replacement. Additionally, their small form-factors help minimize up-front switch system costs.

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What Is a Solid-State Relay (SSR)?

A solid-state relay is constructed using a photo-sensitive metal-oxide semiconductor field-effect transistor (MOSFET) that is controlled using an LED. Light from the encapsulated LED actuates the photo-sensitive MOSFET and allows current to flow through it.



Figure 1. Solid-State Relay (SSR) Construction

SSRs are useful for high-voltage applications because the LED actuation provides a galvanic isolation barrier between the control circuitry and the MOSFET. Because the MOSFET is doing the switching, however, there is no galvanic barrier between its contacts. When there is no gate drive on the MOSFET, the drain-source channel on the MOSFET has a very high resistance, which provides the disconnection between the contacts. Because SSRs use MOSFETs, which are solid-state devices, to switch states instead of a mechanical arm, they have infinite mechanical lifetime. SSRs also offer faster switching speeds than electromechanical relays. Switching time for SSRs is dependent on the time required to power the LED on and off, approximately between 0.5 ms and 1 ms. This is faster than the time required for the mechanical arm in electromechanical relays to move between states.  

The NI PXI-2533 and PXI-2534 256-crosspoint matrix modules are examples of high-density switch products that incorporate SSR technology. Both modules offer unlimited mechanical lifetime and unlimited simultaneous connections while providing the ability to switch up to 55 V at 1 A on all channels.

Figure 2. NI PXI-2533 256-Crosspoint SSR Matrix Modules

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What Is a Field-Effect Transistor (FET) Switch?

Like SSRs, FET switches are not mechanical devices and therefore have an unlimited mechanical lifetime. FET switches use a series of CMOS transistors to connect and disconnect circuits. Unlike SSRs, the control circuitry drives the gates of the transistors directly instead of driving an LED. Direct drive of the transistor gate allows for much faster switching speeds because the power-on and power-off time of the LED is not an issue. Also, because there are no mechanical parts or LEDs in the packaging, FET switches can be very small. One major limitation of the FET switch, however, is that it lacks a physical isolation barrier, so you can use it only with low-voltage signals.

FET switches are most often used in multiplexer and matrix configurations for higher-speed, low-voltage applications. The PXI-2535 and PXI-2536 544-crosspoint matrix modules are examples of FET-based switch modules. These modules offer switching speeds up to 50,000 crosspoints per second.          

 

Figure 3. NI PXI-2536 544-Crosspoint FET Switch Module

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Advances in FET and SSR Technology

 Until recently, FET and SSR devices have had a reputation for inducing measurement errors in test systems due to their high path resistance, which can sometimes exceed 1 kΩ. This characteristic has impeded these devices’ entry into the automated test market. But recent advancements in transistor technology have made the path resistance of FETs and SSRs comparable to that of electromechanical relays. The NI PXI-2533 256-crosspoint SSR relay for example has a path resistance of 1 Ω, which is less than or equal to the path resistance of most electromechanical relay modules. This substantial technological breakthrough has now made it possible to leverage the inherent advantages of FET and SSR technology, which include unlimited mechanical lifetime and faster switching speeds while not suffering the consequences of higher path resistance.

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PXI FET and SSR Switch Modules Reduce Test Costs

FET and SSR devices help reduce cost in automated test systems by lowering up-front cost, increasing switch system lifetime, and minimizing test times.

The compact size of FETs and SSRs helps reduce the up-front cost of PXI switch systems. The cost of a PXI switch module is based on the cost of relay components, back-end circuitry, and materials such as the printed circuit board (PCB) that are used to build the module. The small form factor of FETs and SSRs facilitates the building of very high-density single-slot PXI switch modules. This helps reduce the number of PXI modules, and, consequently, PXI slots used in a chassis, when building high-density switch systems such as those used in semiconductor validation testers. By using fewer modules, you incur less expense on raw materials and the back-end architecture. The PXI-2535 544-crosspoint matrix is an example of a very-high-density PXI switch module built using FET technology.

The unlimited mechanical lifetime and faster switching speeds of FET switches also help minimize costs in test systems. Consider the example of a system that is used to conduct 10 tests on a chip with 500 I/O points. The chip is used in numerous devices and its cumulative sales are estimated at 1 million per month. The test system, which is built using a single NI PXI-4130 source measure unit (SMU) and a switching front end that is used to route all 500 points to the SMU, is required to run continuously without interruption. The cost comparison of using an FET-based switch product versus an electromechanical relay-based product is as follows.

Using the 50,000 channels per second scanning speed of the PXI-2535 544-crosspoint FET switch, you can test all 1 million chips in less than 12 days. Because FETs have unlimited mechanical lifetime, you incur no switch module replacement costs during the process.

 

  Figure 4. Chip testing with SMU and 544-Crosspoint FET Switch

If you used an electromechanical relay-based switch module with the same density, expenses would be much higher. Electromechanical relays have a typical lifetime of 1 million closures and speed of 250 channels per second. Because each relay is closed 10 million times during the process of testing all 1 million chips, the relay module needs to be replaced 10 times. This would increase the total expenses incurred on maintaining the system. The slower speed of electromechanical relays also adds costs in comparison to the FET-based solution. The time taken to test 1 million chips using electromechanical relays is 231 days. Thus using electromechanical relays would add the cost of maintaining and running a production floor for 219 additional days in comparison to the FET-based module. Longer test times also introduce challenges when managing inventory and shipping products to customers.

While hypothetical, this example shows the real cost savings you can achieve due to the benefits of FET and SSR technology.

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Conclusion

There is no single solution for routing signals in automated test systems. In fact, the number of solutions available to the market is continually increasing. FETs and SSRs are examples of switching solutions that have always been available to the market but have become viable options only recently due to advances in transistor technology. With these advances, you can now reap the benefits of solid-state switching, which include faster switching speeds and unlimited mechanical life, to build better, faster, and more economical test systems.

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