The Unsung Value of the End of Line Production Test

If you create a product, you need to test it to make sure it works. Seems simple enough, right? All too often, however, companies will fail to properly test each item that they produce, resulting in numerous issues down the road for them and their consumers. The key point here is that many companies making products with radios in them don’t know they need to test them in the production process.

Back in the 1970s and 1980s, the electronics industry was particularly problematic in this regard. Essentially, the first person to actually test a device was the consumer. For some companies, products would fail out of the box 60-70% of the time. This concept is called “infant mortality” and has contributed to many bankruptcies over the years. A lot of things have changed to ensure this doesn’t happen anymore, and one of those things is end-of-line production test, also known as functional testing.

What is a production test?

For any device development, there are two major types of testing. Design verification tests (DVTs) determine if a design does what it was designed to do. This happens as part of the development process and is normally done by the engineers designing the product. This ensures the design meets the requirements captured in the Product Requirements Document. Once the design of the product passes these tests, there’s no need to perform it again.

Production tests, on the other hand, verify that each copy of the product does everything the user expects it to do. Does it function properly from the point of view of the user? These tests need to be conducted on every copy of that device or product after it’s assembled. You want to test only the inputs and outputs of the product, but also the things the user will interact with. A production tester is NOT a diagnostic system. It should spend no extra time or effort on determining WHY a given copy of the product doesn’t work. If you’re already getting certain pieces of useful info as part of the test, no harm in recording it for diagnostic purposes later, but don’t build in any more complexity than is necessary.

Production test systems are as unique as the products themselves. It is essential that the product and related production tester are developed together. If they aren’t, you will end up holding up production later in the project. When do you start work on the production tester? You can start on the same day you start on the actual hardware design of the product. If you think you can wait, then later in the development process, you WILL wait. You’ll have the product done but you can’t ship because you have no production tester yet.

What do you need to test?

In production tests, you don’t need to test most of what was tested in design verification. For instance, you might feel like you want to test all the internal system voltages, but you don’t need to. The exact voltages of the internal power supply system will impact the things you DO test, so they’re covered, you just don’t have a specific test of system voltages. You WOULD check max power consumption in the system’s highest use case; this will tell you if something is seriously wrong in the power system. You would check sleep power consumption in the system’s lowest power use case; this will ensure the device actually meets the users run-time expectations. You may have other intermediate power states you need to check, again to meet user expectations of how the device is supposed to behave.

Subassemblies

If you are using purchased subassemblies, such as a radio module or System-on-Module (SOM), these components were tested by the manufacturer so there is no need to re-test. You just need to test where those components interact with your device.

• For radio modules, the behaviors and modulation has already been tested. Usually you are adding voltage/power supply, a USB or serial comm interface, RF path, and sometimes an antenna. These interfaces are all you need to check.

• For SOMs, you don’t need to check the ethernet modulation or USB comm protocols. Just test for the input power supply, comm interfacing, and any shielding or cooling that has been added to it.

Checks for the added components listed above are relatively straightforward. Power supply check is a simple voltage & current measurement. The comm interface check is having the main CPU talk to the module and verifying it works. An RF path/antenna check is as simple as feeding RF signal through path and measuring losses that it incurs.

Antennas

For products with internal antennas, performance can be hindered by misplaced components, wrong values, physical damage, or anything else that can impact the device during assembly. You need to test performance to ensure everything works as expected. You’ll need to measure radiated receive sensitivity or radiated transmit power. Measure signal strength at the margin where it is close to failure, typically within 6 Db of its margin limits. This has to be done in a controlled environment and for some radios you do have to do it inside an RF shielded enclosure depending on how the test is done.

Radios

If you have a radio in the product, you will need to test its performance as well. But only those aspects that were not tested by the manufacturers of the components you are implementing. Once you assemble the product, measure conducted or radiated receive sensitivity or transmit power. Some radios like GPS receivers require a specialized signal generator to perform the testing. Radiated power measurements, on the other hand, can be difficult to get consistent. At F3 we have techniques that can help.

Conclusion

As we work with customers to help address certification, performance, and other factors, we often encounter problematic end of line production test systems. They sometimes don’t test the radio part of the product at all or they test the wrong things and fail to ensure quality products. Beyond the radio, crystal frequency, low power current consumption, and even LEDs often are insufficiently tested, if at all.

F3 has a team of dedicated production test experts who can review your system and help to get your product tested in a way that ensures the best quality. We have an array of reusable test elements, both hardware and software, in a toolbox called Thrall. With Thrall, you can potentially remove up to 70% of the development timeline for a customized production tester and dramatically lower development costs.