Why Production Test Jigs Are Essential for Efficient Electronics Manufacturing

You have been working on your latest electronic module design project.The design was finished, manufacturing documentation was released and two weeks later, the manufactured prototypes arrived at your desk. You took the technical specification and ran through the design verification process to ensure everything worked as specified. You even delivered the beta batch to the field and eventually ensured everything worked as expected.

That’s all great, but this is only one side of engineering work. The other aspect that we need to keep in mind from the very start of an electronic module design is testability.

Overview of the Electronics Manufacturing Process

Before we dive deeper into the production testing, let’s look at the entire process. The electronic module manufacturing process is roughly illustrated in the flow diagram below.

Inputs of the process are raw PCB and electronic components. They go through the electronic manufacturing process (done by EMS – electronic manufacturing service). The output of this process (and the first milestone of the overall process) is the electronic module, a.k.a. PCB assembly (PCBA). PCBA needs to go through the above-mentioned testing procedure (highlighted in purple color in the above flow diagram).

This procedure is the focus of this blog. After PCBA is tested and confirmed it is defect-free, it gets integrated into the rest of the system (connection with other PCBAs in the system, fitting into the enclosure etc.).

In this phase, integrational testing occurs, which is the last step before packing the complete product and shipping. However, this may be covered in another discussion.

In one of our blogs, we discussed Design for Manufacturing (DFM), an approach that makes this complete process as easy (and cheap) as possible. As discussed, one of these points is design for testing, an approach that makes the purple box above easier (or even possible in some cases).

The production testing goal is to ensure the modules are manufactured according to the documentation and there are no defects in the manufacturing process. Typical defects that can occur are:

• Bad soldering joints (open or shorts),
• Missing or broken components,
• Wrong oriented or misaligned components,
• Wrong value components,
• Foreign object debris…

Now, why would someone spend money on developing automated testing equipment – testing jigs?

Reducing Testing Time with Production Jigs

If the product is still in the development or testing phase and we are looking at smaller production batch, we will most likely develop a production testing protocol that will utilize a manual labor – a testing technician with all the needed tools and instructions on what point to measure and what steps to take. Depending on the module complexity, this might take about 5 to 60 minutes per module! And that’s fine if we are not talking about higher volume series.

But once we reach a higher volume series, it is beneficial to develop automated testing equipment that will automate this process (production test jig). This will result in lower testing time and effort needed. With the testing jig, we are talking about 30 seconds up to 3 minutes for more complex testing procedures.

Eliminating Human Error in Testing Procedures

No matter how careful the testing technician is, manual production testing process is always prone to subjectivity in decision making (specifically, we are talking about pass/fail criteria). Also, with a large quantity and tight deadlines, it is possible that human errors appear (e.g. misreading the test result, measurement at the wrong location, counting errors, flawed labelling, etc).

While using automated test equipment, the testing procedure is automated within a testing script. This means that pass/fail criteria for each test are also documented in a configurable file, so human factor is eliminated. Also, any serial number reading or labeling for each device can be automated.

Automated Data Logging with Testing Jigs

All units are typically tracked by serial number or similar unique ID. The testing script can automatically log each testing session of a unit together with all important parameters – operator name, testing time, evaluation of the testing etc.

Cost Efficiency of Production Testing Jigs

Even though the initial price of testing jig development is not negligible, we need to consider a complete return of investment. Of course, a testing jig will be utilized for the current PCBA we want to test.

Further, almost any revision of the module can be tested with the same jig, only with minor modifications of a DUT-specific electromechanical part. Even a new variant of the product might reuse the existing parts of the jig. That’s one of the reasons Byte Lab approaches testing jig design in a modular manner.

Who Pays for the Development of Testing Jigs?

That said, who should cover the costs of developing testing jig and production procedure? After all, the electronic manufacturing service (EMS) companies use it. Well, the answer is not that simple. In some cases, the EMS will develop the jig and the procedure, as it will reduce the time and effort needed (so it will increase their margin).

Sometimes (when the jig and procedure are more complex), the EMS doesn’t have the necessary resources and the design house would do it to reduce the manufacturing cost, ensure quality or enable the manufacturing at the less sophisticated EMS.

Popular Production Testing Methods

There are multiple possible ways to conduct production testing, to name only a few:

• In-circuit testing (ICT)
• Flying probe testing
• Functional testing (FCT)

Of course, each of these methods can further use a spectrum of different testing technologies, but the complete testing approach can be characterized as one of the above.

We will briefly analyze each of these testing approaches.

In-Circuit Testing (ICT): Advantages and Disadvantages

This method requires a bed-of-nails fixture.

This fixture consists of:

• The common part – same for all PCBAs the would be tested (devices under test – DUTs) and
• DUT-specific part – part that contains the specific needle fixture that corresponds to the DUT test points. This is the part where the electrical connection between the testing jig and DUT is established.

It also requires each net on the DUT to have an exposed test point (which can be troublesome for a very complex design, sometimes even impossible).

ICT consists of many simple tests. The testing sequence is written for a specific DUT, and it measures connection from each net to a neighbor net and does either of the listed measurements:

• Measures impedance between two neighbor nets (identifies whether a good passive component is mounted, and makes sure the soldering joints are good)
• Does a diode testing from the ground to the IC pin (makes sure the IC pin is soldered correctly)

The testing sequence can be written by a testing engineer simply by inspecting the electrical schematics or in some cases empirically (by measuring it on a golden sample – the DUT specimen we are certain is 100% correct).

In short, the pros of ICT are:

• Very good test coverage,
• Simply to write the testing sequence,
• In case of test failure, it’s easy to pinpoint the exact failure source (we can immediately tell which component is not good).

The cons of ICT are:

• Test points on all nets are mandatory. This can be quite a large number, and it can cause difficulties in a complex design.
• Adding test points to some circuits can be problematic as these copper structures need to be minimized – e.g. RF path, switching mode power supply nets, sensitive analog nets…
• Some impedances are very difficult to measure. For example, very small capacitance or inductance (as their value can be very close to the parasitic values of the needles themselves).

Flying Probe Testing: A Flexible Solution

This method is like ICT in terms of test sequence, but it does not require a DUT-specific testing jig. In contrast to the ICT, it is the automated test equipment (ATE) that uses the measurement probes that are movable (“flying”) and can probe the arbitrary PCBA coordinates. It doesn’t even have to be a test point (it can probe directly at the components pads and uncovered vias), which is one more improvement compared to the ICT.

This is very useful, since this means that a product can go through a redesign in a certain phase of a lifetime (doesn’t need to have a fixed location test points). This ATE is not even bonded to a specific product but can be used for any design that uses suitable components. This makes it a long-term investment. However, this also comes with a price – a flying probe ATE can cost approximately 15-20 times more than an ICT testing jig.

Functional Testing (FCT): A Comprehensive Testing Method

Various OEMs and EMSs imply various methods by the term “FCT”. The most generic definition of FCT would be (as the name implies) to simply provide electrical inputs to the DUT and check that it functions as expected. There are multiple possible levels of this testing – sometimes it could be as simple as turning on the DUT and checking the power consumption or a more detailed functional test of each subsystem with checking every functional aspect. Byte Lab uses this approach in the majority of cases, as we see it as an optimal tradeoff between the price and test coverage.

Of course, the FCT that Byte Lab designs does not test the complete product as a black box. We stimulate and measure outputs of each subsystem alone (and finally complete picture altogether). This provides test coverage that is comparable to the ICT, but without the necessity of adding 100+ test points to the design.

The testing sequence is almost always specified by the same engineers that are doing the design itself. This enables the complete design process of the product (including DFM and DFT), to be optimal – without an additional unneeded effort to add test points that are not mandatory. The engineer who is writing the tests sequence knows which test points are mandatory and does not automatically add test points to each net (as a generic requirement for ICT).

ATE used in this approach is mechanically almost the same as in the case of ICT.

The benefits compared to the ICT are:

• It doesn’t require test points on every net in a design.
• RF and other sensitive circuits can be tested with good reliability (ICT is not capable of doing this as the measured impedances here are typically too low to be measured without an additional Vectored Network Analyzer integrated into the test jig).

The cons of the FCT are:

• A more complex testing sequence than a simple RLC or diode measurement
• In case of a test failure, it is not 100% straightforward to pinpoint the failure source (if the engineer does not conduct the testing, which is typically not a case). That’s why we believe that a mandatory output during a tester design (among the tests sequence and testing procedure) is a troubleshooting guide. This guide enables the EMS technicians to locate the failure source.

Why It’s Important to Plan Production Testing Early

To conclude, it is important to consider the production testing of the module at the earliest design stage. The sooner we start implementing the testing support in the design, the more natural it is, and it comes with a less price (in terms of engineering effort and time).

However, it is never too late to consider it and make your product testing easier and cheaper. There are multiple testing approaches and each of those has its benefits and tradeoffs. Therefore, feel encouraged to discuss your product manufacturing and testing with your design house anytime, it’s never too soon!