Smart Tests

Did you ever receive circuit boards that did not work as expected, and you do not have a clue why? In the traditional electronic industry usually only ⅔ of the circuit boards survive an electrical test. But did you know that with a few design tweaks you can improve the survival rate immensely?

Manufacturers are cagey about providing detailed test information as this could reduce their workload. Instead, they let you work with an imperfect design which will crash in series production later.

Not with us. 100% of our Beautiful Boards are electrically tested. With our Smart Tests we provide you with a detailed test report of your order directly after production. This gives you a fantastic opportunity to identify and improve weaknesses in your design.

About this article

This article explains everything you need to know about our Smart Tests. You have received a faulty test report and are now wondering what to do next? Or you are simply curious and want to learn more? Then you have come to the right place.

Basics about E-Test

Unlike many other products, where a complete check of an entire batch is too costly and/or would lead to the destruction of individual products, the high efficiency of today’s flying probe testers enables a complete electronic check of each individual board before it leaves the manufacturer.

The test procedure is always identical: For each measurement, two different measuring points are approached simultaneously by two probes. A potential difference is applied, and the resulting resistance value is measured and documented.

But how does the flying probe testing machine “know” between which points to measure and how to interpret the results?

The basis for the test program is formed by the nets and pads defined in the respective PCB design, which are derived from the circuit diagram. They determine which points must be conductively connected and which must not.

Pads are designated and exposed surface areas at which components can be soldered or mount onto the PCB. A net is a group of pads that are conductively connected to each other.

The logical connection between nets and pads is hereby as follows:

All pads of a net must be conductively connected to each other. Conversely, pads from different nets must not be connected. This logically also leads to the fact that each pad can only belong to one single net.

During a test run, both pads of the same net and pads of different nets are approached. Our Smart Test checks for open connections or short circuits.

OPEN: The measured resistance value exceeds a pre-defined threshold for pads of the same net. This means the two pads are considered as not connected.

SHORT: The measured resistance value between pads of two different nets falls below a pre-defined threshold. Thus, the to probe points are considered as connected.

However, now the testing machine needs to know, if the found open connection or short circuit is intentional (e.g. if two pads of the same net are tested, we are expecting a short) or not. To specify the expected connectivity between two probe points, we are using so called IPC data.

IPC-D-356 and Gerber

Unlike the other PCB data, the detailed test specifications are not written and transferred in Gerber, but in a standardised IPC-D-356 format, or IPC for short. We extract this file from any CAD file automatically when you upload your project files to AISLER.

Please note that we ignore any IPC file that is included in zipped Gerbers. If you want Smart Tests please upload your CAD file or contact our support.

As already mentioned, the logical relationship between the listed pads and nets in the IPC file is based on the schematic in your CAD application.

**For a meaningful test, it is therefore necessary that the schematic has also been completely implemented by the developer in the corresponding PCB design. **

Test data and PCB design must match before manufacturing. If two pads of the same net are already not connected to each other in the original copper layers, an open connection will certainly be detected in the e-test. In this particular case, there is no manufacturing defect, although the test result in itself suggests otherwise. It was merely tested “incorrectly” based on the board design, respectively the Gerber data. From the point of view of the circuit diagram, an intended connection was not implemented. In such a case, it is up to the developer to decide whether the design produced in this way is functional or whether an adaptation of the design and a re-manufacturing is necessary.

How we manufacture your PCBs

As you may have noticed, the minimum order quantity at AISLER is three PCBs; larger quantities must always be multiples of three. This is because a batch always consists of three identical panels. Depending on the size of your order, we may also have to split it into multiple batches.

For example, if you have ordered 27 PCBs, each panel will normally contain 9 instances of your PCB.

9 PCBs (instances) / panel × 3 panels = 27 PCBs

After production, each panel is electrically tested, and the Smart Test report will be attached to your project and will be visible under the Tests tab of your respective project. We will also inform you by mail as soon as the results are available. The test results are imported on a panel level, so that all boards on the same panel will show up subsequently. This can help in identifying, what might have gone wrong.

The Smart Tests page

Once the test results of the manufactured panels containing your PCBs become available, we process the results in the background and display them for you in a clean and convenient way: once you open the Tests tab of your project and select the corresponding revision, the results will be displayed grouped by PCB instance. So for each PCB (board) instance will either display “No issues detected” when all tests for this particular board instance have passed, or “Issues detected” when something seems off. You can then see the measurements for the particular PCB instance by clicking on the Details button.

Here you can find the type of the failed test, the source and target pads and nets as well as the measured resistance.

In the above example, 12 PCBs were ordered in total. This means that four PCBs were placed and manufactured on each panel. This, then results to boards 1-4, 5-8 and 9-12 each residing on the respective same panel.

How to interpret the test results?

If there are no issues: Amazing! :tada: Everything went well during production and the results all match the expectations. This also means that your design is perfect to manufacture. Good job!

If errors are found in your order, there is no reason to panic! In many cases, the PCBs themselves have been manufactured correctly and the detected errors can be traced back to deviations between test data and PCB design.

In any case, all boards will be sent to you without any impact on the dispatch date.

Before you start measuring all the affected connections of the boards, we recommend that you take a closer look at the Smart Test results. Often, the type of defect, the defect consistency and the frequency can already be used to assess whether a manufacturing defect is likely or not.

To assist you with this task, we will give you some clues:

Identical opens on all PCBs

Please check your design first. It looks like the manufacturing data deviates from the test data.

  • Are there any unrouted wires / missing copper connections?
  • Are all vias in the right place?
  • Have vias been removed before manufacturing? Please compare your design once again with our BoardViewer!
  • Is your soldermask data correct and no pads are accidentally covered?

If everything seems fine: Please wait for the boards and check if all visible vias in the BoardViewer are correctly manufactured.

Identical opens affecting only certain boards

If only some boards show issues, while others don’t, please check for the following cases.

Random boards affected

Example: Boards 1,3,8,12 have issues, while the others are OK.

Please check your production data and test data for any mismatches like above. Furthermore, upon arrival of the boards please check if all visible vias displayed in the BoardViewer are manufactured.

Subsequent boards affected

Example: Boards 1-4 show issues, while the rest is OK.

If all defective boards are subsequent, this might indicate a manufacturing defect of a panel or instance due to one or more missing vias. Please check the boards after the receipt.

Identical shorts on all PCBs

Please check your design first. It looks like the manufacturing data deviates from the test data.

  • Did you use one or more netties in your design? Please note, that nets connected by a nettie are interpreted as separate nets.
  • Does each of your nets have a name?
  • Is every pad assigned to a net?
Shorts and / or opens are randomly detected on one or more instances

This is an indication that one or more boards have not been manufactured correctly. Please inspect the PCBs optically upon receipt.

This may be due to a conventional manufacturing defect. However, you should also check whether your design complies with our Design Rules before reproducing it. The cause may also be in the design, which is difficult to realise from a production point of view (e.g., with exceptionally long and thin trances). Please also have a look on our Ultimate Guide to a robust PCB Design.

Okay, my PCBs were manufactured incorrectly! What happens next?

We are sorry if we have made a mistake here. In this case, please use our contact form and describe the problem. We will do our best to identify the cause of the error and find a quick and satisfactory solution for you.

Help! I still don’t understand what’s wrong with my Boards…

Fear not! In this case too, we are happy to assist you. Please contact us using our contact form contact form and let us know, what your problem is. Furthermore, we are open for suggestions, so if you find something confusing in the Smart Tests or this documentation, let us know, so that we can improve!