DFT

PCB DFT (Design for Testability), as the name implies, does require specific design actions to be performed before it can be applied during the PCB manufacturing stage of product development. This generally means adding test points, headers, LEDs, test traces and undocumented features to the board such that parameters of interest can be calculated or measured for direct determination, which can speed up validation testing.

 

Test Points

 

If your design gets ICT (in-circuit testing), you will have test-points for the fixture. You will probably try to arrange these to minimize any signal integrity issues. But you can also arrange them to make your design easier to probe. For example, aligning them for a differential probe tip at the interesting end of the signal trace. Or, by adding ground and power test-points in strategic places where I know I’ll be probing a lot.

 

Headers

 

You can put on lots of headers that never make it onto the final bill of materials (BOM). Putting in headers for main voltage rails, just to have an easy place to stick a DMM or connect a cable for chamber testing, etc. Also, any time have a JTAG port on programmable logic, putting on a header that will match the programmer. I²C busses can tolerate some SI sloppiness, so I put headers on them. Unused I/O from a CPLD, FPGA, or microcontroller can be routed to a header for debugging or later experimentation.

 

LEDs

 

You can put some LEDs where can make them useful. For example, a DONE signal on a power supply sequencer, to quickly let me know that the thing powered up. Or debug LEDs on FPGAs, microcontrollers, etc., which can use to debut code issues. They can be stripped off the BOM later, but are really nice when you have a prototype that may not be quite ready for prime-time, so you can quickly see what is happening with it.

 

Test Traces

 

If there is room, you might just layout traces that mimic other sensitive traces, so you can experiment with them. Put test-points or connectors on them to connect to scopes, TDRs, signal-generators, etc., so that you can see what your other signals potentially look like. Or, if you have spare clock outputs, run one to a connector; it might be useful for triggering or syncing during testing.

 

Undocumented Features

 

Perhaps, if you have room and time, you might even design and place an entire experimental circuit or extra function that you don’t intend to ship. It might allow you to test out a new idea. Maybe you tried this once to experiment with some minimum loadings for a power supply. Or it might allow you to have better access for debugging, like a USB port for your embedded device. This does take up board space and design time, though.

 

For PCB DFT, there are basically two tests that fall under DFT: Flying Probe Test and Bed of Nails Test.

 

Flying Probe Test

 

The flying probe test is a non-invasive Inc-Circuit Test (ICT) to measure open/short circuits, values for passive components and continuity. This test is designed to be applied for prototypes and low production runs. Utilizing the flying probe test is a joint activity requires the following steps by the PCB designer and contract manufacturer.

• PCB Designer Steps

Step 1: Determine what parameters you want to measure. Parameters may include values for passive devices, such as resistors, continuity for devices like diodes and connectivity, including open and short circuits.

Step 2: Add test points to your PCB layout where probe access points, such as vias and through holes, do not already exist.

• Contract Manufacturer Steps

Step 3: Program flying probe tester to stop at the required test points on the PCB assembly.

Step 4: Run the test and gather test result data.

 

For full coverage of your PCB, you can use the following guidelines to determine where and if test points are needed.

• Use thru-hole pins as test points
• Use standard and blind vias as test points
• Use surface mount pads as test points
• Add test points to otherwise inaccessible nets

The flying probe test can require substantial time, perhaps several days, for setup, programming and running. However, even if a redesign is needed, changes to the test program are usually minimal.

 

Bed of Nails Test

 

The steps required for the PCB designer and contract manufacturer are similar for both the bed of nails test and flying probe test. However, although both tests require setup time before they can be performed, deploying the bed of nails test necessitates constructing a dedicated fixture. This adds significant cost as well as time to the PCB manufacturing stage; in fact, developments setup time can extend to several weeks. However, the bed of nails test itself can be performed very quickly and is applicable to production runs.

The bed of nails test can provide more detailed information than is usually gleaned from the flying probe test. This includes analog and digital data, along with some integrated circuit functional data. However, physical requirements can occasionally prevent you from using the bed of nails test for some soldered pads. Therefore, it is important to consult with your contract manufacturer early in the process to ensure that any such restrictions are accounted for as part of your PCB DFT.

 

Is PCB DFT Necessary?

 

The answer to this question depends on a number of factors. For simple circuits, the time and costs associated with DFT are probably not justified. On the other hand, if your PCB boards are complex and will reside in highly sensitive systems, the reassurance that comes from performing PCB DFT may be appealing. If you opt to employ DFT into your PCB development process, the following table provides a head-to-head comparison for attributes that may be helpful in choosing between the flying probe or bed of nails tests:

 

Attributes	Flying Probe Test	Bed of Nails Test
Shorter Setup Time	✓
Shorter Setup Time Needed for PCB Redesign (if new fixture required)	✓
Programming Required for PCB Redesign	✓	✓
Faster Run Time		✓
Lower Cost/ Unit (Run Time)		✓
Types of Measurement	Flying Probe Test	Bed of Nails Test
Open/Short Circuit	✓	✓
Passive Component Values	✓	✓
Analog/Digital Data		✓
IC Testing		✓

 

The most important part of the PCB development process is the manufacturing of your PCB boards. As having your boards function as required is absolutely essential, you should employ all reasonable options at your disposal to ensure PCB functionality. Your options include the degree of your contract manufacturer’s involvement in verifying your PCB’s functionality via experimentation or testing. For highly specialized and sensitive applications, the reassurance that accompanies DFT may be a logical option, despite the additional time and cost involved. If you choose to utilize PCB DFT, it should be in addition to other QA and DFM procedures that should always be included in your PCB development process.

 

PCB DFT for Functional Circuit Testing

 

FCT is a black-box approach that targets the overall output or functionality of your PCBs, meaning that the complexity of the test depends mainly upon your test procedure, rather than the minutia of the board itself. When you request FCT for your project, during the quoting stage of the process, we will ask you to submit a written test procedure, which is analyzed to determine cost and lead time requirements. The most efficient test procedures require only a few steps, such as powering the board and looking for a specific message to be displayed on a display, or a certain set of LEDs to turn on.

Some clients will include LEDs in their design whose only purpose is to identify correct operating voltages at specific points on the board. While this strategy does take some space on the board, it can mean that your project can be verified by a simple FCT procedure, rather than a more in-depth ICT check.

 

PCB DFT for In-Circuit Testing

 

ICT is a white-box approach, where our test engineers monitor individual voltage and current levels on a finished PCB, possibly even performing step-by-step program execution on the board’s firmware. This method is much more in-depth than FCT, and normally requires considerably more in terms of both time and cost, but it is very effective at pinpointing any potential issues on a board, down to a single component.

ICT is most efficient in Prototype PCB Assembly projects since the lower quantities of these orders mean that the greater time requirement of ICT is not multiplied overmuch by a large number of boards. That being said, it is possible to optimize your higher-quantity order to ICT by designing a test fixture. A fixture allows for us to test your boards quickly, by simply pressing them down one-by-one on a collection of test pins, similar to Bed of Nails PCB Testing for bare PCBs, rather than meticulously probing with a DMM or oscilloscope. It should be noted that you need not actually design a mechanical fixture, but simply indicate to us in a drawing which pads should be tested and what their electrical characteristics should be at a given point in time. Where probing is necessary, the process can be made more efficient by the inclusion of test points on the board for easy connection to our test equipment.

At MADPCB, we understand that high quality PCB Assembly means getting it right the first time. We always do our best to get you your boards at the lowest possible cost, and with the shortest possible lead time, but quality and functionality remain our highest priorities at all times. This mindset is the reason that our PCB Assembly Process includes multiple stages of testing, including E-Test, Visual Inspection, AOI, and AXI for lead-less components. All of these testing methods are included by default when applicable, but our clients can also request additional testing at their discretion. These optional testing methods are known as In-Circuit Testing (ICT) and Functional Circuit Testing (FCT), and both involve a post-assembly check for the desired functionality of your boards by our testing engineers.

If you still have any questions about PCB DFT (Design for Testability) for your project with MADPCB, you can contact us at any time and send in your questions by email, or to get an official quote on your PCB design files.