When I was in college, one of my colleagues and I would constantly debate the importance of simulation and experimentation. He’d insist that designs are proven during simulation, while I’d counter that experimentation is the end-all of development. Although we never came to an agreement back then, experience has proven that both simulation and experimentation are essential for successful and efficient product design and development. In fact, when developing PCBs, it is best to view the entire development cycle as an integrated process as opposed to separate stages. To implement this broad perspective, your board design must anticipate and incorporate key elements to facilitate the experimentation or testing stage.
Supplementing your operational design with elements and test points to facilitate the functional testing of your board is known as design for testability (DFT). DFT is not to be confused with design for manufacturing (DFM), although both are designer activities based on your CM’s equipment and process capabilities. DFM is the employment of board parameter specifications and restrictions that enable your PCB to be manufactured. Instituting good DFM can shorten the turnaround time for PCB manufacturing, while DFT can make board bring up simpler and easier; although, there will be an increase in the time, and possibly cost, of manufacturing your boards. Therefore, before opting to use DFT, you need to ask whether it is really necessary for your design. But to adequately answer this question, we first need to examine how DFT supplements the standard testing performed by CMs.
Quality Assurance Testing
The electrical testing methods utilized by CMs can be classified by purpose. Tests conducted to measure your board’s connectivity fall under quality assurance or control. These quality checks may include:
- Impedance testing is performed to ensure there are no open circuits along traces or breaks between soldered connections.
- Continuity testing is done primarily to test the direction of current flow through components, such as diodes.
- Invasive tests, such as sawing, are employed to inspect the board’s internal structure (e.g. layers, vias).
Most CMs utilize manual or automated inspections as part of their QA regimens. To have any of the above testing methods performed, a request may be necessary. However, none of the above tests require any dedicated design activity prior to manufacturing.
Design for Testability (DFT)
DFT, as the name implies, does require specific design actions to be performed before it can be applied during the manufacturing stage of product development. This generally means adding test points to the board such that parameters of interest can be calculated or measured for direct determination. There are basically two tests that fall under DFT: the flying probe test and the bed of nails test.
Flying Probe Test
The flying probe test is a non-invasive in-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 that requires the following steps by the PCB designer and CM:
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.
Step 3: Program flying probe tester to stop at the required test points on the PCB.
Step 4: Run the test and gather data.
For full coverage of your PCB, you can use the following guidelines to determine where and if test points are needed.
◎ Use through hole pins as test points.
◎ Use standard and blind via holes 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 CM are similar for both the bed of nails test and the 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, development 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 CM early in the process to ensure that any such restrictions are accounted for as part of your DFT.
Is DFT Really 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 boards are complex and will reside in highly sensitive systems, the reassurance that comes from performing DFT may be appealing. If you opt to employ DFT into your PCB development process, the following table provides a head-to-head comparison of attributes that may be helpful in choosing between the flying probe or bed of nails tests:
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
Passive component values
The most important part of the PCB development process is the manufacturing of your boards. As having your boards function as required is absolutely essential, you should employ all reasonable options at your disposal to ensure board functionality. Your options include the degree of your CM’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 DFT, it should be in addition to other QA and DFM procedures that should always be included in your PCB development process.
|Tempo‘s Custom PCB Manufacturing Service|
At Tempo Automation, we specialize in leveraging DFM to produce high-quality PCBs and will partner with you throughout your PCB development process, beginning on day 1. Additionally, we enable you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on design for testability (DFT) or other CM functionality testing, contact us.