Bittele's Essential Design for Testing (DFT) Tips for PCB Assembly
At Bittele Electronics, 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 Electrical Testing, Visual Inspection, AOI Testing, and X-Ray Inspection 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 Quality Assurance engineers. The cost and time requirements of the ICT and FCT procedures is largely dependent upon the specifics of the project in question, and there are a few strategies that you can employ to make these procedures more efficient. We use the term “Design for Testing (DFT)” to describe the optimization of a project for efficient ICT and FCT, and this article aims to give our clients a basic overview of best practices in DFT when it comes to PCB Assembly projects.
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, your Bittele account manager 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.
DFT for In-Circuit Testing
ICT is a white-box approach, where Bittele’s 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.