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NOVEMBER 2020 I DESIGN007 MAGAZINE 37 In some cases, vias are also used to transfer heat. Since a circuit board with all its differ- ent internal layers and metal planes makes for an excellent heatsink, adding vias under hot components will dissipate that heat through- out the board. This can bring a lot of heat re- lief to those parts instead of allowing the heat to build up in those areas of the board where the parts are located. Circuit board test is another important use of a via. To test the connectivity of a fabricat- ed board—as well as the assembly integrity of a fully manufactured board—a variety of tests are conducted on the PCB. These include us- ing test fixtures with probes for every net on the circuit board that contact all the points designated as testpoints simultaneously. Another method is known as a flying probe test, which is a machine that probes each test- point individually. The important part of these different testing methods, however, is that vias usually serve as the testpoints which the probes come in contact with. To facilitate this testing, a PCB designer will use some specialized features in their CAD tools to flag specific vias as testpoints. With a via marked as a testpoint, the location of that via and the net it is attached to then can be ex- tracted from the CAD database in order to cre- ate a testpoint file. This data is then used for building test fixtures and programing the test machines. In some cases, the size and shape of the testpoint vias will even be changed on the layout to visually identify them as a testpoint on the fabricated circuit board. Additionally, these testpoint vias are usu- ally subjected to additional design rules and constraints than a regular via is. For obvious reasons, they can't be placed under compo- nents, and they need to have specified clear- ances from each other to ensure that the probes on the bed-of-nails test fixture can easily access them. Considering all that they are used for and what is required of them, vias are actually sub- jected to a lot more action and adventure than I would have expected. I wouldn't put them on the same level of excitement as SG-1, but vias do have a lot of intricacies to them that commands respect. There is a lot to consider when working with them to ensure that your PCB design performs up to specification and is manufacturable the way you intended it to be. Thankfully, there are people in the de- sign community who are a whole lot smart- er than I am about the best way to work with vias. You have the opportunity to learn from them in this edition of Design007 Magazine. I'm looking forward to reading myself what I-Connect007 and its experts have to say about vias when I receive my own copy. Until next time then, keep on designing, and vias, you have a go! DESIGN007 Tim Haag writes technical, thought- leadership content for First Page Sage on his longtime career as a PCB designer and EDA technologist. To read past columns or contact Haag, click here.

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