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Rough Guidelines to facilitate fixturing,,
One of the common question we receive regarding testing of PCBAs is, "What guidelines should we adhere to when layout of boards is done to facilitate testing." Aspen Test
Engineering has a testability video and it has a great deal of information. But for simple rules of thumb here you go.
When you design your boards there are a few simple rules that will make you ability to test a given board alot easier, faster and more reliably.
Provide Access to all of the nets on the board. It seems obvious but that is the case with everything. In order to design and build an effective test fixture the nets on the board to be available. If ALL of the nets are made available then the test fixture designers can pick and choose where they want to put probes. Please do not conformal coat over the test points and vias. If these positions are left available it will provide options and that is the key to making the fixture simple and reliable.
Keep all the test points accessible to the bottom side of the board. When test points are placed on opposing sides of the board it forces the board designers to design a complete
top access. This introduces an entirely new tolerance stackup and essentially will double the price of the fixture. Why? Because with a bottom only test setup the top side of the board only requires the ability to compress the board onto the test points and the accuracy of this function is certainly not difficult to attain with the addition of top access it requires an additional alignment system to assure that the top and bottom of fixture are aligned much more precisely. It also requires a wire nest and transfer grid above the fixture to assure the data is transmitted to the UUT and back.
Board thickness, at first it seems to make sense that the thinner the board is the less expensive it will be. That is until it’s time to test. The dynamics in compressing the board are complex (much more so than one might think). The loads and stresses that are created in a .060 thick board can be managed much more easily than those same loads in a .030 thick PCBA. In addition to this, the scrap that builds up when testing a .030 board are much more significant.
Test point sizes. As long as test point sizes are .030 or larger no special treatment needs to be done to assure that the fixture will make good contact every time. Once they get smaller than that they many times it will require that guided probing is done and this opens a fixture complexity door that will make it very tricky to assure contact.
Test point proximity to other test points. It is recommended that test points are no closer than .050. Smaller probes are available and it is possible to get closer but the probes are much more delicate and flexible leading to registration issues.
Test point proximity to components. It is recommended that the maximum size of the component is allowed for plus .060 clearance for probes to the center of the test point. Per the IPC standard components may vary from their nominal location significantly.
One of the main items that fixture designers face is when a tristate device is attached hard to a voltage or ground line and the net is not directed through a gate. When circuit layout is done all tristate devices should be installed so that all of thier lines can be isolated through gates.
Tooling holes are key to any fixture. .125 dia tooling holes all by themselves make a PCBA much more testable
For more information or if you have questions please feel free to submit them to sales@aspentest.com. Please come visit us at our website www.aspentest.com we have much more information there.