Visual inspections are performed following the manufacturing process of a printed circuit board to identify any potential problems. While that may work fine for a two-layer PCB, for multi-layer PCBs, it is almost impossible to visually inspect the internal layers once the board has been completed. Therefore, visual and machine tests are done on inner layers prior to lamination to find any problems and correct them. However, the only way to truly verify that a multi-layer board is electrically sound is to conduct an electrical test. Electrical testing on printed circuit boards is a common operation performed during the PCB manufacturing process as one of the last steps prior to completion.
Two Main Types of Electrical Tests
There are two typical types of electrical tests done on a bare printed circuit board: Flying Probe and Universal Grid. Since the probes move around to all the PCB test points, flying probe does not require a fixture. Universal grid requires a fixture to conduct the test and is sometimes called a clam shell or bed of nails test.
The PCB test of a board will use capacitance and resistance. A capacitance test will test for opens and shorts by sending a charge on the net (trace or plane) and then probe each net to measure the induced capacity. Resistance test measures resistance as the current flows through a conductor (trace), in ohms.
A good conductor has LOW resistance. At regular temperatures, the best conductor is silver. The second-best conductor is copper and due to this conductivity, the manufacturing of printed circuit boards relies heavily on copper. The amount of resistance depends on how long, how wide and how tall the trace is. For instance, if you have two traces of the same material and the same height but one is twice as long as the other, then the longer one will have twice as much resistance. If you have two circuits that are the same length but one is thicker, the thicker trace will be half the resistance. Resistance dissipates power in the form of heat.
Net: An entire string of points or connections from the first source point to the last point including component lands and vias.
Net List: A list of alpha-numeric locations that are used to describe a group of two or more points that are electrically common.
Shorts Test: Check to make sure that NO current flows between separate nets by measuring the amount resistance between them.
Opens Test: Check to make sure there is current flow from one “node” to the next for every net on the board.
100% Net List Test: Check of every “node” on every net on the board.
Optimized Net List Test: Check every “end of net” for all nets on the board and selected intermediate nodes on the board.
Guidelines on how bare circuit boards should be tested are provided by IPC via the standard, “IPC- 9252 Guidelines and Requirements for Electrical Testing of Unpopulated Printed Boards” which can be accessed directly from IPC.
Section 4.4 States: “One hundred percent continuity and isolation electrical test is the confirmation that the actual electrical interconnect of conductive nodes matches a proven reference source, including but not limited to CAD/CAM digital data, master pattern artwork, or released drawings.”
Essentially, an “Optimized Net List Test” is sufficient to meet the guideline for the testing of a bare printed circuit board.
At Bay Area Circuits, we test for opens and shorts based on optimized data from the customer-provided Gerber files. When a bare board passes electrical testing, assurance is provided that the board is electrically sound. However, that relies upon the assumption that the board has been designed accurately. A circuit board that has a problem with the design can still pass a bare board electrical test and not function correctly. We measure closely how many boards do not pass electrical test as a measure of the quality from our manufacturing team which helps our continuous improvement efforts.
This video covers much of the PCB Testing process: