Following the manufacturing process of the PCBs we can perform a visual inspection in order to find any problems with a board. That works fine for a two layer pcb. For multi-layer PCBs it is almost impossible to visually inspect the internal layers once the board is completed. Visual tests are done on inner layers prior to lamination to find any problems and correct them prior to lamination. The only way to verify a multi-layer pcb is to conduct an electrical test on it.
First there are two typical types of electrical tests done on bare PCBs.
Flying Probe and Universal Grid
Flying probe does not require a fixture since the probes move around to all of the test points. Universal grid requires a fixture in order to conduct the test. Universal grid is sometimes called a clam shell or bed of nails test.
The test of the 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). That is measured in ohms.
A good conductor has LOW resistance. At regular temperatures the best conductor is silver. The second best conductor is of course copper. This is why copper is used in the manufacturing of printed circuit boards. 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 the same length but one is thicker, the thicker trace will be half the resistance. Resistance will dissipate power in the form of heat.
Here are some of the key terms that you need to know.
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.
IPC provides guidelines on how bare circuit boards should be tested. Here is a copy of the guideline.
“IPC- 9252 Guidelines and Requirements for Electrical Testing of Unpopulated Printed Boards”
“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.”
Basically what it is saying is that an “Optimized Net List Test” is sufficient to meet the guideline for testing of bare printed circuit boards.
We test for opens and shorts based on optimized data from the gerber files.
Ideally when a bare board is tested then you can be guaranteed that it is 100% a good board. That assumes though that the board has been designed correctly. A circuit board can still pass a bare board electrical test and still not function correctly because of a problem with the design. This does not happen very often but occasionally it does.
We measure closely how many boards do not pass electrical test as a measure of the quality from our manufacturing team. If for some reason we see an increase in the number of bare boards failing test we can go back and review our processes to improve the quality.
If you have questions about bare board electrical test please ask them in the discussion box below.
[Updated with Video 11/6/14. Original Post 4/13/12.]