The quality of printed circuit boards and components are major determinants in the process yield of printed circuit board assemblies. The sooner defects are identified the less costly the remedial activities. Therefore, it is critical that PCB quality is verified before the assembly process.
So, what do PCB manufacturers do to ensure the manufacturing quality of the printed circuit boards?
The final stage in the printed circuit board fabrication is testing. During this step, potential manufacturing issues are detected to prevent defective boards from being shipped to the customer.
One of the most common quality control measures is to perform a visual inspection. This might be an acceptable approach on single layer PCBs, but it not accurate or definitive.
Electrical testing is the reliable method used to verify the quality of all types of PCBs.
Bare boards are electrically tested using information from their netlist.
- Flying probe test
- Universal grid test
Flying Probe Testing
The flying probe test method is the latest solution in electrical testing. It uses a generic board holder along with one or more probes. Under software control, these probes access all the nodes on the board one by one. This allows testing for opens, near opens and shorts.
Flying probe testers can now perform onboard memory module programming along with checking for opens and shorts, and test components like diodes and transistors junction. Some advantages of the flying probe test are that requires no test fixture and little set-up time.
The drawbacks to this test are:
- lower test speeds compared to in-circuit
- incompatibility with the complex circuits
Bed of Nails / Universal Grid Testing
The universal grid is also known as clamshell or bed of nails fixture test. Just as the name describes, the electronic fixture for universal grid testing has numerous pins inserted into holes in an epoxy phenolic glass cloth laminated sheet.
These holes are then aligned to the test points on the printed circuit board using tooling pins. This type of testing is used when there is a large production of PCBs. The universal grid process, all the test points are checked at the same time.
|Electrical Test Pitch ( flying Probe)||0.004”|
|Electrical Test Pitch ( Bed of nails)||0.020”|
|Minimum continuity Resistance||0.1 Ohms|
|Maximum Test Voltage||1000 Volts|
|Isolated Resistance Range||25 M Ohms- 2 G Ohms|