What is PCB Grid Testing / Bed of Nails Testing?

Grid testing or Bed of Nails testing is a process used to check the performance of components mounted on a PCB board. This test uses a frame/fixture that contains various pins inserted into an epoxy phenolic glass cloth laminated sheet (G-10) in order to access all the PCB test points. These pins act as sensors which are aligned to make contact with the test points on the PCB board and are also connected with a measuring unit through wires. The position of the pins is designed and customized for each PCB based on the components or points on the board that need to be tested.

A grid testing machine has three building blocks - a fixture, a bed of nails and software, to control the overall functionality of the machine. It usually has two cameras which are placed on the top and bottom of the machine in order to scan the whole board.

Advantages of PCB Grid Testing:

• Very Reliable method of PCB testing as all the points on the PCB board can be tested simultaneously.
• Offers very good accuracy since it uses two cameras
• Can easily detect manufacturing defects
• An in-circuit tester is easy to program
• Interpretation of test results is quite easy
• It checks for Open, shorts, mission components, Wrong polarity, defective components, and current leakages in the circuit
• Most of the parameters can be checked without applying power to the PCB under test

Limitations of PCB Grid Testing:

• Fixtures used in bed of nails testing are expensive since they are mechanical and require a different wiring assembly for a different PCB board
• Fixtures are mechanical so any change in the number of pins or their position require extra cost
• This test doesn’t test continuity through connectors so there are the chances that the connectors’ faults may remain unnoticed.
• Faults related to solder joints, excess solder, solder quality, and solder void also remain unidentified

This type of testing is called In-Circuit Testing. Another test process used for In-Circuit testing is Flying Probe Testing.