Unpopulated boards are usually bare-board tested for "shorts" and "opens". A short is a connection between two points that should not be connected. An open is a missing connection between points that should be connected.
For high-volume production a fixture or a rigid needle adapter is used to make contact with copper lands on the board. Building the adapter is a significant fixed cost and is only economical for high-volume or high-value production.
For small or medium volume production flying probe testers are used where test probes are moved over the board by an XY drive to make contact with the copper lands.
The CAM system instructs the electrical tester to apply a voltage to each contact point as required and to check that this voltage appears on the appropriate contact points and only on these.
Statement: This post is only the personal view of the author and does not represent the opinions of ALLPCB.com.