PCB design glossary
Test Point
AKA: testpoint, tp, test pad
An exposed pad placed on a PCB specifically so a probe can attach to it during bring-up or production test.
Test points are small exposed pads (sometimes a labeled through-hole) used to probe a net with a scope or multimeter, or to fixture the board on a bed-of-nails tester.
Adding test points to every meaningful net is one of the cheapest debugging investments you can make on a prototype board.
Why boards need test points
Test points let you measure a board without probing tiny pins. During bring-up they make it easy to check power rails and key signals; in production, a test fixture presses spring-loaded pins onto them to test every board automatically.
Where to put them
- Every power rail and ground.
- Reset, boot and programming signals.
- Communication lines such as UART, I2C and SPI.
- Any signal you expect to debug.
Sizes and spacing
A round pad of about 1 mm diameter is easy to probe by hand and with spring pins. Placing test points on one side of the board, on a regular grid such as 2.54 mm, keeps a production fixture simple and cheap. Leave soldermask open on test points, and label them on the silkscreen.
Leave room to probe the board you just designed
Ask for test points on the rails and buses you will measure when you describe the board, and they are placed and routed with everything else.
Describe a board with test points