With the introduction of surface mount technology, the packaging density of PCB board increases rapidly. Therefore, even for some PCB boards with low density and few quantity, the automatic detection of PCB boards is basic. In the complex PCB circuit board inspection, the needle bed test method and the double probe or flying needle test method are two common methods.

1. Needle bed test method

This method consists of spring-loaded probes connected to each detection point on the PCB. The spring forces each probe to a pressure of 100-200g to ensure good contact at each test point. Such probes are arranged together and are called “needle beds”. Test points and test signals can be programmed under the control of the test software. Although it is possible to test both sides of the PCB using the pin bed test method, when designing the PCB, all test points should be on the welded surface of the PCB. Needle bed tester equipment is expensive and difficult to maintain. Needles are selected in different arrays according to their specific application.

A basic general-purpose grid processor consists of a drilled board with pins spaced 100, 75, or 50mil between the centers. Pins act as probes and make direct mechanical connections using electrical connectors or nodes on the PCB board. If the pad on the PCB matches the test grid, a polyvinyl acetate film, perforated according to the specification, is placed between the grid and the PCB to facilitate the design of specific probes. Continuity detection is achieved by accessing the end points of the mesh, which have been defined as the X-y coordinates of the pad. Since every network on the PCB is continuously inspected. In this way, an independent detection is completed. However, the proximity of the probe limits the effectiveness of the needle-bed method.

2. Double probe or flying needle test method

The flying needle tester does not rely on a pin pattern mounted on a fixture or bracket. Based on this system, two or more probes are mounted on tiny, freely movable magnetic heads in the X-Y plane, and the test points are directly controlled by CADI Gerber data. The two probes can move within 4mil of each other. The probes can move independently and there is no real limit to how close they can get to each other. The tester with two arms that move back and forth is based on capacitance measurements. The PCB board is pressed against an insulating layer on a metal plate that acts as another metal plate for the capacitor. If there is a short circuit between the lines, the capacitance will be greater than at a certain point. If there are circuit breakers, the capacitance will be smaller.

For a general grid, the standard grid for boards and surface mount equipment with pin components is 2.5mm, and the test pad should be greater than or equal to 1.3mm. If the grid is small, the test needle is small, brittle and easily damaged. Therefore, a grid larger than 2.5mm is preferred. The combination of a universal tester (standard grid tester) and a flying needle tester enables accurate and economical testing of high-density PCB boards. Another method is to use a conductive rubber tester, a technique that can be used to detect points that deviate from the grid. However, the different heights of the pads with hot air leveling will hinder the connection of the test points.

The following three levels of detection are usually carried out:

1) Bare board detection;

2) Online detection;

3) Function detection.

The universal type tester can be used to test PCB boards of one style and type, and also for special applications.