Two common methods of circuit board inspection

With the introduction of surface mount technology, the packaging density of circuit boards has increased rapidly. Therefore, even for low-density and average-number circuit boards, the automatic detection of circuit boards is not only basic, but also economical. In complex circuit board inspection, two common methods are the bed of needle test method and the double probe or flying probe test method.

1. Needle bed test method

This method consists of a probe with a spring connected to each detection point on the circuit board. The spring makes each probe have a pressure of 100-200g to ensure good contact between each detection point. Such probes are arranged together and are called "needle beds". Under the control of the detection software, the detection points and detection signals can be programmed, and the inspector can get the information of all the test points. In fact, only the probes of the test points that need to be tested are installed. Although it is possible to test on both sides of the circuit board at the same time using the bed of needle test method, when designing the circuit board, all the inspection points should be on the soldering surface of the circuit board. The needle bed tester is expensive and difficult to repair. According to the specific application of the needle, the probes in different arrangements are selected.

A basic general purpose grid processor consists of a drilled board with pins centered at 100, 75, or 50 mils. The pins act as probes and use electrical connectors or nodes on the circuit board to make direct mechanical connections. If the pads on the circuit board match the test grid, the polyester film perforated in accordance with the specification will be placed between the grid and the circuit board to facilitate the design of specific probes. Continuity detection is achieved by accessing the end point of the grid (which has been defined as the x-y coordinate of the pad). Since every network on the circuit board is tested for continuity. In this way, an independent test is completed. However, the proximity of the probe limits the effectiveness of the bed-of-needle test method.

2 Double probe or flying probe test method

The flying probe tester does not depend on the pin pattern installed on the fixture or bracket. Based on this system, two or more probes are mounted on a tiny magnetic head that can move freely on the x-y plane, and the test points are determined by CADI

Gerber data direct control. The dual probes can move within a distance of 4 mils from each other. The probes can move independently, and there is no real limit to how close they are to each other. The tester with two arms that can move back and forth is based on the measurement of capacitance. The circuit board is pressed tightly and placed on the insulating layer on a metal plate as the other metal plate of the capacitor. If there is a short circuit between the lines, the capacitance will be larger than at a certain point. If there is an open circuit, the capacitance will become smaller.

Test speed is an important criterion for selecting a tester. The bed of needle tester can test thousands of test points, while the flying probe tester can only test two or four test points. In addition, the needle bed tester may only cost 20-305 for a single-sided test, depending on the complexity of the board, while the flying probe tester requires Ih or more time to complete the same evaluation. Shipley (1991) explained that even if manufacturers of high-volume printed circuit boards consider the mobile flying probe test technique to be slow, this method is still a good choice for manufacturers of lower-volume complex circuit boards.

For bare board testing, there are special test instruments (Lea, 1990). A more cost-optimized method is to use a general-purpose instrument. Although this type of instrument is initially more expensive than a dedicated instrument, its initial high cost will be offset by the reduction of individual configuration costs. For general grids, the standard grid for boards with pin components and surface mount equipment is 2.5mm. At this time, the test pad should be greater than or equal to 1.3mm. For Imm grids, the test pad is designed to be larger than 0.7mm. If the grid is small, the test needle is small, brittle, and easily damaged. Therefore, choose a grid larger than 2.5mm. Crum (1994b) stated that the combined use of a universal tester (standard grid tester) and a flying probe tester can make the inspection of high-density circuit boards both economical. Another method he suggested is to use a conductive rubber tester, which can be used to detect points that deviate from the grid. However, the height of the pads that are leveled by hot air is different, which will hinder the connection of the test points.

usually carry out the following three levels of testing:

1) Bare board inspection;

2) Online testing;

3) Function detection.

Using a universal type of tester, it can test a class of styles and types of circuit boards, and can also be used for special applications.