One, electrical test

PCB in the process of production, hard to avoid the short circuit, open circuit and leakage caused by external factors, such as electrical flaws, coupled with the continuous toward high density PCB, fine spacing and the evolution of multi-level, and failure to timely to bad plate screening out, and let it flow into the process, is bound to cause waste more cost, so in addition to the improvement of the process control, Enhanced testing techniques can also provide PCB manufacturers with solutions to reduce scrap rates and improve product yield.

In the production process of electronic products, the cost loss caused by defects has different degrees in each stage, and the earlier the discovery, the lower the cost of remediation. The Rule of 10 “s is commonly used to estimate The cost of remediation when a PCB is found to be defective at different stages of The process. For example, after the completion of empty plate production, if the board in the circuit can be detected in real time, usually only need to repair the defect, or at most the loss of an empty plate; However, if the circuit is not detected, the board is shipped to the downstream assembler to complete parts installation, and the furnace tin and IR remelting, but the circuit is detected at this time, the general downstream assembler will ask the empty board manufacturing company to compensate for the cost of parts, heavy industry fee, inspection fee, etc. If the more unfortunate, the defective board is still not found in the assembly industry’s test, but into the overall system of finished products, such as computers, mobile phones, automobile parts, and so on, then the test to find the loss, will be empty board timely detection of a hundred times, a thousand times, or even higher. Thus, electrical testing for PCB manufacturers is all about early detection of defective boards.

The downstream operator usually requires the PCB manufacturer to perform 100 percent electrical testing and therefore agrees with the PCB manufacturer on specifications for testing conditions and methods, so both parties will first clearly define the following:

1. Test data source and format

2, test conditions, such as voltage, current, insulation and connectivity

3. Production method and selection of equipment

4. Test chapter

5, repair specifications

There are three stages in PCB manufacturing that must be tested:

1. After etching the inner layer

2. After etching the outer circuit

3, the finished product

Each stage will usually have 2~3 times of 100% test, screen out the bad plate for heavy processing. Therefore, the test station is also the best data collection source to analyze the process problem points. According to the statistical results, you can obtain the percentage of open circuit, short circuit and other insulation problems, and then test after reengineering. After sorting out the data, use the quality control method to find out the root of the problem and solve it.

Two, electrical measurement method and equipment

Electrical test methods include: Dedicated, Universal Grid, Flying Probe, e-beam, conductive cloth, Capacity and ATG-scan MAN are the three most commonly used devices. They are special test machine, general test machine and flying needle test machine. In order to better understand the functions of each device, the features of the three major devices are compared below.

1. Dedicated testing

Fixtures (such as the pins and dials used to test circuit boards) only work with one material number. Boards with different material numbers cannot be tested and recycled. In terms of test points, a single panel can be tested within 10,240 points, and both sides within 8,192 points. In terms of test density, due to the thickness of the probe head, it is more suitable for boards above pitch.

2. Universal Grid testing

The basic principle of general-use test is that the layout of PCB circuit is designed according to the Grid. Generally, the so-called line density refers to the distance of the Grid, which is expressed by Pitch (sometimes can also be expressed by hole density), and general-use test is based on this principle. According to the hole position, a G10 substrate is used as a Mask. Only at the hole position, the probe can pass through the Mask for electrical measurement, so the manufacturing of the fixture is simple and fast, and the probe can be reused. The standard Grid fixed large needle tray with many measuring points can be used to produce movable probe needle tray according to different material numbers. As long as the movable needle tray is changed during mass production, it can be used for mass production test of different material numbers. In addition, in order to ensure the circuit system of completed PCB board is unobstructed, it is necessary to conduct Open/Short electrical test on the board with the needle plate of specific contact point on the general-purpose electric measuring machine with high voltage (such as 250V) multi-measuring points. This kind of universal TesTIng machine is called “AutomaTIc TesTIng Equipment” (ATE).

General-use test points are usually more than 10,000 points, and the test density is called on-grid test. If it is used in high-density boards, the spacing is too close, and it has been separated from on-grid design, so it belongs to off-grid test, and the fixture must be specially designed. The test density of the generic test can reach QFP.

3. Flying Probe test

The principle of the flying needle test is very simple. Only two probes are required to move x, y and Z to test the two ends of each line one by one, so there is no need to make another expensive fixture. However, due to the endpoint test, the measurement speed is very slow, about 10~40 points/ SEC, so it is suitable for samples and small volume production; In terms of testing density, the flying needle test can be applied to very high density plates (), such as MCM.