Introduction to PCB electrical testing methods

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Introduction to PCB electrical testing methods

1. Electrical Testing

In the process of PCB production, it is inevitable that external factors may cause electrical defects such as short-circuit, open circuit, and leakage. In addition, the PCB continues to evolve towards high density, fine spacing, and multi-layer, if the Board is not screened out in time and left in the process, it will inevitably cause more cost waste. Therefore, in addition to the improvement of process control, the improved testing technology can also provide PCB manufacturers with solutions to reduce scrap rates and improve product yield rates.

In the production process of electronic products, the loss of cost due to defects occurs at different stages. The earlier the discovery, the lower the cost of remediation. "The rule of 10''' s" is often used to assess the remediation costs when a PCB is found defective in different process phases. For example, after an empty board is created, if an open circuit in the board can be detected in real time, it usually only needs to be fixed to improve the defect, or at most one blank board is lost. However, if an open circuit is not detected, when the board is shipped to the downstream Assembly manufacturer to complete the installation of parts, the furnace tin and IR are remelted. However, at this time, it is detected that the line is broken, generally, downstream assemblers will request compensation for parts, heavy industry, and inspection fees from empty board manufacturing companies. Unfortunately, the defective board has not been found in the Assembly operator's test, but enters the overall system finished products, such as computers, mobile phones, car parts, etc, at this time, the loss discovered only after testing will be times, times, or even higher than the empty board detection in a timely manner. Therefore, for the PCB operator, electrical testing aims to detect functional defects of the line as soon as possible.

Downstream vendors usually require PCB manufacturers to perform electrical testing. Therefore, they will reach an agreement with the PCB manufacturer regarding the testing conditions and methods, therefore, both parties will clearly define the following items:

1. Test Data Source and format

2. Test conditions, such as voltage, current, insulation and connectivity

3. device production method and selection

4. Test chapter

5. Patch specifications

In the PCB Manufacturing Process, there are three phases that must be tested:

1. inner layer after Etching

2. After the outer line is Etching

3. Finished Products

Each stage usually has 2 ~ 3 tests of 100%, filtering out bad boards and then performing heavy industry processing. Therefore, the test station is also the best data collection source for analyzing process problems. Through the statistical results, the percentage of circuit breaking, short circuit and other insulation problems can be obtained, and then the heavy industry will conduct further testing, after the data is organized, the quality control method is used to identify the root cause of the problem and solve it.

Ii. Electric Test methods and equipment

The electrical testing methods include: special type (dedicated), general type (universal GRID), flying probe, non-contact electron beam, and conductive tape) capacitive (capacity) and brush test (ATG-SCAN man), among which the most commonly used equipment are three, respectively dedicated testing machine, generic testing machine and flying needle testing machine. To better understand the functions of various devices, we will compare the features of the three main devices.

1. Dedicated type (dedicated) test

The purpose of this test is to use a special fixture (fixture, for example, a card for electrical testing on a circuit board, boards with different item numbers cannot be tested and cannot be recycled. In terms of test points, a single Panel can be tested within 10,240 points and each 8,192 points on both sides. In terms of test density, due to the relationship between the thickness of the probe header, the single panel is more suitable for pitch and above.

2. Universal grid Test

The basic principle of generic testing is that the layout of the PCB line is designed based on the grid. Generally, the line density refers to the distance of the grid, that is, the distance (pitch) (in some cases, the pore density can also be used for representation). The general test is based on this principle, and the position of the hole is represented as a mask with a G10 substrate, the probe can pass through the mask for electrical testing only at the position of the hole, so the preparation of the fixture is simple and fast, and the probe can be reused. Generic testing provides a standard grid fixed large needle tray with many measurement points, which can be used to create a needle tray for the active Probe Based on different numbers. During mass production, you only need to change the active needle tray, you can test the mass production of different materials. In addition, in order to ensure the smooth completion of the PCB circuit system, you must use a high-voltage electrical Tester (such as 250 V) with multiple measuring points, an open/short electrical test is performed on the board using a needle tray with a specific contact. This generic testing machine is called an automatic testing machine 」
(Ate, automatic testing equipment ).

The number of universal test points is usually above 10 thousand points. The test density or test is called the on-grid test. If the test density is applied to high density boards, the on-grid design has been removed because the distance is too dense, therefore, it is an off-grid test, and its fixtures must be specially designed. Generally, the testing density of generic tests can reach QFP.

3. Flying probe test

 

The Flying needle test principle is very simple. Only two probes are needed for moving X, Y, and Z to test the two endpoints of each line one by one. Therefore, there is no need to create expensive fixtures. However, because it is an endpoint test, the speed is extremely slow, about 10 ~ 40 points/sec, so it is suitable for sample and small mass production. In terms of test density, the flying needle test can be applied to very high density boards (), such as MCM.

 

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