PCB Blog - Analysis of PCB board electrical measurement technology

1. Electrical test

In the production process of PCB board, it is inevitable that electrical defects such as short circuits, open circuits, and leakage will be caused by external factors. In addition, the PCB boards continue to evolve towards high density, fine pitch, and multi-level. If the board is screened out and allowed to flow into the process, it will inevitably cause more cost waste. Therefore, in addition to the improvement of process control, improving the testing technology can also provide PCB manufacturers with reduced scrap rates and improved product yields. solution. In the production process of electronic products, the loss of cost due to defects has different degrees at each stage. The earlier it is found, the lower the cost of remediation. "The Rule of 10's" is one that is often used to evaluate the cost of remediation when a PCB board is found to be defective at various stages of the process. For example, after the empty board is made, if the open circuit in the board can be detected in real-time, it is usually only necessary to repair the line to improve the defect, or at most one empty board is lost; but if the open circuit cannot be detected, the board will be shipped. When the downstream assembler completes the installation of parts, it also undergoes furnace tin and IR remelting, but at this time it is detected that there is an open circuit in the circuit, and the general downstream assembler will ask the empty board manufacturing company to compensate for the cost of parts and rework. , inspection fee, etc. If it is more unfortunate, the defective board has not been found in the test of the assembler, and it enters the finished system of the whole system, such as computer, mobile phone, auto parts, etc., and the loss is found only after testing at this time will be the empty board detected in time 100 times, 1000 times, or even higher. Therefore, for the PCB board industry, electrical testing is to find boards with circuit functional defects as soon as possible.

Downstream manufacturers usually require PCB board manufacturers to conduct 100% electrical testing, so they will agree with the PCB manufacturers on test conditions and test methods. Therefore, both parties will first clearly define the following items:

1) Test data and format

2) Test conditions such as voltage, current, insulation, and continuity

3) Equipment production method and selection point

4) Test chapter

5) Patch Specifications

In the manufacturing process of the PCB board, there are three stages that must be tested:

1) After the inner layer is etched

2) After the outer layer circuit is etched

3) Finished product

There are usually 2~3 100% tests in each stage, and the defective boards are screened out and then reworked. Therefore, the test station is also a data collection for analyzing process problems. Through statistical results, the percentage of open circuits, short circuits, and other insulation problems can be obtained. After rework, the test is carried out. After sorting out the data, use the quality control method to find the problem. the root cause to be resolved.

2. Methods and equipment for electrical measurement

Electrical testing methods include: Dedicated, Universal Grid, Flying Probe, Non-contact Electron Beam (E-Beam), Conductive Cloth (Glue), Capacity And brush test (ATG-SCAN MAN), there are three kinds of commonly used equipment, namely special test machine, general test machine, and flying probe test machine. In order to better understand the capabilities of various devices, the following will compare the characteristics of the three main devices.

1. Dedicated test is a dedicated test mainly because the fixture used (Fixture, such as the needle plate for electrical testing of the circuit board) is only suitable for one part number, different Part number boards cannot be tested and cannot be recycled. In terms of the number of test points, single-sided boards can be tested within 10,240 points and double-sided with 8,192 points each. In terms of test density, due to the thickness of the probe head, it is more suitable for boards with the pitch above.

2. Universal Grid test

The basic principle of the general-purpose test is that the layout of the PCB circuit is designed according to the grid. Generally, the so-called circuit density refers to the distance of the grid, that is, it is expressed by the pitch (sometimes can also be used for hole density. The universal test is based on this principle. According to the hole position, a G10 base material is used as the mask. Only at the position of the hole can the probe pass through the mask for electrical measurement. Therefore, the production of the fixture is simple and fast, and Probes are reusable. The standard grid fixed large dial with a large number of measuring points for general-purpose testing can make movable probe dials according to different material numbers, and can be mass-produced for different material numbers as long as the movable dial is replaced during mass production test. In addition, in order to ensure the unobstructed circuit system of the completed PCB board, it is necessary to conduct Open/Short electrical tests on the board with the needle plate of specific contacts on the general-purpose electrical measuring machine using high-voltage electricity (such as 250V) with multiple measuring points. This kind of general-purpose testing machine is called an "automatic testing machine" (ATE, Automatic Testing Equipment). The number of general-purpose test points is usually more than 10,000 points, and the test with a test density of or is called an on-grid test. If it is used for high-density boards, the spacing is too dense and it has been separated from the on-grid design, so it belongs to the off-grid. For testing, the fixture must be specially designed. Usually, the test density of general-purpose tests can reach QFP.

3. Flying Probe test

The principle of the flying probe test is very simple. It only needs two probes to move x, y, and z to test the two end points of each line one by one, so there is no need to make another expensive fixture. However, because it is an endpoint test, the measurement speed is extremely slow, about 10~40 points/sec, so it is more suitable for samples and small mass production; in terms of test density, the flying probe test can be applied to very high-density boards ( ), such as MCM.

3. Technical comparison

A typical flying probe test yield is between 1 and 20. If the hole density is known, it can be converted into the total area tested per hour ( ), and the test area is in the range of about 15 (20 and 32 probes) to 0.04 (probe 1 and 600 boards), the difference of 375 times is due to the density and spacing of the boards. Generally, the output of flying probe test equipment with better performance is maintained between 10 and 15, which can be applied to commercial boards with a density of 30 to 600 high-density boards. The total area tested by the flying probe tester is about 3,000 to 5,000 square meters per year. The Bed-of-Nails test equipment, such as the special type and the general type, is less used for the test of the high-density board because the test capability of the high-density board is not as good as that of the flying probe test. However, in theory, the output area of the dial type can reach 200~400, but in terms of the current production situation, the actual production line is 30~100 for the special type, while the general type is 15~50 (comparison of the two). The basis is that the special-purpose type is usually used in mass production, while the general-purpose type is mostly used in small and medium-sized mass production). The difference between theory and practice may include production management problems in addition to the factors of the equipment itself, which will not be detailed here. described. In general, there are about 300,000 special-purpose test equipment and 150,000 general-purpose test equipment per year. However, the output of each device may vary significantly due to the production plan of the PCB board manufacturer; for example, if a mobile phone board is tested with ATE of -pitch CSP, the test rate is only about 1/4, and the output of each test equipment is 150,000 per year. Based on the above introduction, it can be concluded that: First of all, in terms of the applicable purpose of the test technology, the flying probe test is currently electrical test equipment suitable for use in small-scale production and samples, but if it is to be used in medium-to-large-scale production, due to The slow test speed and expensive equipment will greatly increase the test cost. No matter what level of the board the general-purpose type and special-purpose type are used for, as long as the output reaches a certain number, the test cost can reach the standard of economies of scale, and It only accounts for about 2~4% of the price, which is the main reason why the general-purpose and special-purpose models are the current mass-produced test models. However, as the speed of change of electronic products accelerates, the product life cycle of a single circuit design version is shortened (for example, the current life cycle of a mobile phone PCB board is about 6 months).