PCB proofing and electrical measurement technology

I. Electrical test In the production process of PCB, it is inevitable that electrical defects such as short circuit, open circuit and leakage will be caused by external factors. In addition, PCB continues to evolve towards high density, fine spacing and multi-level. If the defective boards are not screened out in time and allowed to flow into the manufacturing process, it will inevitably lead to more cost waste. Therefore, besides the improvement of manufacturing process control, improving testing technology can also provide PCB manufacturers with solutions to reduce scrap rate and increase product yield. In the production process of electronic products, the cost loss caused by defects has different degrees in each stage. The earlier it is discovered, the lower the cost of remediation. “TheRuleof10’s” is a method that is often used to evaluate the repair cost when PCB is found to be defective at different stages of the manufacturing process. For example, if the open circuit in the board can be detected in real time after the empty board is made, the defect can usually be improved by repairing the line, or at most one empty board can be lost; However, if the open circuit is not detected, after the board is shipped to the downstream assembler to complete the installation of the parts, it will also be remelted by furnace tin and IR. However, at this time, it is found that there is an open circuit in the circuit. Generally, the downstream assembler will ask the vacant board manufacturing company for compensation for the parts cost, heavy work cost, inspection cost and so on. Even more unfortunately, if the defective board has not been found in the test of the assembler, but enters the finished products of the whole system, such as computers, mobile phones, auto parts, etc., then the loss discovered after the test will be 100 times, 1000 times or even higher than that of the empty board detected in time. Therefore, electrical testing is for PCB operators, in order to find the boards with circuit functional defects as early as possible. Downstream operators usually require PCB manufacturers to conduct 100% electrical tests, so they will reach an agreed specification with PCB manufacturers on test conditions and test methods. Therefore, both parties will clearly define the following matters first: 1. Source and format of test data 2. Test conditions, such as voltage, current, insulation and connectivity. 3. Manufacturing method and selection of equipment 4. Test chapter 5. Repair specifications In the manufacturing process of PCB, there are three stages that must be tested: 1. After the inner layer is etched 2. After the outer circuit is etched 3. Finished products There are usually 2~3 times of 100% testing in each stage, and the defective boards are screened out and then reworked. Therefore, the test station is also the best data collection source for analyzing the manufacturing process problems. Through statistical results, the percentage of open circuit, short circuit and other insulation problems can be obtained. After rework, the test will be carried out. After sorting out the data, the quality control method will be used to find out the root of the problems and solve them. Second, the method and equipment of electrical measurement Electrical testing methods include Dedicated, UniversalGrid, FlyingProbe, non-contact electron beam, conductive cloth (glue), capacitance and ATG-scanner. Among them, there are three kinds of most commonly used equipment, namely, dedicated testing machine, non-contact electron beam, capacitance and ATG-scanner. In order to better understand the functions of various devices, the following will compare the characteristics of three main devices. 1. Dedicated test The special-purpose test is special, mainly because the Fixture (for example, the needle plate for the electrical test of circuit board) is only suitable for one material number, and the boards with different material numbers cannot be tested and recycled. In terms of test points, the single panel can be tested within 10,240 points and 8,192 points on both sides. In terms of test density, due to the thickness of probe head, it is more suitable for boards above pitch. 2. UniversalGrid test The basic principle of universal testing is that the layout of PCB circuit is designed according to Grid. Generally, the so-called line density refers to the distance of grid, that is, it is expressed by Pitch (sometimes it can also be expressed by hole density). Universal testing is based on this principle, and a G10 substrate is used as a Mask according to the hole position. Only at the hole position can the probe pass through the Mask for electrical measurement. Therefore, the jig is easy and fast to make, and the probe can be reused. The universal test uses the standard Grid with many measuring points to fix the large needle disk, and the needle disk of the movable probe can be made according to different material numbers. In mass production, the mass production test of different material numbers can be carried out only by changing the movable needle disk. In addition, in order to ensure the smooth circuit system of the completed PCB board, it is necessary to carry out the Open/Short electrical test on the board by using the needle dial with specific contacts on the universal electric measuring mother machine with multiple measuring points using high voltage (such as 250V). This universal testing machine is called “Automatic Testing Equipment” (ATE). The number of universal test points is usually more than 10,000, and the test density is in or out of the on-grid test. If it is applied to a high-density board, it is out of the on-grid design due to the too close spacing, so it belongs to the off-grid test, and its fixture must be specially designed. Usually, the test density of universal test can reach QFP. 3. FlyingProbe test The principle of flying probe test is very simple, only two probes need to move in X, Y and Z to test the two endpoints of each circuit one by one, so there is no need to make an expensive jig. However, because it is an end-point test, the speed measurement is extremely slow, about 10~40points/sec, so it is more suitable for sample and small-scale production. In terms of testing density, the flying probe test can be applied to extremely high density boards.