1. Bed of Nails Test Method
This method connects a probe with a spring to each test point on the circuit board. The spring gives each probe a pressure of 100-200g to ensure good contact at each test point. Such probes arranged together are called "bed of nails". Under the control of the test software, the test points and test signals can be programmed. In fact, only the probes of the test points that need to be tested are installed. Although it is possible to test both sides of the circuit board at the same time using the bed of nails test method, when designing the circuit board, all test points should be on the soldering side of the circuit board. The bed of nails tester equipment is expensive and difficult to repair. The needle head selects probes with different arrangements according to its specific application.
A basic universal grid processor consists of a drilled board with pins with a center spacing of 100, 75 or 50 mil. The pins act as probes and make direct mechanical connections using electrical connectors or nodes on the circuit board. If the pads on the circuit board match the test grid, a polyester film punched according to the specification will be placed between the grid and the circuit board to facilitate the design of specific detection. Continuity detection is achieved by accessing the end points of the grid (defined as the x-y coordinates of the pad). Since each 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 nails test method.
2. Observation of the circuit board
The circuit board is small in size and complex in structure, so professional observation instruments must be used to observe the circuit board. Generally, we use a portable video microscope to observe the structure of the circuit board. Through the video microscope camera, we can clearly see the very intuitive microstructure of the circuit board from the microscope. In this way, it is easier for us to design and test the circuit board. The portable video microscope currently used on the factory site uses the portable video microscope MSA200 and VT101, because it can achieve "observation at any time, detection at any time, and multi-person discussion" which is more convenient than traditional microscopes!
3. Dual-probe flying probe test method
The flying probe tester does not rely on the pin pattern mounted on the fixture or bracket. Based on this system, two or more probes are mounted on tiny magnetic heads that can move freely on the x-y plane, and the test points are directly controlled by CADI Gerber data. The dual probes can move within 4 mils of each other. The probes can move independently, and there is no real limit to how close they can be. Testers with two arms that can move back and forth are based on the measurement of capacitance. The board is pressed against an insulating layer on a metal plate, which acts as the other plate of the capacitor. If there is a short between the lines, the capacitance will be greater than at a certain point. If there is a break, the capacitance will be smaller.
Test speed is an important criterion for selecting a tester. A bed of nails tester can accurately test thousands of test points at a time, while a flying probe tester can only test two or four test points on a board at a time. In addition, a bed of nails tester may take only 20-30 seconds to perform a single-sided test, depending on the complexity of the board, while a flying probe tester may take 1 hour or more to complete the same evaluation. Shipley (1991) explains that even though high-volume printed circuit board manufacturers consider the moving flying probe test technology slow, this method is still a good choice for manufacturers of complex circuit boards with lower production volumes.
For bare board testing, there are dedicated test instruments. A more cost-optimized approach is to use a universal instrument, which, although initially more expensive than a dedicated instrument, will be offset by the reduction in individual configuration costs. For universal grids, the standard grid for boards with leaded components and surface mount devices is 2.5 mm. In this case, the test pads should be greater than or equal to 1.3 mm. For a 1 mm grid, the test pads should be designed to be greater than 0.7 mm. If the grid is smaller, the test needles are small and brittle and easily damaged. Therefore, it is best to use a grid greater than 2.5 mm. The combination of a universal tester (standard grid tester) and a flying probe tester can make the detection of high-density circuit boards both accurate and economical. Another method he suggested is to use a conductive rubber tester, a technology that can be used to detect points that deviate from the grid. However, the different heights of the pads treated with hot air leveling will hinder the connection of the test points.