**Shen’an PCB Circuit Board Production Automatic Detection Technology**
With the advent of surface mount technology, the packaging density of circuit boards has increased significantly. As a result, even for low-density, general-quantity circuit board manufacturing, automatic inspection has become not only a fundamental requirement but also a cost-effective solution. In the processing and inspection of complex circuit boards, two common methods are used: the needle bed test method and the double probe or flying probe test method.
1. **Needle Bed Test Method**
In this method, a spring-loaded probe is connected to each inspection point on the PCB. The spring applies a pressure of 100-200g, ensuring reliable contact at each test point. These probes are arranged together and referred to as a “bed of needles.” Controlled by specialized detection software, the test points and signals can be programmed, allowing the inspector to access information for all the test points. In practice, only the probes for the specific test points that need to be checked are installed. Although the bed-of-needle test method can test both sides of the PCB simultaneously, it is recommended that all test points be located on the soldering surface during the PCB design phase. The needle bed tester is costly and challenging to maintain. Depending on the specific application, probes with different arrangements are selected.
1. **Basic General-Purpose Grid Processor**
A basic general-purpose grid processor typically consists of a drilled board with pin center spacings of 100, 75, or 50 mils. The pins serve as probes, utilizing electrical connectors or nodes on the circuit board to establish direct mechanical connections. If the pads on the circuit board align with the test grid, a polyester film, perforated according to the specifications, is placed between the grid and the printed circuit board to facilitate specific detection designs. Continuity testing is performed by accessing the grid’s endpoints, which correspond to the x-y coordinates of the pads. Since every network on the PCB is tested for continuity, this method ensures that an independent inspection of the circuit board production is completed. However, the proximity of the probes limits the effectiveness of the bed-of-needle testing method.
2. **Double Probe or Flying Probe Test Method**
A flying probe tester does not rely on a pin pattern installed on a fixture or bracket. In this system, two or more probes are mounted on a small magnetic head that can move freely on the x-y plane. The test points are controlled directly by CAD or Gerber data. The dual probes can move as close as 4 mils to each other, and they can operate independently, without a fixed limit on how close they can get. The tester, which features two arms capable of moving back and forth, works based on capacitance measurements. The printed circuit board is pressed firmly onto an insulating layer on a metal plate, with the other metal plate acting as the capacitor’s second plate. If a short circuit occurs between the lines during the board’s production, the capacitance increases at that point. Conversely, if an open circuit is present, the capacitance decreases.
The speed of circuit board processing and testing is a key factor when selecting a tester. A bed-of-needle tester can accurately test thousands of points simultaneously, while a flying probe tester can only test two or four points at a time. Additionally, a bed-of-needle tester may cost between $20–30 for a single-sided test, depending on the complexity of the PCB, whereas a flying probe tester can take longer and may cost more to complete the same evaluation. Shipley (1991) noted that although high-volume PCB manufacturers consider the mobile flying probe test method slow, it remains an excellent choice for low-volume, complex circuit board production.
For bare board testing in PCB manufacturing, specialized instruments are available (Lea, 1990). A more cost-effective approach, however, is to use a general-purpose instrument. Although such instruments may have a higher initial cost than dedicated ones, this cost is often offset by the reduction in individual configuration expenses. For standard grids, the typical grid for boards with pin components and surface-mounted devices is 2.5 mm. In this case, the test pad should be at least 1.3 mm in size. For grids as small as 1 mm, the test pad is designed to be larger than 0.7 mm. If the grid is smaller, the test needle becomes more fragile and prone to damage. Therefore, it is preferable to use a grid larger than 2.5 mm. Crum (1994b) stated that combining a universal tester (standard grid tester) with a flying probe tester can provide an accurate and cost-effective solution for detecting issues in high-density PCBs. Another approach he suggested is using a conductive rubber tester, which can detect points that deviate from the grid. However, variations in the height of pads processed by hot air leveling can hinder the connection of test points.
With the advent of surface mount technology, the packaging density of circuit boards has increased significantly. As a result, even for low-density, general-quantity circuit board manufacturing, automatic inspection has become not only a fundamental requirement but also a cost-effective solution. In the processing and inspection of complex circuit boards, two common methods are used: the needle bed test method and the double probe or flying probe test method.
1. **Needle Bed Test Method**
In this method, a spring-loaded probe is connected to each inspection point on the PCB. The spring applies a pressure of 100-200g, ensuring reliable contact at each test point. These probes are arranged together and referred to as a “bed of needles.” Controlled by specialized detection software, the test points and signals can be programmed, allowing the inspector to access information for all the test points. In practice, only the probes for the specific test points that need to be checked are installed. Although the bed-of-needle test method can test both sides of the PCB simultaneously, it is recommended that all test points be located on the soldering surface during the PCB design phase. The needle bed tester is costly and challenging to maintain. Depending on the specific application, probes with different arrangements are selected.
1. **Basic General-Purpose Grid Processor**
A basic general-purpose grid processor typically consists of a drilled board with pin center spacings of 100, 75, or 50 mils. The pins serve as probes, utilizing electrical connectors or nodes on the circuit board to establish direct mechanical connections. If the pads on the circuit board align with the test grid, a polyester film, perforated according to the specifications, is placed between the grid and the printed circuit board to facilitate specific detection designs. Continuity testing is performed by accessing the grid’s endpoints, which correspond to the x-y coordinates of the pads. Since every network on the PCB is tested for continuity, this method ensures that an independent inspection of the circuit board production is completed. However, the proximity of the probes limits the effectiveness of the bed-of-needle testing method.
2. **Double Probe or Flying Probe Test Method**
A flying probe tester does not rely on a pin pattern installed on a fixture or bracket. In this system, two or more probes are mounted on a small magnetic head that can move freely on the x-y plane. The test points are controlled directly by CAD or Gerber data. The dual probes can move as close as 4 mils to each other, and they can operate independently, without a fixed limit on how close they can get. The tester, which features two arms capable of moving back and forth, works based on capacitance measurements. The printed circuit board is pressed firmly onto an insulating layer on a metal plate, with the other metal plate acting as the capacitor’s second plate. If a short circuit occurs between the lines during the board’s production, the capacitance increases at that point. Conversely, if an open circuit is present, the capacitance decreases.
The speed of circuit board processing and testing is a key factor when selecting a tester. A bed-of-needle tester can accurately test thousands of points simultaneously, while a flying probe tester can only test two or four points at a time. Additionally, a bed-of-needle tester may cost between $20–30 for a single-sided test, depending on the complexity of the PCB, whereas a flying probe tester can take longer and may cost more to complete the same evaluation. Shipley (1991) noted that although high-volume PCB manufacturers consider the mobile flying probe test method slow, it remains an excellent choice for low-volume, complex circuit board production.
For bare board testing in PCB manufacturing, specialized instruments are available (Lea, 1990). A more cost-effective approach, however, is to use a general-purpose instrument. Although such instruments may have a higher initial cost than dedicated ones, this cost is often offset by the reduction in individual configuration expenses. For standard grids, the typical grid for boards with pin components and surface-mounted devices is 2.5 mm. In this case, the test pad should be at least 1.3 mm in size. For grids as small as 1 mm, the test pad is designed to be larger than 0.7 mm. If the grid is smaller, the test needle becomes more fragile and prone to damage. Therefore, it is preferable to use a grid larger than 2.5 mm. Crum (1994b) stated that combining a universal tester (standard grid tester) with a flying probe tester can provide an accurate and cost-effective solution for detecting issues in high-density PCBs. Another approach he suggested is using a conductive rubber tester, which can detect points that deviate from the grid. However, variations in the height of pads processed by hot air leveling can hinder the connection of test points.
