2022-09-09
Why does PCBA need to be tested?
The necessity of PCBA testing
As a platform for many components and circuit signal transmission, printed circuit board (PCB) has always been regarded as a key part of electronic information products, and its quality determines the quality and reliability of the final product. Due to the development trend of high density, lead-free and halogen-free environment requirements, various failure problems such as poor wettability, cracks, delamination, etc. may occur if professional and timely PCBA inspection is not carried out.
In order to ensure the high quality and reliability of PCBA, PCBA manufacturers must inspect circuit boards at different stages in the manufacturing and assembly process to eliminate surface defects. In addition, timely and professional inspection can lead to defects exposed prior to electrical testing and facilitate data accumulation for statistical process control (SPC).
The widespread use of surface mount technology (SMT) has increased inspection requirements because SMT solder joints must withstand more stress than applying through-hole plating (PTH) technology. Since the device leads that depend on the SMT have to carry more structural loads, the device will not be soldered firmly to the board without enough solder. Therefore, the long-term electrical reliability of circuit boards on which surface mount devices are assembled depends heavily on the structural integrity of the solder joints, which increases the necessity of PCBA inspection.
PCBA inspection method
To date, in addition to visual inspection, a variety of structural inspection techniques are available, with varying cost, performance, and defect coverage. Automatic inspection techniques include optical inspection, laser triangulation, X-ray inspection and X-ray lamination technology. Generally, PCB assembly inspection techniques are divided into two types: visual inspection and automatic process inspection. This is also the inspection method currently used in our factory.
1. Visual inspection
Visual inspection can be used after a large number of steps in the PCBA processing process, and visual inspection equipment is selected according to the location of the inspection target. For example, after solder paste printing and device placement, inspectors can visually detect obvious defects such as contaminated solder paste and missing parts. The most common visual inspection is the ability to inspect reflow solder joints by looking at light reflected from a common prism from different angles. Typically, this inspection takes only one second to test 5 joints.
The effectiveness of visual inspection depends on the competence of inspectors, the consistency and applicability of inspection standards. Inspectors must fully understand the technical requirements of each solder joint, as each type of solder joint may contain up to 8 defect criteria, and there may be more than 6 solder joints on different assembly equipment. Therefore, visual inspection is not suitable for quantitative measurement of effective structural process control. Also, visual inspection is not suitable for inspection of hidden solder joints such as J-lead devices with high density packaging, ultra-fine quad flat devices, surface array flip-chip or BGA (ball grid array) devices. Based on uniform and specific rule establishment, visual inspection is considered a low-cost, easily accessible technique for large-scale defect detection.
2. Structural Process Test System (SPTS)
As a platform for many components and circuit signal transmission, printed circuit board (PCB) has always been regarded as a key part of electronic information products, and its quality determines the quality and reliability of the final product. Due to the development trend of high density, lead-free and halogen-free environment requirements, various failure problems such as poor wettability, cracks, delamination, etc. may occur if professional and timely PCBA inspection is not carried out.
In order to ensure the high quality and reliability of PCBA, PCBA manufacturers must inspect circuit boards at different stages in the manufacturing and assembly process to eliminate surface defects. In addition, timely and professional inspection can lead to defects exposed prior to electrical testing and facilitate data accumulation for statistical process control (SPC).
The widespread use of surface mount technology (SMT) has increased inspection requirements because SMT solder joints must withstand more stress than applying through-hole plating (PTH) technology. Since the device leads that depend on the SMT have to carry more structural loads, the device will not be soldered firmly to the board without enough solder. Therefore, the long-term electrical reliability of circuit boards on which surface mount devices are assembled depends heavily on the structural integrity of the solder joints, which increases the necessity of PCBA inspection.
PCBA inspection method
To date, in addition to visual inspection, a variety of structural inspection techniques are available, with varying cost, performance, and defect coverage. Automatic inspection techniques include optical inspection, laser triangulation, X-ray inspection and X-ray lamination technology. Generally, PCB assembly inspection techniques are divided into two types: visual inspection and automatic process inspection. This is also the inspection method currently used in our factory.
1. Visual inspection
Visual inspection can be used after a large number of steps in the PCBA processing process, and visual inspection equipment is selected according to the location of the inspection target. For example, after solder paste printing and device placement, inspectors can visually detect obvious defects such as contaminated solder paste and missing parts. The most common visual inspection is the ability to inspect reflow solder joints by looking at light reflected from a common prism from different angles. Typically, this inspection takes only one second to test 5 joints.
The effectiveness of visual inspection depends on the competence of inspectors, the consistency and applicability of inspection standards. Inspectors must fully understand the technical requirements of each solder joint, as each type of solder joint may contain up to 8 defect criteria, and there may be more than 6 solder joints on different assembly equipment. Therefore, visual inspection is not suitable for quantitative measurement of effective structural process control. Also, visual inspection is not suitable for inspection of hidden solder joints such as J-lead devices with high density packaging, ultra-fine quad flat devices, surface array flip-chip or BGA (ball grid array) devices. Based on uniform and specific rule establishment, visual inspection is considered a low-cost, easily accessible technique for large-scale defect detection.
2. Structural Process Test System (SPTS)
Digitization and analysis systems for real-time and automatic video capture can significantly improve the tolerance and repeatability of visual inspections. Therefore, structural process testing systems rely on some form of emitted light, such as visible light, laser beams, and X-rays. All of these systems process images to obtain information to find and measure defects related to solder joint quality. Similar to visual inspection, SPTS is implemented without physically touching the board. However, unlike visual inspection, SPTS has such high repeatability and eliminates the subjectivity of defect measurement.
3. Automatic/Automatic Optical Inspection (AOI)
AOI systems rely on multiple light sources, a library of programmable LEDs, and a few cameras to illuminate the solder joints and take pictures. In reflected light, leads and solder joints act as reflections, reflecting most of the light, while PCBs and SMDs reflect very little. The light reflected from the solder joint does not provide actual height data, while the pattern and intensity of the reflected light provide information about the curvature of the solder joint. Professional analysis is then performed to determine if the solder joint is complete, if the solder is adequate, and if poor wetting has occurred. In addition to this, the AOI system also checks for solder bridging and missing components or displacement before or after reflow soldering.
AOI equipment operates at 30-50 joints per second and is relatively low cost. However, it fails to check certain solder joint parameters, such as the height of the solder joint and the solder in the joint, and it fails to check for hidden solder joints, such as those belonging to BGA, PGA and J-lead devices that are essential for solder reliability of solder joints. In summary, AOI testing performs BEST when examining ICs and gull-wing devices with a pitch greater than 0.5mm.
4. Automatic Laser Test (ALT) measurement
ALT is a more direct technique for testing height and shape solder joints or solder paste deposition. The system is used to measure the height and reflectivity of some surface features when the image of the laser beam is focused on one or more position-sensitive detectors held at an angle to the laser beam.During ALT measurements, the surface height is determined from the position of the light reflected by the position-sensitive detector, while the surface reflectivity is calculated from the power of the reflected beam. Due to secondary reflections, the beam may strike a position-sensitive detector at multiple locations, which requires a scheme to distinguish correct measurements. Furthermore, the reflected beam may be shielded or interfered with by interfering materials as the light travels along the position sensitive detector. To eliminate multiple reflections and prevent shielding, the system should test the reflected laser beam along a regulated, independent optical path. During multiple height measurements of solder joints, the ALT system OPTIMAL is used for solder paste deposition volume and location alignment prior to component assembly. It provides data for real-time structural process control of solder paste printing, including viscosity, alignment, cleanliness, flow and extrusion speed and stress.
5. X-ray Fluoroscopy System
3. Automatic/Automatic Optical Inspection (AOI)
AOI systems rely on multiple light sources, a library of programmable LEDs, and a few cameras to illuminate the solder joints and take pictures. In reflected light, leads and solder joints act as reflections, reflecting most of the light, while PCBs and SMDs reflect very little. The light reflected from the solder joint does not provide actual height data, while the pattern and intensity of the reflected light provide information about the curvature of the solder joint. Professional analysis is then performed to determine if the solder joint is complete, if the solder is adequate, and if poor wetting has occurred. In addition to this, the AOI system also checks for solder bridging and missing components or displacement before or after reflow soldering.
AOI equipment operates at 30-50 joints per second and is relatively low cost. However, it fails to check certain solder joint parameters, such as the height of the solder joint and the solder in the joint, and it fails to check for hidden solder joints, such as those belonging to BGA, PGA and J-lead devices that are essential for solder reliability of solder joints. In summary, AOI testing performs BEST when examining ICs and gull-wing devices with a pitch greater than 0.5mm.
4. Automatic Laser Test (ALT) measurement
ALT is a more direct technique for testing height and shape solder joints or solder paste deposition. The system is used to measure the height and reflectivity of some surface features when the image of the laser beam is focused on one or more position-sensitive detectors held at an angle to the laser beam.During ALT measurements, the surface height is determined from the position of the light reflected by the position-sensitive detector, while the surface reflectivity is calculated from the power of the reflected beam. Due to secondary reflections, the beam may strike a position-sensitive detector at multiple locations, which requires a scheme to distinguish correct measurements. Furthermore, the reflected beam may be shielded or interfered with by interfering materials as the light travels along the position sensitive detector. To eliminate multiple reflections and prevent shielding, the system should test the reflected laser beam along a regulated, independent optical path. During multiple height measurements of solder joints, the ALT system OPTIMAL is used for solder paste deposition volume and location alignment prior to component assembly. It provides data for real-time structural process control of solder paste printing, including viscosity, alignment, cleanliness, flow and extrusion speed and stress.
5. X-ray Fluoroscopy System
