How to determine the necessity of PCBA detection? PCBA test method

One-stop PCBA intelligent manufacturing manufacturers today will tell you why PCBA should be tested? The inspection method and necessity of PCBA testing. Speaking of PCB, everyone is familiar with it, do you know what PCBA is? PCBA refers to the assembly of PCB boards for SMT placement, DIP plug-ins, and testing to make a finished product. We can understand it as a finished circuit board. The PCBA test is to detect the conductivity and output and input values of the PCBA board, so why should the PCBA be tested?

The necessity of PCBA detection

As a platform for signal transmission of many components and circuits, printed circuit boards (PCBs) have always been regarded as a key part of electronic information products, and their quality determines the quality and reliability of the final product. Due to the development trend of high-density, lead-free and halogen-free environmental requirements, if professional and timely PCBA testing is not carried out, various failure problems may occur, such as poor wettability, cracks, stratification, etc.

In order to ensure the high quality and reliability of PCBA, PCBA processing manufacturers must inspect circuit boards at different stages of the manufacturing and assembly process to eliminate surface defects. In addition, timely and professional inspections can lead to defects exposed before electrical testing, and are conducive to the accumulation of statistical process control (SPC) data.

The widespread application of surface mount technology (SMT) has increased the requirements for inspection, because SMT solder joints must withstand more stress than the application of through-hole electroplating (PTH) technology. Since the leads of the SMT-dependent device must withstand more structural loads, if there is not enough solder, the device will not be soldered firmly to the circuit board. Therefore, the long-term electrical reliability of the circuit board assembling the surface mount device depends to a large extent on the structural integrity of the solder joints, which increases the necessity of PCBA inspection.

PCBA inspection method

So far, in addition to visual inspection, there are a variety of structural inspection techniques to choose from, with different costs, performance and defect coverage. Automatic inspection technology includes optical inspection, laser triangulation, X-ray inspection and X-ray lamination technology. In order to achieve the best process inspection, PCBA processing manufacturers should understand the advantages and disadvantages of each inspection method and clarify the best performance of each type. Generally, PCB assembly inspection technology is divided into two types: visual inspection and automatic process inspection.

1. Visual inspection

After a large number of steps in the PCBA processing process, visual inspection can be used, and visual inspection equipment can be selected according to the location of the inspection target. For example, after the solder paste is printed and the device is placed, the inspector can find obvious defects with the naked eye, such as contaminated solder paste and missing parts. The most common visual inspection can check the reflow soldering joints by observing the light reflected from ordinary prisms from different angles. Generally speaking, this kind of inspection can test 5 joints in just one second.

The effectiveness of visual inspection depends on the ability of the inspectors, the consistency and applicability of the inspection standards. The inspector must fully understand the technical requirements of each solder joint, because each type of solder joint may contain up to 8 defect standards, 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. In addition, visual inspection is not suitable for hidden solder joint inspection, such as J-lead devices with high-density packages, ultra-fine square flat devices, surface array flip chips or BGA (ball grid array) devices. Based on the establishment of unified and specific rules, visual inspection is considered a low-cost, easy-to-access technology suitable for large-scale defect detection.

2. Structural Process Test System (SPTS)

The digitization and analysis system of real-time and automatic video capture can significantly improve the tolerance and repeatability of visual inspection. Therefore, the structural process test system relies on certain forms of emitted light, such as visible light, laser beams, and X-rays. All of these systems obtain information by processing images to identify and measure defects related to the quality of solder joints. Similar to visual inspection, the implementation of SPTS does not require physical contact with the circuit board. However, unlike visual inspection, SPTS is so highly repeatable and eliminates the subjectivity of defect measurement.

3. Automatic/automatic optical inspection (AOI)

The AOI system relies on multiple light sources, a programmable LED library, and some cameras to illuminate the solder joints and take pictures. Under the reflected light, the leads and solder joints reflect, reflecting most of the light, while the PCB and SMD reflect very little light. The light reflected from the solder joint cannot provide actual height data, while the pattern and intensity of the reflected light provide information about the curvature of the solder joint. Then conduct a professional analysis to determine whether the solder joints are complete, whether the solder is sufficient, and whether poor wetting occurs. In addition, the AOI system also checks for solder bridging and missing components or displacement before or after reflow soldering.

AOI equipment runs at a speed of 30-50 connectors per second and has a relatively low cost. However, it failed to check the parameters of certain solder joints, such as the height of the weld and the solder in the solder joints, and failed to check hidden solder joints, such as those that are essential for the welding reliability of BGA, PGA and J-shaped lead devices. In short, the AOI test is performed best when inspecting ICs and gull wing equipment with a spacing greater than 0.5mm.

4. Automatic laser test (ALT) measurement

ALT is a more direct technique used to test the height and shape of solder joints or solder paste deposits. When the image of the laser beam is focused on one or more position-sensitive detectors at an angle to the laser beam, the system is used to measure the height and reflectance of some surface components. During ALT measurement, the surface height is determined by the position of the light reflected by the position-sensitive detector, and the surface reflectance is calculated by the power of the reflected beam. Due to secondary reflection, the beam of light may illuminate position-sensitive detectors in multiple locations, which requires a scheme to distinguish correct measurements. In addition, when the light travels along the position-sensitive detector, the reflected beam may be shielded or disturbed by interfering materials. In order to eliminate multiple reflections and prevent shielding, the system should test the reflected laser beam along an adjusted independent optical path. During multiple height measurements of solder joints, the ALT system OPTIMAL is used to align the amount and position of solder paste deposition before component assembly. It provides data for real-time structural process control of solder paste printing, including viscosity, alignment, cleanliness, fluidity, and extrusion speed and stress.

5. X-ray fluoroscope system

The X-ray perspective system emits a beam of light from a single-point light source and passes vertically through the circuit board. As this process continues, solder joints weaken the intensity of light to a greater extent than other materials. The intensity change of light energy is converted into a digital X-ray pattern with a grayscale of 256. The grayscale X-ray graphics of some solder joints are actually density images that represent the thickness, distribution, and internal integrity of the solder joints. On a single-sided PCB, the X-ray perspective system can accurately check for welding joint defects, such as welding joint defects (including cracks, insufficient welding, bridging, misalignment, voids, etc.) that occur on the J-shaped wiring device, gull wing equipment or passive chips. In addition, it can also check for missing components and reverse tantalum capacitors. However, when it comes to double-sided PCBs, the X-ray fluoroscopy system cannot accurately inspect those defects caused by the possible overlap of the X-ray images of the solder joints on both sides of the board.

6. X-ray lamination system

Compared with the X-ray perspective system, the X-ray lamination system produces the focal plane of the horizontal cross-sectional area by scanning or rotating synchronously with the X-ray detector. The off-axis image generated on the detector then causes a cross-sectional image with a surface thickness of 0.2-0.4mm to be generated by a single swing or multiple swings, which leads to homogenization. In addition, the components on the front and rear sides of the focal plane become defocused in the laminated image, causing the solder joints in the focal plane to detach from other materials on the PCB.

According to the laser rangefinder, the X-ray lamination system plots the surface position of the plate relative to the focal plane and corrects the warping of the plate. After that, the circuit board moves in small vertical increments so that it passes through the focal plane, after which different parts of the same solder joint can be checked. It is suitable for BGA and PTH solder joint detection. The double-sided PCB moves vertically in large increments to pass through the focal plane to check the solder joints on both sides of the board.

By modifying the scanning radius of the beam and the focal plane of vertical movement, different magnification coefficients or visual area sizes can be set. The X-ray lamination system can measure the parameters of all physical solder joints on different focal planes, which can provide process defect coverage. Due to the indication relationship between the cross-sectional image of the X-ray and the given solder paste volume, the grayscale reading can be converted to the actual size by the specified standard unit or metric unit. After analyzing the measurement results, the data will be provided for characterization and assembly improvement.

For example, changes in the average solder paste thickness or solder paste volume of solder joints can make people aware of the quality level of solder paste printing and defect sources. The X-ray lamination system runs at an inspection speed of 30-40 joints per second. It operates through a flexible sampling method to ensure 100% coverage of critical equipment inspection, but cannot cover 100% of equipment with an assembly cycle of less than 45 seconds. The X-ray lamination system is the most expensive of all inspection methods, but it greatly shortens the search and rework time.

How to determine the PCBA processing detection method?

Although there are many types of detection methods, there are big differences between AOI inspection and X-ray inspection. Three elements should be taken into account when determining the inspection method: defect type, cost and inspection speed.

When it comes to defect type AOI and X-ray coverage, AOI is usually used to test the inner layer before lamination. Defect items include the amount of solder paste, component location, deletion and polarity, and solder joint defects. However, the former focuses on fine and microscopic defects after lamination, and can test wiring components, semiconductor packaging, BGA welding defects, solder joint voids, and high-mixing, low-capacity assembly.