BGA is a typical high-density packaging technology. Its characteristic is that the chip pins are distributed under the package in the form of ball solder joints, which can make the device smaller, with more pins, larger pin spacing, higher finished product assembly rate and better electrical performance. Therefore, the application of such packaged devices is becoming more and more extensive. However, since the BGA solder joints are hidden on the bottom of the chip, it is not conducive to inspection after soldering and assembly. On the other hand, the country or industry has not formulated BGA welding quality inspection and acceptance standards, so BGA welding quality inspection technology is the main problem in the application of such equipment.
BGA soldering quality inspection methods are very limited. Common inspection methods include visual inspection, flying probe electronic test, X-ray inspection, dye inspection and biopsy inspection. Among them, dyeing and section inspection are destructive inspections, which can be used as failure analysis methods but are not suitable for welding quality inspection. In NDT, a visual inspection can only detect solder balls at the edge of the device, not internal defects in the solder balls. The false alarm rate of the flying probe electronic test is too high; the X-ray inspection machine can detect the hidden objects under the equipment well by using the X-ray transmission characteristics. The soldering condition of the solder balls is an effective method for BGA soldering quality inspection.
Electron X-ray machine real-time imaging systems are divided into two types: 2D imaging and 3D tomography. The principle is to use an X-ray to penetrate the sample to be tested, and then received by the image receiver and converted into an image signal, and the image shows obvious grayscale contrast. A region with a larger grayscale in the image indicates a large attenuation of the X-ray energy, indicating that the material in that region is thicker or that the material has a larger atomic number. Two-dimensional imaging observes the top view of the part under test, which has the advantage of fast imaging. 2D X-ray image of a common BGA solder ball. Since the solder ball is made of a tin alloy, it absorbs more X-rays and has a larger gray level relative to the surrounding material. 3D tomography uses the rotation of mechanical equipment in the device to scan the sample from various angles, and the software analyzes and processes it to form the 3D shape of the sample being measured. This test method can reflect the real state of the tested sample more realistically and clearly.
The quality of BGA soldering includes continuous soldering, missing solder balls, moving solder balls, solder ball voids, virtual soldering and the pincushion effect. These defects will affect the reliability of the circuit. Some of them are immediately apparent, such as a short circuit when connecting the solder balls. Some show up in use, such as the pillow effect. In use, the solder balls can easily break on the pillow and form fake solder. With some testing, we can easily troubleshoot real-time performance defects, while non-real-time performance defects are more harmful to electronic systems, so we should strengthen detection and timely investigation.
Electron x-ray inspection machines are generally considered to be only capable of detecting defects including continuous soldering, solder ball loss, solder ball displacement and voids. The introduction of 3D tomography technology enables X-ray inspection to cover all common defects in BGA soldering. In particular, the detection of incorrect soldering and pincushion effects no longer depends only on destructive inspection methods. On the other hand, in practical engineering applications, in order to consider detection efficiency, it is necessary to combine 2D imaging and 3D tomography. Quickly inspect the overall soldering quality through 2D imaging, check for continuous soldering of solder balls, solder ball loss, solder ball displacement and solder ball voids, and initially identify false solder. According to the actual situation, use 3D tomography to confirm the existence of false welding and the pincushion effect. The comprehensive application of the two technical means can complete the welding quality inspection of the bga machine and provide reliable quality assurance for the application of the bga machine.
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