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www.us - tech.com


March, 2024


New Technology to Enable Flawless Solder Inspection


By Jay Lee, Deputy General Manager, MIRTEC


joints for high-end electronics — “ART,” a system capable of inspecting solder joints to IPC Class 3 standards. ART stands for “Anti- Reflection Tech nology. In high-end markets, product reliability


M


and performance are more important than mass production capacity, result- ing in significant investments being made in inspection processes to maintain quality. Solder joint inspection is par-


ticularly important in the high-end market where reliability and dura- bility are critical. A failure at the solder joint


can be catastrophic. When it comes to solder joint inspection, we see the performance limitations of cur- rent 3D AOI. The main and most important phenomenon is that the measurement repeatability is not maintained at a high level, and sometimes, the 3D image is simpli- fied compared to the actual solder geometry.


Solder Joint Inspection The main reason why accu-


rate 3D inspection of solder joints is difficult is because of their high- ly reflective surface and irregular shape. After the reflow process, solder joints have a polished sur- face, which is one of the key factors that hin- der pattern light recognition. This is essen- tial for reconstructing the shape of an object during 3D inspection. In general, when 3D patterned light is


projected onto a material surface, some of the light is absorbed. Some is reflected at an angle equal to the incident angle at which the patterned light was projected relative to the normal of the surface (this is called spec- ular reflection). The remaining scattered light is collect- ed by the camera, which serves as the sensor


ART’s optical setup with five cameras. If the surface of the object to be inspect-


ed is orthogonal to the camera (i.e., the cam- era is positioned in the direction normal to the surface of the object to be inspected), the amount of light that can be perceived by the camera is small, making it difficult to recog- nize patterns. If the surface of the object to be inspect-


ed is at an inclined angle, the amount and intensity of light received will exceed the sen- sor’s tolerance, leading to light saturation and the sensor’s inability to recognize the pattern shape. If the object to be inspected


IRTEC has developed a solution that solves a current inspection challenge regarding 3D inspection of solder


in the optical system. Subsequently, this col- lected light is converted into a pattern image. This pattern image serves as the source data needed to reconstruct 3D through mathemat- ical calculation. However, if the surface reflectivity of the


object is very high, both the absorbed light and scattered light decrease, and most of the light is reflected.


has a highly reflective surface, there will be spots where pattern recognition is not possi- ble depending on the angle between the cam- era and the object’s surface. The irregularity of a solder joint’s shape


results in varying reflection angles across the entire area of the solder joint. The combina- tion of high surface reflectivity and irregular- ity characteristics results in the phenomenon that when 3D measuring solder joints, some parts of the solder joint can acquire pattern data nor- mally, while other parts lose data due to light saturation, and still other parts lose data due to lack of light. The 3D source data is pieced with holes. This phenomenon is common


regardless of whether the height measurement method of 3D AOI is laser triangulation or PSP (Phase Shift Profilometry, commonly known as Moiré in the industry). No matter how high the dynamic range (the range in which bright and dark areas can be recognized) of the camera applied to the optical system, it can only reduce the problem. The physical structure of the


solder joint creates vanishing points in the 3D measurement pat- tern. As mentioned earlier, for 3D AOI to reconstruct the geometry of an inspected object, the patterned light from the projector must be


reflected from the object and a portion of that light must reach the camera. However, if sol- der has not risen above the component due to excess soldering, and instead, the solder has formed normally, then the surface slope of the component at the point where it contacts the solder is in the same direction as the camera. Therefore, when the patterned light


projected by the projector reaches the junc- tion surface of the solder and the component body, the reflected light is blocked by the


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