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www.us-tech.com
October, 2024
Reliable 3D THT Solder Inspection Based on Real Measurement Data
By Jay Lee, Deputy General Manager, MIRTEC Co., Ltd.
he importance of through-hole technol- ogy (THT) solder inspection is increas- ing, driven by a greater number of THT components on PCBs. The problem is that today’s 3D AOI systems were developed for SMT inspection, which leaves THT solder inspection vulnerable.
T
THT was once considered an outdated technology that slowly faded away from the
cal durability. Considering the industries that are driving the growth of electronics manufacturing in recent years are automo- biles and large appliances, we can see how these characteristics are driving an increase in THT component usage.
Structural Issues
Today’s 3D AOI systems operate with two main techniques: laser triangulation and phase shift profilometry — commonly known as Moiré. Since both techniques are based on triangulation, they use an optical system with cameras and projectors (or laser emit- ters) mounted at fixed positions to create two fixed angles.
While they are a reasonable solution for fast and precise inspection compared to other 3D technologies, the fixed angle of the sensor and pattern light source means that there is a risk of measurement failure due to reflec- tions, occlusions, etc.
MIRTEC’s Anti-Reflection Technology (ART) system.
mainstream of the electronic manufacturing industry. SMT is much better at reducing product size and increasing productivity than THT.
SMT is also better than THT in terms of minimizing parasitic inductance and resist- ance, which leads to increased performance of components and frequency of capacitors. However, THT components have their advantages.
THT parts have a higher mechanical bond strength to the PCB than SMT parts. Also, an electrolytic capacitor has higher absolute capacitance than an MLCC. Of course, MLCCs beat electrolytic capacitors in capacitance per unit area, but electrolytic capacitors are used if high capacity is needed. At the product stage, these two advan- tages translate into high physical and electri-
Unlike 2D technology, it requires the sensor to be able to recognize patterns, which can be a limitation when inspecting reflective surfaces or objects that reflect little light. The problem with these materials can be solved by changing the color/wavelength of the light source, but the issue with angles cannot be solved in the same way. Here’s a simple example to illustrate how 3D AOI properly measures the height of an object. The light from the projector (or laser emitter) must hit the object being inspected, and then the light reflected from the surface must hit the sensor of the camera. It’s impor- tant to note that the projected light is specu- lar, meaning that only a specific portion of the light reflected from the surface must be direct- ed to the camera, not all of it.
If a significant portion of the reflected light is directed at the camera, the camera is overwhelmed with too much light for it to dis- tinguish patterns. This condition is called light saturation, which causes pattern recog- nition failure. This light saturation may or may not occur depending on the angle between the projector, the surface being
A mixed PCB with both THT and SMT components.
ponents, the solder joint is only on one side. In contrast, THT components have a blind spot that encompasses the entire 360-degree area around the pin.
Inspection Difficulties
Whether it’s light saturation or occlu- sion, the height of the point where the prob- lem occurs is unknown. In 3D inspection, it will show up as noise in the form of a hole or a spike upward in the area, which means that if the measurements are correct, the THT pin solder joint should show up in the 3D image as a hollow area around the pin rim.
However, this probably won’t be seen under normal configurations because 3D AOI manufacturers are already aware of this optical limitation and use 3D algorithms to restore the void in those areas. The problem is that if there is a hole in a slope where light
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inspected, and the camera. Since solder joints are curved, light saturation will likely occur somewhere on the surface.
There is another phenomenon besides light saturation called occlusion. Occlusion is when the camera is unable to read the pro- jector’s pattern due to gaps or obstructions. In the case of THT solder joints, the entire joint around the pins is a blind spot causing occlusion.
SMT solder joints also suffer from this issue, but with a key difference: in SMT com-
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