The ideal, qualified BGA X-ray image will clearly show that the BGA solder balls are aligned with the PCB pads one by one. The solder ball image shown is uniform and consistent, which is an ideal reflow soldering result. Conversely, the deformed solder ball is mainly caused by the following reasons: low reflow temperature, warpage of the PCB or deformation of the PBGA plastic substrate. It may also be caused by printing defects in SMT processing.
Qualified solder joint
The definition of simple and obvious defects such as bridging, short circuit, lack of ball, etc. in X-ray inspection is very clear, but there is no more in-depth definition of complex and unobvious defects such as virtual welding and cold welding. The densely packed components on the double-sided board often cause shadows. Although the X-ray head and the table of the workpiece to be measured are designed to rotate,
Solder joint inspection
It can be detected from different angles, but sometimes the effect is not obvious. In order to effectively judge complex and unobvious defects, some equipment manufacturers have developed "signal confirmation" software. For example, the true meaning of the X-ray image is evaluated and judged based on the change in size and uniformity of the solder ball in the X-ray pattern after reflow soldering. The following describes how to determine certain welding defects based on the changes in the solder ball diameter and the uniformity of the X-ray image in the three stages of the BGA and CSP reflow soldering process.
BGA package diagram
In the A stage (150 ℃ heating stage, the solder ball is not melted), the BGA standing height is equal to the solder ball height.
In the B stage (beginning of the collapse stage or once sinking), when the temperature rises to 183 ℃, the solder ball begins to melt and enter the collapse stage, at which time the standing height of the solder ball drops to 80% of the initial solder ball
In the C stage (the final collapse stage or second subsidence), when the temperature rises to 230 ° C, the solder ball is fully melted and melted with the solder paste, forming a bonding layer at the upper and lower interfaces of the solder ball. The standing height of the solder ball is reduced to 50% of the initial solder ball height, and the diameter of the ball on the X-ray image is increased to 17%, resulting in a 37% increase in the protruding area.
(2) Uniformity of X-ray image
If the X-ray images of all the balls are uniform and the area of the circle is equal to the area of the ball or varies within the range of 10% to 15%, this situation is very good. There are no defects in reflow soldering, which is called "uniform and consistent". In the use of X-ray inspection, uniformity provides the most important feature for the rapid determination of BGA welding quality. From a vertical angle, BGA solder balls are regular black dots. Bridging, insufficient or excessive soldering, solder spatter, no alignment, and air bubbles can all be quickly detected.
The inspection of the virtual welding is analyzed by a certain principle. When the X-ray is tilted to observe the BGA at a certain angle, a well-welded solder ball will undergo a secondary field collapse, instead of a spherical projection, but a trailing shape. If the X-ray projection of the BGA solder ball after welding is still a circle, it means that the ball has not been welded and collapsed, so that it can be presumed that the solder joint is virtual or an open circuit structure. It can be observed from the figure that the solder balls that are still spherical are open solder joints.
X-rays can also be used to detect internal damage to printed circuit boards, component packages, connectors, solder joints, etc.
