PCB News - Evaluation and judgment of X-ray inspection in the production process of PCB boards

Evaluation and judgment of X-ray inspection in the production process of PCB boards

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An ideal, qualified BGA x-ray image will clearly show that the BGA solder balls are aligned with the PCB board one by one. The solder ball images shown are uniform and consistent, which is an ideal reflow soldering effect. On the contrary, the main causes of solder ball deformation are: low reflow temperature, PCB board warpage or PBGA plastic substrate deformation. It may also be due to printing defects in SMT processing.

Qualified welded joints

In X-ray inspection, the definition of simple and obvious defects such as bridging, short circuit, and missing balls is very clear, but there is no more in-depth definition for complex and inconspicuous defects such as virtual welding and cold welding. The dense components on the double-sided board often cause shadows. Although the x-ray head and the worktable of the workpiece to be measured are both rotating,

Solder joint inspection

It can be detected from different angles, but sometimes the effect is not obvious. In order to effectively judge complex but not obvious defects, some equipment manufacturers have developed "signal confirmation" software. For example, the real meaning of the x-ray image can be evaluated and judged by the changes in the size and uniformity of the solder balls in the x-ray image after reflow soldering. The following describes how to determine certain soldering defects based on the change in the diameter of the solder ball 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 degree Celsius heating stage, the solder ball is not melted), the station height of the BGA is equal to the station height of the solder ball.

In the B phase (the beginning of the collapse phase or once the sinking phase), when the temperature rises to 183 degree Celsius, the solder ball begins to melt and enter the collapse phase, at this time the standing height of the solder ball drops to 80% of the initial solder ball

In the C stage (the last collapse stage or the second settling stage), when the temperature rises to 230°C, the solder balls are completely melted and melted together with the solder paste, forming a bonding layer on the upper and lower interfaces of the solder balls. The standing height of the solder ball is reduced to 50% of the initial solder ball height, and the diameter of the solder ball on the x-ray image is increased to 17%, which increases the protruding area by 37%.

(2) Uniformity of x-ray image

If the X-ray images of all the balls are uniform, the area of the circle is equal to the area of the ball, or varies in the range of 10%-15%, this situation is very good. Reflow soldering has no defects, which is called "uniformity". When using X-ray inspection, uniformity is the most important feature to quickly determine the quality of BGA soldering. From a vertical perspective, BGA solder balls are regular black spots. Bridging, under or over soldering, solder spatter, misalignment and bubbles can all be detected quickly.
A certain principle analysis is carried out on the detection of virtual welding. When the X-ray is oblique to a certain angle to observe the BGA, the well-welded solder ball will experience secondary field collapse, not the spherical projection, but the tail shape. If the X-ray projection of the BGA solder ball after soldering is still a circle, it means that the ball has not been soldered and collapsed, so the solder ball can be considered to be a virtual or open circuit structure. It can be seen from the figure that the solder balls that are still spherical are open solder joints. iPCB is happy to be your business partner. Our business goal is to become the most professional prototyping PCB manufacturer in the world. With more than ten years of experience in this field, we are committed to meeting the needs of customers from different industries in terms of quality, delivery, cost-effectiveness and any other demanding requirements. As one of the most experienced PCB manufacturers and SMT assemblers in China, we are proud to be your best business partner and good friend in all aspects of your PCB needs. We strive to make your research and development work easy and worry-free.