Theideal, qualified BGA X-ray image will clearly show that the BGA solder ballsare aligned with the PCB pads one by one. The solder ball image shown is uniformand consistent, which is an ideal reflow soldering result. Conversely, thedeformed solder ball is mainly caused by the following reasons: low reflowtemperature, warpage of the PCB or deformation of the PBGA plastic substrate.It may also be caused by printing defects in SMT processing.

Qualifiedsolder joint

Thedefinition of simple and obvious defects such as bridging, short circuit, lackof ball, etc. in X-ray inspection is very clear, but there is no more in-depthdefinition of complex and unobvious defects such as virtual welding and coldwelding. The densely packed components on the double-sided board often causeshadows. Although the X-ray head and the table of the workpiece to be measuredare designed to rotate,

Solderjoint inspection

Itcan be detected from different angles, but sometimes the effect is not obvious.In order to effectively judge complex and unobvious defects, some equipmentmanufacturers have developed "signal confirmation" software. Forexample, the true meaning of the X-ray image is evaluated and judged based onthe change in size and uniformity of the solder ball in the X-ray pattern afterreflow soldering. The following describes how to determine certain weldingdefects based on the changes in the solder ball diameter and the uniformity ofthe X-ray image in the three stages of the BGA and CSP reflow solderingprocess.

BGApackage diagram

Inthe A stage (150 ℃ heating stage, the solder ball is not melted), the BGAstanding height is equal to the solder ball height.

Inthe B stage (beginning of the collapse stage or once sinking), when thetemperature rises to 183 ℃, the solder ball begins to melt and enter thecollapse stage, at which time the standing height of the solder ball drops to80% of the initial solder ball

Inthe C stage (the final collapse stage or second subsidence), when thetemperature rises to 230 ° C, the solder ball is fully melted and melted withthe solder paste, forming a bonding layer at the upper and lower interfaces ofthe solder ball. The standing height of the solder ball is reduced to 50% ofthe initial solder ball height, and the diameter of the ball on the X-ray imageis increased to 17%, resulting in a 37% increase in the protruding area.

(2)Uniformity of X-ray image

Ifthe X-ray images of all the balls are uniform and the area of ​​the circle isequal to the area of ​​the ball or varies within the range of 10% to 15%, thissituation is very good. There are no defects in reflow soldering, which iscalled "uniform and consistent". In the use of X-ray inspection,uniformity provides the most important feature for the rapid determination ofBGA welding quality. From a vertical angle, BGA solder balls are regular blackdots. Bridging, insufficient or excessive soldering, solder spatter, noalignment, and air bubbles can all be quickly detected.

Theinspection of the virtual welding is analyzed by a certain principle. When theX-ray is tilted to observe the BGA at a certain angle, a well-welded solderball will undergo a secondary field collapse, instead of a sphericalprojection, but a trailing shape. If the X-ray projection of the BGA solderball after welding is still a circle, it means that the ball has not beenwelded and collapsed, so that it can be presumed that the solder joint isvirtual or an open circuit structure. It can be observed from the figure thatthe solder balls that are still spherical are open solder joints.

X-rayscan also be used to detect internal damage to printed circuit boards, componentpackages, connectors, solder joints, etc.