Application of X-RAY in BGA welding quality inspection

The full name of BGA is Ball Grid Array (PCB), which is a pin packaging method for large components, similar to the four sided pins of QFP, both of which are connected to the circuit board by SMT solder paste soldering. Its difference lies in the single row pins listed in the surrounding "one degree space", such as gull wing shaped extension pins, flat extension pins, or retracted J-shaped pins at the bottom of the abdomen; Change to a full or local array at the bottom of the abdomen, adopt a two-dimensional spatial area based solder ball pin distribution, and use it as a soldering interconnect tool for chip packaging to circuit boards. It has the characteristics of small packaging area, increased functionality, increased number of pins, high reliability, good electrical performance, and overall low cost.

Although BGA devices have many advantages, there are still shortcomings that cannot be changed: after welding, due to the fact that all the solder joints of BGA devices are below the bottom of the device body, traditional visual inspection methods cannot be used to observe and inspect the welding quality of all solder joints, nor can AOI (Automatic Optical Inspection) equipment be used to evaluate the appearance of solder joints. Currently, the commonly used method is to use X-RAY inspection equipment to inspect the physical structure of BGA device solder joints.

X-RAY testing principle

X-ray inspection equipment is based on the imaging principle of X-rays. The X-ray generator emits X-rays to irradiate the inspected printed circuit board group and BGA devices. X-rays cannot penetrate high-density and thick substances such as tin and lead, and can form dark images. However, they can easily penetrate low-density and thin substances such as printed circuit boards and plastic packaging without forming images, achieving quality inspection of BGA device welding points.

Solder joint defects

a. Types of solder joint defects

The main defects of BGA solder joints include solder bridging, solder beads, voids, misalignment, open circuits, loss of solder balls, cracking of solder joints, and virtual soldering.

b. Solder bridging

Due to the electrical short circuit caused by solder bridging, there should be no solder bridging between adjacent solder balls after BGA device welding. This defect is more obvious when using X-RAY inspection equipment, and continuous connections between solder balls can be seen in the image area, making it easy to observe and judge.

c. Solder bead

Solder beads are one of the main defects in surface mount processes, and there are many reasons that can cause solder beads. This type of defect is also easy to identify in the X-RAY imaging area. When using X-RAY inspection equipment to observe and measure solder beads, the main attention should be paid to their size and position requirements, and the minimum electrical gap requirements should not be violated.

d. Hollow

Voids are the most common after BGA device soldering, as many BGA devices may have voids or pores in their solder balls. In the reflow soldering process, if the reflow curve is not set properly, it is more likely to produce voids. Both GJB 4907-2003 and IPC-A-610E standards provide evaluation criteria for voids, but there are differences in the evaluation scales. According to GJB 4907-2003, the void in the solder joint should not exceed 15% of the solder joint volume, while IPC-A-610E specifies that the void in any solder ball in the X-RAY imaging area should not exceed 25%.

e. Virtual soldering

Generally, virtual soldering is caused by insufficient reflow soldering process. During the reflow soldering process, the solder ball and solder paste did not melt well, making it impossible to form a well wetted eutectic. Virtual soldering is a defect that cannot be ignored, which can easily cause device detachment and affect the electrical performance of the device. Virtual welding defects must also be inspected by rotating the X-ray angle to take timely and effective measures to avoid their occurrence.