PCB News - Reasons for poor welding of BGA and countermeasures

Reasons and solutions for poor welding of BGA

With the continuous improvement of people's awareness of environmental protection, lead-free PCBs have become popular.

However, everything has two sides. Without the assistance of lead, the problem of poor soldering of PCB, especially BGA, is gradually exposed.

Next, we invite engineers from the Shenzhen Bo PCBA factory of Benqiang Circuits to talk about their analysis and solutions to this problem.

1. The problem of BGA solder ball falling off and soldering part deformation of the solder ball

After the BGA component is reflowed, the solder ball deforms and falls off at the BGA end, causing electrical non-conduction.

Cause Analysis:

The BGA structure is poorly designed, and its insulating resin is too thick.

During the soldering process of BGA, the volume of the solder ball becomes larger due to the incorporation of the applied solder paste, and the shape of the solder ball changes to form an ellipse due to melting. At this time, because the insulating resin on the BGA side is too thick, and the surface tension of the lead-free solder is larger than that of the lead solder, the surface tension causes the solder ball to fall off from the BGA side.

solution:

1. The BGA supplier improves the design structure

2. Control of solder paste printing volume

3. Improve the accuracy of tin printing and placement

2. BGAVoid (void)

There are voids in the solder balls of the BGA components after reflow. In the subsequent high and low temperature cycles, high temperature and high humidity, vibration and drop tests, the BGA appears to be electrically non-conducting, as shown in Figure 5. According to 8.2.12.4 of IPC-A-610D, the BGAVoidX-ray image area of class 1, 2, and 3 cannot exceed 25% of the total area of the solder ball.

Cause Analysis:

The PCBPAD is poorly designed, and there are holes on the PAD.

With the gradual development of electronic products "small, light and thin", their BGA packages are gradually becoming smaller, and BGAs with a pitch of 0.5mm are now very common. In PCB design, holes are often required in the PAD for wiring requirements. In the SMT process, after PAD is coated with solder paste, there is air in the hole, and the gas expands during the Reflow process. The surface tension of lead-free solder is greater than that of leaded solder, and the gas cannot be completely released, thus forming a cavity.

solution:

1. Improved PCB PAD design, using drilling filling technology to fill the holes.

2. Adjust the Reflow profile to increase the preheating time and reduce the peak temperature.

3. The selection of solder paste alloy composition, the higher the Ag content, the less voids.

3. BGA Bridge (Lian Tin)

The solder balls of BGA components are connected to the solder after reflow.

Cause Analysis:

PCB PAD is poorly designed, and there are holes on the PAD

In the SMT process, after PAD is coated with solder paste, there is air in its drill hole. During the Reflow process, the gas expands, causing the solder balls to gradually expand. When two adjacent solder balls expand to a certain extent, a continuous tin is formed, such as Shown in Figure 7.

solution:

1. Improved PCB PAD design, using drilling filling technology to fill the holes.

2. The PCB PAD is poorly designed, the shape of the PAD is irregular, and there is no solder mask between the PADs.

With the development of the electronics industry, the pitch of the BGA is getting smaller and smaller. When the shape of the BGAPAD is irregular, the gap between the PADs will be smaller than the standard value in some places. Figure 8: The pitch of the BGA is 0.5mm and the diameter of the PAD is 0.24mm. Then the standard for the gap between PADs is 0.26mm, but in some places the gap is only 0.12mm.

Four. BGA less tin

BGA solder joints have been inspected by X-ray, and the solder joints have different shapes and sizes, and there are problems with false soldering and soldering reliability.

Cause Analysis:

Poor stencil opening design makes it difficult to demold the solder paste

solution:

1. Modify the size of the steel mesh opening and improve the steel mesh production process

2. Adjust the printing parameters

The traditional hole-opening method of BGA is round. Another hole-opening method is square rounded corners. The hole is made with the same size as the diameter and side length. It is better than the traditional round shape, but it is necessary to evaluate whether the amount of tin will cause the BGA to connect to the tin when the square round corner is used for the opening method. For 0.5mm pitch BGA, due to its small opening size, it is recommended that the steel mesh be made by laser cutting and electropolishing.

3. The particle size of the metal powder in the solder paste is too large, causing difficulty in demolding.

Countermeasures:

1. Select the appropriate metal powder particle size type according to the needs of the product

There are different sizes of the metal powder particle size in the solder paste, and the following types exist according to the J-STD-005 powder particle size.

Five. BGA offset

The BGA solder joints are inspected by X-ray, and the solder joints deviate from the PAD position, and there are problems with false soldering and soldering reliability.

Cause Analysis:

The placement accuracy of the placement machine is not enough

It is well known that the self-correction ability of lead-free solder paste is worse than that of lead-free solder paste. In the lead-free process, the components that are misaligned cannot be corrected by their own correction ability during the Reflow process.

solution:

Improve the placement accuracy of the equipment.

Six.BGA soldering interface separation (between solder ball and PCB)

The BGA component is electrically conductive after reflow, but in the subsequent high and low temperature cycle, high temperature and high humidity, vibration and drop test, the connection of the BGA appears INT phenomenon.

Cause Analysis:

1. The Reflow Profile is not properly set, and the BGA solder ball and PCB board have not formed a good soldering.

solution:

1. Refer to the Reflow Profile of the solder paste and BGA to re-adjust the profile to extend the time above 220 degrees and increase the Peak Temp. When using SAC305 solder paste, it is recommended that the temperature is above 220 degrees for 40 to 60 seconds, and the Peak Temp is 235 to 245 degrees.

2. PCB electroplating is poor, such as the "blackpad" phenomenon in ENIG finish PCB, which causes the bonding strength between the PCB and the solder to decrease. After a slight force, the PCB and the solder break, as shown in Figure 12.

2. Before PCB production, it needs to pass the verification of welding strength, use tinned copper wire soldering iron to solder on the BGAPAD, and then test its tensile force.

3. Establish a PCB manufacturer management table, count all PCB bad records, and select PCB manufacturers with stable quality and strong comprehensive strength such as after-sales service.

In summary:

Due to the need to manufacture miniaturized high-density packaging electronic devices, BGA and CSP have now become mainstream packaging forms. But its invisible solder joint forming process puts forward higher requirements for assembly and rework. In addition, the high-density substrate technology required for BGA and CSP packaging and the implementation of lead-free make process control more complicated. The lead-free soldering of BGA is worthy of our more in-depth research and analysis to improve the reliability of the product.