Detailed explanation of the causes of non-wetting open soldering and improvement measures

Detailed analysis of Non Wet Open (NWO) and suggestions for improvement

I.the definition and typical characteristics

Non Wet Open (NWO) refers to the PCB (Printed Circuit Board), BGA (Ball Grid Array) pads do not achieve a good wet open solder joints.Typical characteristics of its slicing diagram for all or part of the PCB pads on the lack of solder wetting, as shown in Figure 1-1 (not attached here, but a clear description).

图 1-1 无润湿开焊焊点切片图

II. Causes

BGA warpage causes solder paste to pull up

Formation stage: No wetting open solder joints usually begin to form in the temperature rise stage of reflow soldering (160 ~ 190 ℃).

Formation mechanism: As shown in Figure 1-2, BGA warpage occurs, the solder paste will be brought to the BGA solder ball, due to the separation of the solder paste and pads, resulting in the formation of good solder joints.

Figure 1-2 Mechanism of non-wetted open solder joint formation

Other conditions that lead to open solder joints

Solder Paste Leakage: Solder paste is not properly printed onto the pads.

Oxidation of solder pads: Oxidation on the surface of the pads affects the wettability of the solder.

Dirt on the pad or flux process problems: Dirt on the pad surface or improper use of flux, resulting in the solder not being able to wet well.

Root cause

The root cause of this defect is the deformation of the BGA and the pulling up of the solder paste.Huawei's Zhu Ailan et al. conducted an in-depth study on the causes of solder paste pull-up, and found that there is no direct correlation between the phenomenon of solder paste pull-up and its low-temperature activity, high-temperature adhesion and adhesion stability.However, through the low temperature over the furnace (below the melting point of the solder paste) direct pull-up method to study the solder paste pull-up phenomenon, the results show that the probability of the solder paste is pulled up for 0~7.6%, at least show that this phenomenon exists.Warpage and the probability of NWO failure can be significantly reduced by lowering the reflow temperature.

III. the identification method

Non-wetting open soldering can be identified by X-Ray.As the solder paste is covered with solder balls, the solder joints are usually significantly larger than the surrounding solder joints.If this phenomenon corresponds to the failed solder joints, it is basically confirmed that there is no wetting open soldering phenomenon.

Suggestions for improvement

For non-wetting open soldering phenomenon, need to analyse specific cases, but generally should follow the following recommendations for improvement:

PCB on the surface before cleaning

Ensure that the PCB on the line before a thorough surface cleaning to remove the pad surface of the oxide layer, dirt, etc., to improve the wettability of the solder.

Use SPI to monitor solder paste printing quality

SPI (Solder Paste Printing Inspection) is used to monitor the quality of solder paste printing to prevent leakage of printed veneers from flowing into subsequent processes.The SPI allows real-time detection of the amount of solder paste printed, the printing position and other parameters to ensure that the quality of solder paste printing meets the requirements.

Other Improvement Measures

Optimise soldering process parameters: according to the characteristics of PCB and BGA, optimise the temperature profile, time and other parameters of reflow soldering to ensure that the solder can fully wet the pad.

Enhance the fixing of BGA: Increase the fixing measures of BGA on PCB, such as the use of glue, bracket, etc., to reduce the warping and deformation of BGA in the welding process.

Regular inspection and maintenance of equipment: Regular inspection and maintenance of soldering equipment to ensure the accuracy and stability of the equipment to reduce soldering defects caused by equipment problems.