Formation of Solder Joints during BGA Reflow Soldering

The formation of solder joints during BGA (Ball Grid Array) reflow soldering is a complex and delicate process that involves a number of phases, including preheating, solder melting, ball collapsing, auto-alignment, and so on. The following is a detailed analysis of the process:

Solder Joint Formation Process

1. Preheating Stage: During this stage, the solvents in the solder paste begin to evaporate and the flux begins to wet the solder balls and remove oxides from their surfaces. This is to ensure that the solder ball and pads can form a good intermetallic connection between.

2. Primary Collapse: As the temperature rises, the solder begins to melt and the BGA assembly begins to sink due to gravity, known as primary collapse. This process makes the initial contact between the solder ball and the pad, and prepares for the subsequent soldering process.

3. Secondary Collapse and Auto-alignment: When the temperature continues to rise, the solder ball completely melts and fuses with the molten solder. At this point, the BGA assembly further sinking and automatic alignment, that is, the second collapse. The key to this process is that the oxide film on the surface of the solder ball is removed by the high temperature of the molten solder, allowing the solder ball and the solder to be fully integrated. The secondary collapse not only achieves the physical connection of the solder joints, but also realises the automatic alignment of the BGA assembly through the surface tension of the molten solder, which improves the precision and reliability of the soldering.

Reasons for Temperature Differences

1. Effect of Oxide Films:

Although the melting point of a solder alloy is theoretically dependent on its composition, oxide films on the surface of the solder ball can prevent direct contact between the ball and the molten solder. Therefore, higher temperatures as well as chemical activity are required to remove these oxide films, thus achieving a complete fusion of the solder ball with the solder. This is the reason why the temperature required for secondary collapse is 11~12°C higher than the melting temperature of solder paste.

2.  experimental observation

In the experiment, it can be observed that when the oxide film on the surface of the solder ball is penetrated by the molten solder, the solder ball will rapidly fuse with the molten solder. This shows that the oxide film is the main factor hindering the fusion of the solder ball and the solder.

3, the simplification of BGA placement

Due to the automatic alignment feature of the BGA reflow soldering process, the placement position of the BGA component does not require a high degree of accuracy. This feature simplifies the placement process, allowing the operator to place according to the silkscreen frame in the corner of the BGA, without undue concern about positional deviation. In summary, BGA reflow soldering technology achieves efficient and reliable soldering of high-density IC packages through its unique two-collapse and auto-alignment processes. At the same time, the technology also simplifies the placement process and improves productivity.