The formation of BGA solder voids can lead to current concentration effects and reduce the mechanical strength of solder joints. Therefore, from a reliability perspective, it is necessary to reduce or minimize solder voids. To answer this question, it is necessary to explore the causes of void formation.
There are various reasons for the formation of BGA solder voids, such as the crystal structure of solder alloys, PCB design, the amount of solder paste deposition during printing, the soldering process used, and voids trapped in the solder balls during fabrication.
Below, we will discuss the formation and prevention of BGA solder voids from the perspective of solder paste to reduce the number of voids.
- Improper Reflow Temperature Profile Settings
(1) Excessive temperature ramp-up rate during the heating stage may cause rapid gas escape, lifting the BGA off the pads. (2) Insufficient duration of the ramp-up stage may result in gas still escaping during the reflow stage, affecting the effectiveness of the flux system.
- Improper Solvent Composition in the Solder Paste
(1) Rapid gas escape during the heating stage may lift the BGA, causing misalignment and bridging. (2) During the reflow stage, a significant amount of gas may still escape from the flux system. However, due to the confined space between the BGA and the pads, these volatile gases cannot escape smoothly, leading to the squeezing of molten solder joints.
- Inadequate Wetting Ability of the Solder Paste on the Pads
Insufficient wetting of the solder paste on the pads can result in poor cleaning of the oxide layer on the pads, leading to insufficient wetting and solder balls.
- Excessive Surface Tension of the Flux System during Reflow
Mainly caused by improper selection of the carrier (mainly resin) and the surfactant. Certain surfactants not only reduce the surface tension of the flux system but also significantly reduce the surface tension of the molten alloy. Effective coordination between resin and surfactant can fully utilize the wetting properties.
- High Non-volatile Content in the Flux System
Excessive non-volatile content leads to obstruction of BGA solder ball collapse during melting, causing non-volatile substances to invade or encapsulate solder joints.
- Selection of Rosin as the Carrier
For BGA solder paste, selecting rosin with a low softening point is more meaningful than selecting rosin with a high softening point used in ordinary solder paste systems.
- Amount of Rosin Used
Unlike solder paste systems, for BGA solder paste, rosin serves as a carrier for various activators to release them at the appropriate time to play their roles. However, excessive rosin not only hinders the release of these substances but also hinders the collapse of BGA solder balls during the reflow process, resulting in voids. Therefore, the amount of rosin used should be much lower than that used in solder paste systems.
Another cause of BGA voids is the phenomenon of re-entrainment during soldering. The formation of this phenomenon is related to the temperature and duration of action of the active substances in the flux system. During BGA reflow soldering, due to gravity, BGAs are more prone to this adverse phenomenon compared to SMT solder pastes.
The above lists and discusses the reasons for BGA voids caused by solder paste. Similar to the development of solder paste, the development of BGA solder paste is also a process of balancing various influencing factors. Although each factor has a unique effect, they interact with each other in the entire system. Identifying various influencing factors helps solve problems, and finding solutions, especially finding suitable materials, is the ultimate solution.
