Wetting angle and surface tension, Two Critical Parameters for packaging materials

As the electronic industry is transitioning to lead free packages, this leads to significant challenges for solder, flux, and process developments. In flip chip package, solder wetting is a key factor affecting the solder joint reliability. This paper presents some wetting tests methods. These methods can be used in a fast paced materials development environment where a short turn around time for experimental results is needed. These methods have been used for materials and process development screening. The wetting tests were performed first in the lab for materials being developed. Only the materials with good wetting properties went to factory floor for further development. Since the wetting test methods are low cost and have fast turn around time than the factory process, it reduced the materials development time and cost for lead free process.

Lead free solders in flip chip packages have much higher peak reflow temperature than tin lead solder due to its high melting temperature. The wetting properties between tin lead solder and lead free solder is quite different. Tin lead solder has low surface tension and better wetting than lead free solder. Solder wetting and spreading properties are related to the solder joint reliability, such as solder voids and solder non wet problems.

A solder spreading test was used for solder wetting analysis under different reflow conditions. Wetting angles of solder spheres after reflow were calculated by using analytical equations from the solder spreading data. The calculated wetting angles were compared with measured ones. It shows that the calculated contact angles were consistent with the measured one. For very low contact angle, it is very challenge to measure it accurately. The wetting angle calculated from the solder spreading give a good estimate for low contact angle. The wetting properties for different solders and fluxes, and different surface finishes were also studied. The solder ball spreading can be either regular or irregular in shape depending on what fluxes were used. This indicates the wetting and rheology properties of fluxes have an impact on solder spreading properties. So some wetting properties of fluxes, such as surface tension and contact angle, were also collected.

Solder wetting balance test was used to get the maximum wetting force and time at zero wetting force. The solder wetting force from the solder wetting balance test was compared with the solder spreading and wetting angle for the same samples. The correlation between solder bump non wet and solder wetting properties will be discussed. Our results showed that wetting force and wetting angle measurements can provide useful information about the performance of the solder and flux materials and are good tools for quality control and product development.