- Much has been written about void prevention when soldering with lead-free solder pastes containing tin-silver-copper. Excessive solder voids can create a reliability issue especially in applications where the lead-free assembly will be exposed to thermal cycling conditions or in applications where the assembly will be exposed to vibration or flexing during box builds. Also voids can reduce thermal performance and reduce electrical integrity.

- It must also be stated that smaller voids can in cases increase reliability by changing the crack pattern. Studies have shown that there is no reduction in reliability when voids are present to up to 25% by volume in the joint. Voids can act as stress relievers, due in part to the compressive nature of air pockets.

- This is documented in the technical paper, Voiding: Occurrence and Reliability Issues with Lead-free, by Martin Wickham of the National Physical Laboratory.

• Solder paste chemistry

• Solder surface tension effects

• Thermal profile

• Oxidation of the outer surface of solder joints

• Termination geometries, joint shape

• Metallization of finishes for boards and components

• Component board out-gassing during reflow

- Lead-free alloys such as SAC alloys have slight higher surface tensions when compared to 63/37. It is important to select a solder paste which has a flux chemistry designed for the higher preheats and peak temperatures. Choosing a solder paste, which does not contain resins and activators which decompose at these higher temperatures is the primary factor in void reduction. Good solder paste manufacturers are designing flux systems for lead-free alloys. The voiding potential information is often available for use during the paste selection process.

- Optimizing the reflow profile as to remove any volatiles by extending the preheat times and increasing the time above liquidus will also help in reducing void entrapment. Insuring components and boards are free of moisture and plating contaminants will also help to reduce voids. It has been shown that copper OSP tends to produce slightly higher volume of voids when compared to Ni/Au and silver immersion, which produce much less.

- In some cases joint geometries are contributors. Components such as leadless chip carries or large flat surfaces, perpendicular to the board will prevent out-gassing during the soldering process; this results in void increases. Solder flux by-products both.

- liquid and gases, will have to slowly make there way upwards. Component geometries which prevent the proper upward flow will usually result in an increase in voids.

X-Ray, Voids in
BGA ‘s