What is a Solder Void?

Applying appropriate amounts of solder paste to a pad is of paramount importance to ensure good quality solder joints. If this is not done, bad solder joints will be created - cold joints and brittle joints. When less than require solder is applied to a joint, it can result in empty spaces or holes inside the solder joint – these are known as Solder Voids.

A Solder void is generated due to the insufficient availability of solder while creating the joint. Typically, a solder void contains air, but it may also consist of flux residue or cured resin when resin-containing solder paste is used during soldering.

According to IPC (IPC-A-610), solder voids are considered to be a defect when the overall calculated area of all voids in any given solder ball/joint is greater than 25% of the total solder ball area.

Type of Solder Voids

• Macro Voids: Created due to the evolution of trapped gases of fluxes and solder paste. They are 4 to 12 mils in diameter and most likely to appear in SMT processes.
• Planar Microvoids: They are responsible for BGA failure (1 to 2 mils in dia.).
• Shrinkage Voids: Elongated and rough voids starting from the surface of the solder joints. They can be seen in both BGA and through hole solder joints. They look like cracks in the joint.
• Intermetallic Microvoids: They are sub-micron size voids, formed when the board is exposed to elevated temperatures (>100C) for more than 48 hours.
• Microvia Voids: Microvias associated to the BGA joint are responsible for Microvia voids.
• Pinhole Microvoids: 1 to 3 microns in diameter and located between the IMC layer and copper land.

Factors responsible for Solder Voiding

• Stencil Design and Reflow Profile
• Solder Paste and PCB Design
• PCB surface finish & Process Environment

Impact of Solder Voids on Solder Joint Performance

• Impose threat to the reliability of a solder joint
• They cause solder bridging
• Reduce the thermal conductivity of solder joints
• They may cause the solder to flow from pad to another during the reflow process
• Also, affect the current carrying capacity

How can you avoid Solder Voiding?

Solder voiding can be controlled by providing more of an outgassing path so that gases can easily escape and not get trapped inside a board. An indigenously chosen solder reflow profile can give the volatile materials or gasses enough time to escape before the solder joint solidifies. Also, low temperature, lead-free solder paste creates the less voiding issue.

Industry Specifications for Solder Voiding 

IPC-A-610 has defined acceptability criteria for solder voids, which stated that solder voids would be considered as a defect when the cumulative projected area of all voids in any given solder ball is greater than 25% in an x-ray image. IPC-7095 gives more detailed information about the treatment of voiding in BGA solder joints, including voiding sources, calculation of the void percentage, inspection of voiding, and control & preventive measures. IPC-7095 states that; Current industry data suggests that voids in the solder joint are not a reliability concern. In fact, the appearance of a void after assembly reflow is an indicator that the reflow process has taken place and the BGA ball has changed characteristics.

In a nutshell, voids are not actually a reliability concern, but their location is. Voids generated at the intermetallic layers or interfacial surfaces are responsible for cracking during aging and atmospheric stress conditions.