Solder Inclusion
Entrapped gases forming empty spaces within solder joints or underfill materials during thermal reflow processing degrade mechanical and electrical interconnect integrity. Occurring frequently in surface mount assembly, voiding reduces thermal conductivity and electrical path cross-sections under high-power components and ball grid array packages. Acceptability standards defined in IPC-A-610 restrict maximum allowable voiding to 25 percent of total solder joint area for bottom termination components, beyond which joints fail inspection.
X-ray Evaluation
Volatile flux solvents, oxidized surface finishes, or inadequate outgassing pathways during solder paste reflow cause gaseous bubbles to remain trapped inside solidifying solder fillets. Bottom termination components like quad flat no-lead packages exhibit high susceptibility to voiding because large ground pads trap flux vapors beneath the component body. Process adjustments, including optimizing reflow soak time, switching to low-voiding solder paste chemistries, or utilizing vacuum-assisted reflow ovens, significantly reduce gas bubble entrapment.
Vacuum reflow draws air and flux outgassing out of liquid solder before cooling, achieving void levels below ten percent. Inspection of hidden solder joints relies on two-dimensional and three-dimensional X-ray systems capable of measuring individual and cumulative void surface area percentages.
Reliability Risk
Excessive voiding degrades mechanical shear strength, increasing susceptibility to thermal fatigue cracking under operational thermal cycling. High current densities passing around large voids induce localized electromigration and localized heating in power electronics assemblies. Process monitoring enforces strict void limits to ensure long-term solder joint reliability.
