Inclusion Threshold
The upper limit for the volume of entrapped gas within a metallic joint determines the thermal and mechanical viability of an interface. Industry standards like IPC-A-610 define a solder voiding limit as a percentage of the total solder joint area visible in an x-ray image. Voids appear as lighter circular regions within the darker solder mass.
Radiographic Inspection
Non-destructive testing using automated x-ray equipment quantifies the size and distribution of these internal gaps. Large voids in a thermal pad can block heat transfer and lead to the overheating of power components. Small scattered voids are usually considered acceptable as long as they do not exceed 25 percent of the total area.
Joint Integrity
Electrical and structural failure occurs when the cumulative area of these pockets reduces the load bearing capacity of the connection. Thermal cycling causes these gaps to expand and contract, which eventually leads to fatigue cracks in the surrounding metal. Excessive flux outgassing or contaminated component leads often cause these defects during the reflow stage.
Controlling the ramp rate and soak time in the reflow profile allows gases to escape before the solder solidifies. High reliability assemblies require a more restrictive boundary to ensure longevity in harsh environments.
