Quantifying Radiographic Escape Rates for Bottom Terminated Component Solder Joints
Radiographic escape quantification requires correlating grey-scale attenuation thresholds with physical microsections to catch hidden non-wetting defects.
Perimeter Lead Starvation
Fillet Defect
Workmanship anomalies describe a condition where insufficient solder volume is deposited onto the outer terminations of a quad flat pack or discrete component. During reflow, perimeter lead starvation occurs when solder paste is under-deposited or when heat pulls solder away from the lead-pad interface. This lack of solder results in weak joints that cannot withstand normal operational stresses.
Deposition Control
Inadequate solder volume during printing often stems from clogged stencil apertures or incorrect squeegee pressure. Automated solder paste inspection detects these insufficient deposits on the outer pads before component placement. If the printing machine is not properly calibrated, the peripheral pads on large packages can receive less paste than the center pads.
Thermal profiles also contribute to the issue if rapid heating causes the outer leads to reach reflow temperature before the solder paste fully melts, drawing the alloy up the lead and leaving the pad starved of solder.
Joint Fatigue
The insufficient fillet profile decreases the mechanical strength of the solder joint, leading to failure during thermal cycling or mechanical shock. Visual inspection after reflow easily identifies these poorly formed fillets. Boards with this defect must undergo rework to manually add solder paste or wire alloy to the starved leads.