Evaluating Step Stencil Keep out Zones for Mixed Pitch Assemblies
Step stencil keep out zones require a 10 to 1 distance ratio per mil of step height difference to prevent squeegee levitation and solder paste bridging.
Solder Volume Insufficiency
Connection Deficit
Assembly defect conditions occur when the amount of conductive alloy in a joint falls below the minimum requirement for mechanical or electrical reliability. Solder volume insufficiency leads to weak connections that may fail under vibration or thermal stress. This condition is often characterized by a lack of wetting or a thin film that does not form a proper fillet.
It is a major cause of open circuits in surface mount technology.
Causal Mechanism
Clogging of the stencil apertures or improper squeegee pressure frequently results in a lack of material on the pads. When solder volume insufficiency is detected, it usually indicates a problem with the paste release or the stencil thickness. Solder paste that has dried out or exceeded its tack life also contributes to poor deposition.
Components with high lead counts are particularly sensitive to this defect due to their small pad areas.
Inspection Threshold
Detection happens during automated optical inspection where the height and area of the solder are compared to a golden board. If the solder volume insufficiency exceeds the limits defined by IPC-A-610, the board must be rejected or reworked. X-ray inspection may be required to verify the fill levels inside through-holes or under ball grid arrays.
Correcting the stencil design or the printing parameters prevents the recurrence of the issue.