Post Production Change Control in Contract Printed Circuit Assemblies
Post-production change control requires frozen revision baselines, strict PCN risk classification, delta testing, and explicit contractual cost allocation.
Tombstoning Failure Mode
Meaning
Mechanical instability occurs during the reflow process when surface tension forces act unevenly upon a component before the solder reaches a fully molten state. A tombstoning failure mode results when one termination of a passive device loses contact with its pad while the other side pulls the body upward into a vertical position. Disparate wetting speeds across two pads trigger this rotation.
Small chip components like capacitors and resistors remain especially vulnerable to these imbalances during thermal cycling within convection ovens.
Thermal Variance
Asymmetric heat distribution causes the solder paste on one pad to liquify before its counterpart. Molten alloy exerts a surface tension force that drags the metallic end of the component toward the center of the pad. If the opposing side remains solid, the component pivots on that stationary hinge.
Rapid transitions in heating profiles exacerbate this discrepancy.
Layout Geometry
Design features determine the degree of susceptibility to this physical displacement. Wide traces act as heat sinks that draw energy away from a pad, delaying the melting point compared to narrower connections. Disconnected ground planes or thermal reliefs create temperature gradients that drive inconsistent wetting times.
Pads exceeding the dimensions of the component terminations exacerbate the leverage during the vertical lift.
Assembly Control
Solder mask definition and stencil aperture precision limit the volume of material delivered to each connection. Consistent deposition ensures that surface tension forces stay balanced across both ends of the device. Nitrogen inerting atmospheres further assist by preventing premature oxidation of the metallic surfaces.
Proper alignment of the component centering during pick and place operations minimizes the mechanical torque applied during the subsequent reflow stage.