Secondary Source Bring up Cost Metrics and Net Present Value Calculation
Secondary source bring-up generates positive Net Present Value when unit cost savings and risk offsets exceed fixed tooling and qualification expenses.
Tombstoning
Meaning
Surface-mount component displacement occurs when unequal surface tension forces at opposite component terminations lift one end off a printed circuit board pad during reflow. Automated optical inspection systems flag tombstoning on small passive components such as resistors and capacitors after surface-mount assembly passes through the reflow oven. The defect breaks electrical continuity on the elevated side, resulting in an open circuit that halts system operation.
Preventing this structural imbalance requires tight control over pad geometry and solder paste deposition volumes across small component footprints.
Differential Melting
Asymmetrical thermal mass beneath component pads causes solder paste on one pad to reach liquidus state before the adjacent pad melts. Liquid solder exerts surface tension forces that pull the component end downward and inward toward the molten pad center. If the opposing pad remains solid, the rotational torque about the un-melted side lifts the free end completely vertical.
Pad Layout
PCB land pattern designs significantly influence surface tension balance during soldering. Unequal copper trace connections act as heat sinks, causing one pad to heat faster than its neighbor during thermal ramp-up. Utilizing thermal relief connections on power planes equalizes heat dissipation rates between paired pads, preventing tombstoning caused by thermal imbalance.
Trace routing symmetry ensures both component pads absorb heat at identical rates throughout the reflow cycle. Furthermore, excessive pad extension beyond component boundaries increases wetting forces that aggravate lifting tendencies on lightweight passive chips. Proper footprint sizing according to IPC design standards prevents disproportionate liquid solder torque.
Process Mitigation
Controlling solder paste printing volume and component placement accuracy reduces component lifting risks during high-speed SMT assembly. Precise placement prevents component edges from touching outer pad boundaries where unequal wetting forces concentrate.
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.