Quantifying Inner Layer Interconnect Strain during Solder Reflow
Matching dielectric CTE above Tg to copper ductility prevents inner layer interconnect fracture during high temperature reflow thermal profile cycles.
Teardrop Fillets
Junction Geometry
Gradual widening of a conductor track at its entry point into a pad or via land reduces localized stresses and improves the reliability of the electrical connection. This tapered junction, referred to as teardrop fillets, prevents trace breakout during drilling or thermal expansion. The added copper at the intersection provides a robust physical transition that redistributes the tensile forces acting on the connection.
Implementing this design feature is standard practice for high-reliability boards used in aerospace or medical applications.
Pattern Protection
Mitigating the effects of mechanical registration errors during the drilling process is a major advantage of utilizing these features. When the drill bit deviates from the center of the via pad, it can cause the hole to break through the edge of the circular pad, an occurrence known as breakout. If teardrop fillets are present, the extra copper on the entry side preserves the electrical connection even if the hole is misaligned.
This feature also prevents the formation of sharp corners where chemicals can become trapped during the etching process, reducing the risk of trace corrosion. In addition, the geometry helps reduce the concentration of current density at the trace-to-pad transition, which minimizes localized heating under high current loads.
Design Rule
Automated design software generates these geometries based on user-defined parameters such as trace width and pad diameter. Designers specify the teardrop style, which can be straight or curved, and the length of the transition zone where the trace begins to widen. Solder mask clearances must be checked to ensure that the added copper of the teardrops does not interfere with the solder mask dams between adjacent pads.
Thermal relief patterns must also be coordinated with teardrop generation to prevent unintended heat sinking during hand-soldering operations.