Quantifying Resin Flow Shear Dynamics in Multilayer Laminate Buildup Operations
Modelling resin squeeze flow shear kinetics during multilayer pressing prevents inner-layer trace swim and guarantees reliable dielectric clearance.
Trace Swim Mechanics
Conductor Displacement
Internal copper features move from their nominal positions during the heat and pressure cycles of the multilayer lamination process. Trace swim mechanics involves the study of the physical forces that cause these fine conductors to shift laterally within the softening resin. This movement is a primary concern for high density boards with narrow traces and tight spacing requirements.
Understanding these mechanics allows fabricators to adjust their processes and artwork to compensate for the expected displacement.
Dynamic Force
Viscous drag from the flowing epoxy resin applies a mechanical load to the sides of the copper traces as they sit between the laminate sheets. Because the resin is under high pressure and moving toward the edges of the panel, it pushes any unanchored copper features along with it. Trace swim mechanics suggests that the magnitude of the shift is proportional to the resin velocity and the surface area of the trace wall.
Longer traces or those located in areas of high resin flow are more susceptible to this type of movement than short or anchored features. Using high resin content prepreg or increasing the lamination temperature too quickly can aggravate this effect by reducing the viscosity of the fluid. Fabricators often use specialized layup techniques or modified resin systems to increase the stability of the traces during the critical flow stage.
Precision Impact
Uncontrolled movement leads to shorts, opens, misalignment and reduced annular ring widths that compromise the reliability of the finished circuit. Maintaining alignment within a few thousandths of an inch is necessary for the successful drilling and plating of high layer count designs.