Power Distribution Plane Cavity Resonance and Radiated Emissions
Power distribution plane cavity resonances create high impedance peaks that drive edge fringing fields, forcing radiated emissions beyond CISPR 32 limits.
Substrate Tolerance
Material Variation
The allowable variation in the physical and electrical properties of a dielectric material affects the consistency of printed circuit board performance. This substrate tolerance governs parameters such as the thickness of the laminate, the dielectric constant and the loss tangent of the resin-glass matrix. These variations arise from the manufacturing processes of the laminate supplier and the glass-to-resin ratio used in the prepreg.
The tolerance limits apply across each production lot and must be controlled to ensure consistent electrical performance of the finished board.
Impedance Impact
Variations in dielectric thickness and dielectric constant directly alter the characteristic impedance of high speed transmission lines. Since impedance depends on these material parameters, a wide substrate tolerance can cause the trace impedance to deviate from the nominal fifty-ohm target. This deviation causes signal reflections, which degrade the signal integrity of high speed digital interfaces on the assembled board.
To maintain control over these variations, designers must specify tight tolerances and use controlled impedance fabrication services where the manufacturer adjusts the trace widths to compensate for the actual laminate thickness.
Coupon Measurement
Quality assurance teams verify the laminate properties by measuring the impedance of test coupons included on the panel. This measurement ensures that the manufactured board conforms to the specified electrical requirements.