Calculating dielectric height with Speedstack
Application Note AP507
Speedstack H Value calculation
(see also Powerpoint presentation AP507b — Base thickness and isolation dimension definitions in Speedstack)
H Value is a value that is extracted from the stackup and used for the calculation of impedance in controlled impedance structures included in the stackup. It is the effective height of a dielectric substrate after the pressing of the stack.
Speedstack is fully integrated with the Si8000m Controlled Impedance Quick Solver and Si9000e PCB Transmission Line Field Solver so the user can quickly add controlled impedance structures to layers in the stackup. Using the Goal Seek facility of the Si8000m/Si9000e the designer or board fabricator rapidly can arrive at the controlled impedance structure parameters to produce the target impedance.
The H Value calculation produces the dielectric heights required for the Polar SI8000m/Si9000e field solvers.
Finished Thickness is a critical dimension in many applications; Speedstack keeps track of finished thickness and tolerance and allows fabricators the flexibility of adding in-house post-press thickness for prepreg layers. Speedstack also takes into account plating thickness where appropriate.
Finished Thickness is the thickness of a prepreg after pressing between two solid sheets of copper and should be defined in the materials library; it will be a lower value than the Base Thickness. This value will be dependent upon the board shop’s process parameters.
The method for finishing, Copper Coverage or Simple Percentage, can be set under the Speedstack Tools Menu|Set Finishing Method.
Simple Percentage Method
The Simple Percentage Method method is included for compatibility with earlier version of the stackup design systems. Polar Instruments recommends the use of the Copper Coverage method.
Choose Copper Coverage to display the Copper Coverage Based Prepreg Corrections dialog. Copper coverage determines the amount of copper that is embedded in the adjacent prepreg(s).
Set by Layer Type
Specify the percentage of copper for each layer type, signal, mixed or plane, that will be embedded in the adjacent prepreg.
Proportional to Coverage
Specify finishing dependent upon actual copper coverage to allow the values to be modified on an individual basis.
Copper coverage is inversely proportional to the degree of copper embedding. The amount of the copper embedded can be expressed as:
Ce = Cu(1-Cc/100)
The copper coverage for each layer can be set individually under Properties.
Copper may be embedded into the prepreg from either or both sides. Embedding copper in the prepreg reduces the thickness from the Finished Thickness. This reduced thickness is referred to as the Isolation Distance.
Where more than one prepreg layer is involved the reduction is averaged across all the prepregs. A figure for Isolation Distance of less than zero implies an impossible build, i.e. the embedded copper totals more than the thickness of the prepreg.
In the Speedstack graphic below the Isolation Distance is H2 – T1 i.e. the prepreg thickness minus the trace thickness.
The H Value is the sum of the isolation thickness and the full thickness of the adjacent copper(s).
However this does depend upon the controlled impedance structure and the layer type. Where a structure straddles one or more copper layers the full intervening copper thicknesses are added to the sum of the prepreg isolation thicknesses. Power plane copper is not included in the H Value.
Where a structure straddles a mixed layer, the copper of the mixed layer is included in the H Value, but if that mixed layer is the power plane for the structure the thickness of the copper is ignored. The H Value may be larger than the Finished Thickness because those areas of prepreg where there is no copper embedded are subjected to less pressure than they would be if all areas were covered with copper.
An H Value larger than Base Thickness implies an impossible build. The H Value used in a structure can be calculated, i.e. checked, by summing the isolation distances and the intervening copper finished thickness plus the signal trace thickness where applicable.
For further information please talk to your regional Polar office.