pcb assembly-STG Electronic
The result of this effort is the Samtec Golden Standard. PCB Assembly The Samtec Golden Standard has been used for several years internally at Samtec. It has proven to be an invaluable training and investigative aid for evaluating simulation and modeling packages and approaches and for evaluating test procedures and data post processing.
1.2 The Ideal Golden Standard Design and Construction
The Golden Standard is based on a two conductor coupled microstrip design. The
geometries were developed with consideration for future manufacturability. The design
is illustrated in isometric and cross section views below.
The structure can be described briefly as two lines of 0.075 inch width separated by a space of 0.025 inch. Each trace is separated from the top ground plane by 0.025 inch gaps. Conductors are considered ideal lossless copper, 0.0014 inch thick. The upper and lower conductors are separated by a dielectric 0.059 inch thick. The total width is 1.00 inch. The total coupled length is 5.15 inches.
The ideal solution requires an infinite ground plane below pcb assembly and on both sides of the microstrip traces, but very "wide" ground planes are sufficient. Three to four times the width of the traces is found to be acceptable. To reduce the problem space for more universal applications and to match with the physical design of the Golden Standard board, the total width of the reference structure is limited to one inch.
A similar situation occurs with the upper and lower ground planes. In the ideal solution, the upper and lower ground planes are equipotential. Again, in many field solvers, this can easily be constrained.
In the Samtec analysis, material properties were chosen to closely approximate standard FR4 PCB laminate. The values used actually aren’t much of a concern in the virtual model, as long as identical values are used in both the analytical calculations and the model.
1.3 Derivation of Golden Standard Analytical Solutions
In this paper, we focus solely on frequency domain characterization of the Golden
Standard. However, Samtec has performed many time domain simulations and
correlations using several commercially available software packages.
A theoretical discussion of transmission line theory is included in Part II of this document. An in-depth discussion of the calculation of the analytical solutions can be found there as well. An analytical solution can be obtained for the magnitude and frequency of maximum coupling for near-end crosstalk (S31). See Part II, Reference (2) for details.
For the Golden Standard, a maximum coupling of 13.3 dB at a frequency of 324 MHz is
calculated. The coupling maximum should repeat at multiples of 324 MHz. Phase change will also occur in S31 at 324 MHz and related multiples. This provides several easily discernable markers at low to mid frequency for comparing against simulated data. It is interesting to note that the S31 marker is a function of the coupled line length, and the added length of the SMA connectors and “Y” break out region have very little impact on data from the Golden Standard physical boards.
An analytical solution also exists for the maximum coupling frequency for far-end crosstalk or S41. Details are provided in Part II, Reference (3). The S41 marker occurs at a higher frequency (8-13.6 GHz) in the Golden Standard structure, so it serves as a good high frequency validation point.
An important feature of the S41 resonance is that its frequency is highly dependent on the lossy characteristics of the dielectric material. For example, for a lossless dielectric, the resonance should occur at 13.6 GHz. Losses tend to lower this frequency. For example, using our assumptions for the FR4 material properties, we calculated the resonance would occur at 8.3 GHz. Thus, the S41 marker is an excellent test for validating material property assumptions.