RF and wireless design: Substrates and laminates

Yesterday we spent an absolutely intense two hours in discussions of substrates for RF and high frequency design with a couple of experts. Frequencies from about 1 Ghz to 77 Ghz were in play. The amazing part of the discussions was the level of parameters to be considered, not only in the manufacture of the laminates but also the layout of the interconnect, filters, transmission lines, and heat sinking.For high speed digital the control of the impedance/constant line width was more of a factor, unlike in RF where multiple line widths and shapes are in common use. A multitude of transmission lines are used in a bewildering array of combinations. Other parameters such as the glass weave and its impact on impedance was a discussion worth having. Three laminates emerged as winners for the a large number of applications in design. The venerable FR4 was buried under the the new requirements at 77 Ghz and even at 24 Ghz.The impact of DF and DK ( buzz words of course to be treated in some detail in subsequent posts). The use of materials and their trade-offs were fascinating. The size of the material sold has also gone through revisions and large sizes are now common. Gone are the limits of 18 X 24. The other very interesting issue that surfaced was the role of, and difficulty in, testing of not only devices but also the substrates themselves. The relationships between the thickness and the width of lines changes from the simple expressions we all knew. The difficulty of modeling has increased and very few CAD tools appear to have the capability to do what is needed. Only one CAD tool was mentioned several times as a recommended one for design and modeling at the high performance levels. Some very interesting numbers for insertion loss and actual measured values of permittivity and loss tangents were presented and argued over. Very interesting empirical design equations and data was presented as well. In this discussion the effect of the roughness factor was presented and emphasized. Finally a detailed discussion on the materials of construction such as resins,fillers and reinforcements ended the presentations. In short a very interesting couple of hours. Interested parties may contact us about these subjects through our website at www.signalpro.biz>>contact.