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Design007-Oct2018

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60 DESIGN007 MAGAZINE I OCTOBER 2018 Cu thickness Dk Conductor width Substrate thickness include substrate thickness, conductor width, copper thickness, and dielectric constant (Dk). As a material supplier, we are often asked to investigate the Dk of our material when a circuit has an issue with incorrect impedance. In reality, the Dk is the least impactful vari- able for impedance. It is much more common for substrate thickness and conductor width to play a significant role in impedance varia- tion. The substrate thickness concern can be an issue related to the cop- per-clad laminate a n d / o r t h e PCB fabricator, depending on how the circuit construction. In general, the sub- strate thickn e s s c o n t ro l is better for a microstrip circuit using the copper-clad lam- inate compared to a microstrip that a PCB fab- r i c a t o r m i g h t construct using a prepreg and copper foil for a foil lamination process. Due to the thickness control issue—assuming a multilayer circuit with a microstrip layer on the top or bottom of the struc- ture—a circuit with a demanding imped- ance specification is usually best when made with a laminate from the material supplier as Different Aspects of Impedance for PCBs Many PCBs are specified to have a controlled impedance value. There are several circuit and material properties that impact the impedance performance of a circuit. Some of these proper- ties are obvious to engineers who have worked with controlled impedance boards over the years. However, even experienced engineers are sometimes surprised to find the level of impact these properties have when look- ing at all of the things that influence PCB impedance performance. Addi- tionally, there are several issues t o c o n s i d e r w h e n m a k i n g impedance measurements to ensure the values are accu- rately captured. A s a re f e r - ence, consider a m i c r o s t r i p t r a n s m i s s i o n line circuit. When an impedance model is gener- ated for a PCB m i c ro s t r i p c o n - struction, the different variables that influence impedance can eas- ily be changed to see the magnitude each variable has for altering the imped- ance value of the circuit. There is a hierar- chy of the influences of these variables on PCB impedance. For a microstrip transmission-line circuit, the most to least influential variables Lightning Speed Laminates by John Coonrod, ROGERS CORPORATION

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