Design007 Magazine


Issue link:

Contents of this Issue


Page 53 of 67

54 The PCB Design Magazine • August 2015 The PCB laminates used for high-frequency applications possess several unique attributes that are distinctly different than FR-4 types of materials. Obviously, the electrical properties are better for high-frequency laminates than FR-4, however there are many other beneficial characteristics which may be less obvious for those unfamiliar with these specialty laminates. These other properties are the reason why sometimes, high-frequency laminates are used for applications that are not high-frequency. Moisture absorption can be problematic for high-frequency circuits (and other applica- tions) for several reasons. When a circuit ab- sorbs moisture from its environment, some wa- ter content becomes part of the material matrix on a molecular level. Water has a high dielec- tric constant, with a value of approximately 70. Most components that make up a laminate have dielectric constant values in the range of about 2–10, so if an appreciable amount of the material matrix has a component of 70, the overall laminate dielectric constant will increase significantly. A change in dielectric constant will cause a change in impedance, propagation velocity, phase response and oth - er circuit issues. Additionally, the change due to moisture is typically a variable based on the environmental conditions in which the circuit is operating. The amount of moisture absorption that can impact circuit performance is unique to the cir- cuit design and application. Some applications are nearly unaffected by moisture absorption is- sues and others will have electrical performance that is greatly affected. The amount of moisture absorption which is considered low for high- frequency applications is somewhat subjective, but as a general rule a value of 0.3% or less is considered good. Moisture absorption also in- creases insertion loss, and as another general rule, the applications that are higher frequency or higher speed are more susceptible to mois- ture effects. column by John Coonrod RoGeRs CoRPoRATion LIGHTNING SPEED LAMINATES A Brief Overview of High-Frequency Laminates

Articles in this issue

Archives of this issue

view archives of Design007 Magazine - PCBD-Aug2015