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PCBD-June2016

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68 The PCB Design Magazine • June 2016 High-frequency and high-speed digital PCBs may not have issues with soldermask. How- ever, depending on their construction, other PCBs can have an issue with soldermask caus- ing degraded electrical performance. PCBs with a stripline structure, in which the signal layer is buried within a multilayer, typically do not have an issue with electrical performance deg- radation due to soldermask. Soldermask can impact PCBs with RF circuitry on the outer lay- ers, which can lessen high-frequency electrical performance. Typically, PCBs with RF traces on the outer layers have minimal or no soldermask in the RF circuitry areas. Many times the soldermask is applied in areas where components are sol- dered to the PCB but the soldermask is devel- oped away in the areas where conductors have critical RF performance. There are many reasons to avoid soldermask coverage on RF conductors, due to inherent soldermask properties. Most soldermask used in the PCB industry is liquid photoimageable (LPI), which is typically high in dissipation factor (Df) and high in moisture absorption, and the thickness can vary due to processing or design. The typical soldermask has a dissipation fac- tor of about 0.025 when tested at 1 GHz, and moisture absorption is about 1–2% depending on the formulation. For comparison, many high-frequency laminates have a Df value of about 0.005 or better and moisture absorption is typically no worse than 0.3%. The higher Df property of soldermask raises the circuit's dielec- tric loss, which causes an increase in insertion loss. The moisture absorption can cause differ- ences in impedance and phase response, but it is typically more problematic for losses where it can cause increased insertion loss. Another point to consider is that RF circuit- ry on the outer layer of a PCB will usually be a microstrip or grounded coplanar waveguide by John Coonrod ROGERS CORPORATION The Dilemma: Soldermask for High-Frequency PCBs LIGHTNING SPEED LAMINATES

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