Minimizing Insertion Loss

Insertion Loss

What Impacts Insertion Loss?

 

PCB Materials

  • Laminate / Resin
  • Glass Style / Prepreg
  • Copper Profile (Surface Roughness)
  • Soldermask (Microstrip Diff Pairs)
  • Surface Finish

Design Attributes

  • Trace Width, Length, Height
  • Microstrip vs Stripline
  • Via Configuration (Via Delay)

Fabrication

  • Trace Etching
  • Inner Layer Prep & Oxide

Signal Loss and the Df of the Material

The relationship between Signal Transmission Loss and the Dissipation Factor of
the Material:

PI = k   tan

where,
PI: absorption power per volume (signal transmission loss),
k: constant
f: frequency
tan: dissipation factor (Df)
Transmission Loss (dB/cm)

Material Classification 

Material Classification

Copper Loss vs Surface Roughness

Copper Loss vs Surface Roughness

Copper Profile

Properties of copper foil
cross section of copper foil

Electrodeposited Copper Foil Structure

 

Electrodeposited copper foil has two surfaces. The drum side is basically an image of
the drum surface and is quite smooth, while the matt side exhibits dendritic features.

  • Copper foil requires a surface treatment prior to bonding prepreg resins
  • Surface treatment can be applied to either side or both
  • Some resins can require different treatments
Electrodeposited Copper Foil Structure
trace length and transmission loss

Different Resin and Foil Combination

Different Resin and Foil Combination

Glass Yarn Properties

Glass Yarn Properties

E-Glass is the predominant glass that is used in the printed circuit industry. S-Glass
is a high strength glass and the NE-Glass is a low loss, low dielectric constant glass
for high performance laminates

Spread Glass

Spread Glass
Spread Glass weave effect

Spread Glass and Non-Spread Glass

Spread Glass and Non-Spread Glass

Design Attributes

 

  • Trace Width makes a large impact (wider is better)
  • Trace Length makes an impact (with Frequency)
  • Trace thickness makes an impact (not as much as width)
  • Wider is preferred over thicker copper trace

Copper Loss vs Trace Width

Copper Loss vs Trace Width

Challenges for Microstrip Insertion Loss

Difficult to Minimize Insertion Loss on External Layers

  • Solder Mask (Df 0.019 – 0.029) & thickness
  • Copper profile (low profile copper foil preferred)
  • Tightly-Coupled Differential pair (1:1 or 1.5:1)
  • Embedded Microstrip or Stripline

            Use HVLP Cu foil 

            Eliminate Solder Mask & Surface Finish

Via Configurations Impact Insertion Loss

Via Configurations Impact Insertion Loss

PCB Back-Drilling: Effects of Via Stub Length

PCB Back-Drilling

Fabrication

  • Trace etching has little impact (if wide trace ½ oz Cu)
  • Inner layer prep & Oxide makes a big impact

Internal Trace Geometry

Internal Trace Geometry

Surface Treatment and High Pressure Lamination

Surface Treatment and High Pressure Lamination

Oxide Treatment (RTF & Shiny Copper)

Oxide Treatment

Oxide Treatments Influence on Insertion Loss

Four Oxide Treatment Influence on Insertion Loss

Insertion Loss

Minimizing PCB Insertion Loss – Summary

 

  • Trace width makes a large impact

• Maintain Trace width of 0.006” (0.004” min)
Higher the speed, Wider the trace
• 25 gigabits = 0.008”

  • Keep trace length short as possible

• Less than 20”
Higher the speed & frequency, shorter the trace length
• 25 gigabits = <12”

 

  • Maintain trace or copper thickness of 1 oz to minimize loss (1/2 oz min)

• Thickness is not as large of an impact as width
(prefer width over thickness)

  • Maintain traces on internal layers (Embedded Microstrip or Stripline)

• Solder Mask (Df 0.019 – 0.029) & SM thickness

  • Shape of trace after etch (top versus base) Skin Effect
  • Choose PCB material with a low Df

• 0.010 = mid loss up to 8 gigabit (depending upon the design)
• 0.004 = low loss up to 25 gigabit (depending upon the design)

  • Utilize a balanced glass construction

• Balanced Glass (lower speeds)
• Spread Glass or Flat Glass for higher speeds

  • Utilize low profile copper (#1 contributor to Insertion Loss)

• Very low profile copper
• Ultra very low profile
• RTF

  • Back-drill – minimize stub
  • The manufacturing process can contribute to loss

• Stable and low etch oxide treatment process

Cerra Systems is one of the leading PCB manufacturing solutions providers with PCB manufacturing plants located in North America, Asia with certifications for MIL, AS, TS, ISO, & NADCAP. Our products are Rigid, Flex, Rigid-flex, HDI, RF, and hybrid PCBs addressing up to 64 layers and 2 mil technology. Cerra Systems  Major  PCB Market is Aerospace, defense, Medical, computing, cellular & Industrial.

We supply a wide range of Printed Circuit Board in India and backplane products from very small, double-sided rigid PCB to high layer count, high-density multi-layer boards to large tabletop size backplanes. We have dedicated and quick-turnaround operations capable of delivering PCB prototypes with a lead time as short as 24 hours. We also offer quick-turn prototyping with a lead time as short as 5 days, depending on the technology and layer counts.

For more details, visit us- http://cerrasystems.com/