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 • f • tan
where,
PI: absorption power per volume (signal transmission loss),
k: constant
f: frequency
tan: dissipation factor (Df)
Transmission Loss (dB/cm)
Material Classification
Copper Loss vs Surface Roughness
Copper Profile
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
Different Resin and Foil Combination
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 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
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
PCB Back-Drilling: Effects of Via Stub Length
Fabrication
- Trace etching has little impact (if wide trace ½ oz Cu)
- Inner layer prep & Oxide makes a big impact
Internal Trace Geometry
Surface Treatment and High Pressure Lamination
Oxide Treatment (RTF & Shiny Copper)
Oxide Treatments Influence on Insertion Loss
Four Oxide Treatment Influence on 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
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Manufacturing Partners Profile includes:
Argus Sytems (AESPL) – PCBA, Cable Assembly, Box Build.
Emsxchange – PCBA, Cable Assembly, Box Build.
