What is PCB insertion loss and when must it be tightly controlled?

Engineering Summary

Insertion loss is the attenuation of a signal as it travels through a PCB transmission line, expressed in dB per unit length (e.g., dB/cm or dB/in).

Must be tightly controlled when:

• ·Link budget margin is tight – e.g., 112G PAM4, 77GHz radar, satellite downlinks
• ·Channel length exceeds a few centimeters – loss accumulates with distance
• ·Operating frequency >10 GHz – dielectric and conductor loss rise sharply
• ·Multiple channels must match – mismatched loss creates amplitude and phase errors in phased arrays
• ·Production repeatability is required – uncontrolled loss variation causes field failures

Rule of thumb: If your link budget allows less than 3 dB total loss, you must actively control insertion loss through material selection, copper roughness, and process validation.

1. What Is PCB Insertion Loss? (Definition & Units)

Insertion loss (IL) is the reduction in signal power when it passes from the input to the output of a PCB transmission line (microstrip, stripline, coplanar waveguide). It includes all loss mechanisms:

• Dielectric loss – energy dissipated in the laminate material (Df, loss tangent)
• Conductor loss – energy lost in copper traces (skin effect, surface roughness)
• Radiation loss – energy that leaks from the line (especially microstrip)
• Reflection loss – energy reflected back due to impedance discontinuities

Insertion loss is typically expressed in decibels per unit length (dB/cm or dB/in) and is frequency‑dependent. A typical specification might be: ≤0.5 dB/cm at 28 GHz for a microstrip line on low‑loss material.

Engineering fact

At low frequencies (<1 GHz), insertion loss is often negligible. At mmWave (>20 GHz), it becomes the dominant factor limiting channel length and system performance.

2. How Insertion Loss Is Measured

Recommended for production: Δ‑Loss coupon (IPC‑TM‑650‑2.5.5.13) because it removes connector and launch variation, giving true PCB‑only loss data.

3. When Must Insertion Loss Be Tightly Controlled?

Not every PCB needs tight insertion loss control. The requirement depends on three factors:

Specific applications that require tight insertion loss control:

Rule of thumb: If your design uses low‑loss laminate (Rogers, Megtron, PTFE) and operates above 10 GHz, you should specify insertion loss limits and verify them with coupons.

4. The Two Main Loss Components: Dielectric vs Conductor

4.1 Dielectric Loss  
Caused by polarization of the laminate material. Quantified by dissipation factor (Df). Low Df → low dielectric loss. Dielectric loss increases linearly with frequency.

Engineering fact: A Df increase of 0.001 adds approximately 0.15 dB/cm loss at 28 GHz and 0.4 dB/cm at 77 GHz (assuming microstrip).

4.2 Conductor Loss  
Caused by resistance of copper traces, amplified by skin effect and surface roughness. Skin depth decreases with frequency: at 28 GHz ≈0.39 µm, at 77 GHz ≈0.23 µm. Rough copper (Ra >0.8 µm) increases effective path length → higher loss.

Key takeaway: At mmWave, copper roughness is as important as dielectric loss. Using HVLP copper can reduce total insertion loss by 20‑40% compared to ED copper.

5. How to Specify Insertion Loss on Your Fabrication Drawing

Do not write generic statements like “low loss required.” Be explicit:

UltroNiu practice: We treat insertion loss as a measured parameter, not an assumption. Every mmWave panel includes Δ‑Loss coupons, and we provide the measured data with shipment.

6. Insertion Loss Budget Example (28GHz 5G Front‑End)

Assume a 30 mm microstrip line from transceiver to antenna.

If the PCB trace alone exceeds 2.0 dB (e.g., 0.8 dB/cm × 3 cm = 2.4 dB), the link fails. That is why you must control loss per centimeter, not just total loss after the fact.

7. Common Mistakes That Inflate Insertion Loss

8. When Insertion Loss Control Is Less Critical

You may not need tight insertion loss control when:

• Frequency <3 GHz – loss is low even with FR‑4
• Trace length <2 cm – total loss under 0.2 dB
• Link budget has >6 dB margin – variation is tolerable
• Single‑ended, non‑critical signals (e.g., control, I2C, GPIO)
• Low‑volume prototypes that will be manually tuned

But be careful: even low‑frequency designs with long traces (e.g., power supply sense lines) can suffer if loss is ignored. Always calculate, don’t assume.

9. Frequently Asked Questions

Related Engineering Resources

References: IPC‑TM‑650‑2.5.5.13 (Δ‑Loss), IPC‑TM‑650‑2.5.5.7 (TDR), ISO 25178 (profilometry), Rogers, Panasonic material datasheets.