Copper Foils for High-Frequency Circuit Materials – 6-layer board uses a RO4350B + IT-180A hybrid PCB Design

Product Description

Copper Foils for High-Frequency Circuit Materials – Selection Guide for Low-Loss PCB Design

 

Q: Why does copper foil selection matter for high-frequency PCBs?

A: The surface roughness of copper foil is a critical factor affecting signal integrity in high-frequency designs, particularly above 1 GHz. Roughness forces high-frequency current—which concentrates near the conductor surface due to the skin effect—to travel a longer effective path, converting electromagnetic energy into heat and increasing insertion loss .

 

At millimeter-wave frequencies, standard electrodeposited (ED) foil can double or triple conductor loss compared to smooth foils . For your 6-layer hybrid PCB combining RO4350B (RF layers) and IT-180A (core layers), selecting the right copper foil is essential to achieve the low-loss performance required for 5G and microwave applications.

 

 

1. Copper Foil Manufacturing – ED vs. Rolled

Copper foils for PCB laminates are manufactured through two fundamentally different processes, resulting in distinct surface characteristics and electrical performance.

 

Electrodeposited (ED) Copper

In the ED process, copper is electrochemically deposited onto a rotating titanium drum from a copper sulfate solution. The drum side (smooth) faces the dielectric, while the matte side (rough) faces outward. The matte side undergoes additional treatment to enhance adhesion .

 

Crystal Structure: ED copper crystals grow lengthwise in the Z-axis of the foil, forming a "picket fence" structure with long crystal boundaries perpendicular to the foil plane .

 

Roughness: ED foils typically have higher surface roughness (Rz = 5–12 µm) compared to rolled copper, with the treated side being significantly rougher than the drum side.

 

Advantage: Lower cost and wider availability.

 

 

Rolled Copper

Rolled copper is produced by successive cold rolling operations to reduce thickness, starting from a pure copper billet. The surface smoothness depends on the rolling mill condition.

 

Crystal Structure: Rolled copper crystals are broken and crushed during cold rolling, resulting in smaller, irregular, spherical shapes aligned nearly parallel to the foil plane .

 

Roughness: Rolled copper is typically the smoothest copper foil type, with very low profile (Rz < 3 µm) .

 

Advantage: Superior for high-frequency applications due to smooth surface and excellent elongation.

 

 

2. Copper Roughness – The Key Loss Mechanism

At DC, current distributes uniformly through the conductor cross-section. As frequency increases, current crowds toward the conductor surface.

 

The skin depth δ is:

δ = 1 / √(π·f·µ·σ)

At 10 GHz in copper, δ ≈ 0.66 µm.

 

Roughness-to-Skin-Depth Ratio

The critical parameter is the ratio of RMS roughness to skin depth (Rrms/δ). As frequency increases and skin depth shrinks, the roughness becomes proportionally larger :

Ratio (Rrms/δ)	Impact on Loss
< 0.1	Negligible effect (<2% additional loss)
0.1 to 1.0	Moderate impact (10–50% additional loss)
> 1.0	Severe impact (50–100%+ additional loss)

 

For standard ED foil (Rrms ≈ 3–4 µm), the ratio exceeds 1.0 above ~5 GHz – precisely the operating range of modern 5G and millimeter-wave systems .

 

The Hall-Huray Model

The IPC and IEEE communities have adopted the Huray "snowball" model as the most physically accurate roughness correction factor. Unlike the older Hammerstad model (which saturates at a 2× correction factor), the Huray model represents the rough surface as a collection of spherical protrusions and accurately predicts roughness loss factors of 2.5–3.5× at millimeter-wave frequencies for standard foil .

 

 

3. Copper Foil Roughness Measurements

Rogers uses white light interferometry to characterize copper foil roughness, reporting the Sq (RMS) profile. Studies show that the treated side profile correlates best with conductor losses .

 

Typical Roughness Values by Foil Type

Foil Type	Weight	Dielectric Side Sq (µm)	Top Side Sq (µm)	Products
Rolled	1 oz	0.4	0.3	RT/duroid, RO3003, RO3035, 6202
Rolled	½ oz	0.4	0.3	Same as above
ED (RO4350B)	1 oz	3.2	0.4	RO4350B, RO4003C, RO4360G2
ED (RO4350B)	½ oz	2.8	0.4	RO4350B, RO4003C
LoPro®	1 oz	0.9	1.3	RO4003C, RO4350B, RO4835
LoPro®	½ oz	0.9	0.8	RO4003C, RO4350B, RO4835
ED Low Profile RT	1 oz	1	1.3	CLTE, DiClad, IsoClad
ED Low Profile RT	½ oz	1	0.8	CLTE, DiClad, IsoClad

 

Source: Rogers Corporation copper foils data sheet

Key Insight: The dielectric side roughness (the surface bonded to the laminate) is the primary contributor to conductor loss. Rolled copper (Sq = 0.4 µm) offers the smoothest surface, while standard ED foil on RO4350B (Sq = 2.8–3.2 µm) has significantly higher roughness.

 

 

4. Hybrid Stack-Up Foil Considerations

Your 6-layer board uses a RO4350B + IT-180A hybrid stack-up:

 

Blind vias (L1-L2, L3-L6) and edge plating add further complexity – requiring precise control of plating parameters and surface preparation .

 

Comparison Table – Copper Foil Options for High-Frequency PCBs

Property	Standard ED	Rolled Copper	LoPro®	HVLP
Roughness (Sq)	2.8–3.2 µm	0.3–0.4 µm	0.9 µm	0.6–1.2 µm
Roughness (Rz)	8–12 µm	< 3 µm	3–5 µm	1.5–3 µm
Insertion Loss @ 10 GHz	High	Very Low	Low	Very Low
Thermal Shock Resistance	Moderate	Excellent	Good	Good
Peel Strength	Higher (rougher = better adhesion)	Lower	Moderate	Moderate
Cost	Low	High	Mid	High
Primary Use	General RF, cost-sensitive	Aerospace, extreme thermal	High-frequency digital, mmWave	25G–112G SerDes, mmWave

 

 

 

Application Areas

• Automotive Radar (77 GHz)
• High-Power RF
• Aerospace / Defense
• Phased-Array Antennas
• High-Speed Digital (56G/112G PAM4)

 

 

Q1: Why does rolled copper have better thermal shock resistance than ED copper?

A: Rolled copper has better elastic elongation before reaching permanent deformation. ED copper may exhibit stress cracks under rapid thermal cycling in narrow conductors .

 

 

Q2: What is LoPro® foil and when should I use it?
A: LoPro® is Rogers' low-profile copper foil with a smooth surface (Sq = 0.9 µm) . It is available on RO4000 series materials and significantly reduces insertion loss at high frequencies compared to standard ED foil .

 

 

Q3: Which foil is recommended for my RO4350B + IT-180A hybrid board?
A: For the RO4350B outer layers (RF critical), we recommend LoPro® or rolled copper to minimize insertion loss. For the IT-180A core layers (power/ground), standard ED copper is sufficient.

 

 

Q4: Does foil selection affect impedance control?
A: Yes. Higher roughness increases the effective dielectric constant, which can shift impedance by several ohms. This must be accounted for in your impedance calculator .

 

 

Contact Us

As a specialized PCB supplier, we offer comprehensive copper foil selection guidance for high-frequency designs. We work with all major foil types and can help you select the optimal option for your specific application.

 

For technical inquiries, stack-up review, or quotation requests, please contact our sales team directly.