Ro4350B Large size stackup

 Actually this is fake 4 layer structure, L2 Layer is an empty layer. To meet the board thickness requirement, we made the adjustment to make it fake 4 layer.

And the alternative stackup:

What is fake 4 layer structure

In fact, a three-layer board is a fake four-layer board, but there is no circuit on one inner or outer layer, and the real three-layer board does not exist. There are three reasons:

1. The same process:

It can be manufactured in PCB factories. Four-layer boards generally use a core with 1 copper foil on each side. Three-layer board testing-side copper foil, in terms of process flow, all need to be pressed.

2. The price is the same

The difference in process cost between the two is that the four-layer board has an extra copper foil and adhesive layer, and the cost difference is not much. When the board factory quotes, generally 3-4 layers are quoted as a grade, and the quotation is defined by an even number (of course, more than multiple layers). For example, if you design a 5-layer board, you will quote it according to the price of a 6-layer board. In other words, the price of your 3-layer design is the same as the price of your 4-layer design.

3. Stable process

In the PCB process, four-layer boards are easier to control than three-layer boards, mainly in terms of symmetry. The warpage of four-layer boards can be controlled below 0.7% (IPC600 standard), but when the size of three-layer boards is large, the warpage will exceed this standard, which will affect the reliability of SMT patches and the entire product. Therefore, designers generally do not design odd-numbered boards. Even if odd-numbered layers are used to achieve functions, they will be designed as even-numbered layers, that is, 5-layer boards will be designed as 6-layer boards, and 7-layer boards will be designed as 8-layer boards.

What is manufacturing process of Ro4350B Large size high frequency pcb

1. Materials and inner layer processing

Pre-inspection of cutting

Check RO4350B plate parameters (Dk=3.48±0.05)

Diamond tool cutting (to prevent ceramic edge collapse)

Inner layer graphics

LDI laser imaging (accuracy ±2μm)

Acid etching (grinding is prohibited)

Brown treatment

Milling copper on the edge of the board to expose 2mm of the base material

Bake at 150℃ for 2 hours (to prevent compression bubbles)

2. Mixed pressure lamination

Layer design

RO4350B signal layer + FR-4 power layer mixed pressing

Vacuum pressing (170℃±2℃, pressure error <3%)

Remove glue and micro-etching

Back drilling resin residue (stub ≤0.2mm)

Test interlayer bonding strength (>1.0N/mm)

3. Hole and outer layer

Precision drilling

New drill bit + aluminum pad (anti-PTFE flash)

Copper deposition and ultrasonic (anti-substrate fragmentation)

Outer layer circuit

Graphic electroplating copper thickness 20-25μm

Secondary etching (knife repair of residual copper is prohibited)

4. Final inspection and packaging

Surface treatment

ENIG gold deposition (gold thickness 0.05μm)

Triple anti-coating (salt spray >500h)

High frequency test

Impedance tolerance ±3% (network analyzer)

Flying probe test + 3D microscope hole inspection

Anti-deformation packaging

Foam isolation packing

‌Key Applications of RO4350B Large-Size HF PCBs‌

‌5G Communication Systems‌

Used in base station antenna arrays and filter components, demonstrating stable performance in smart city traffic systems

Enables FR4 hybrid lamination to reduce 5G small cell size and cost

‌Automotive Electronics‌

Applied in 24GHz collision prevention systems with stable dielectric constant (Dk=3.47) at high frequencies

‌Aerospace‌

Maintains signal reliability in extreme temperatures (-50~150°C) with only 0.4% Dk variation5

‌Technical Specifications‌

Electrical Performance

Dk: 3.48±0.05@10GHz (decreases to 3.47@24GHz)

Loss tangent: 0.0037@10GHz, microstrip insertion loss <0.1dB/inch@4GHz15

Physical Properties

Z-axis CTE: 32ppm/°C with excellent dimensional stability

Supports 4-60mil multilayer designs and FR4 hybrid processes

Manufacturing Advantages

Compatible with standard FR4 processing without plasma treatment, allowing direct solder