Radio-frequency PCB Dielectric Layer Warpage and Loss
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Solution Overview
Problem
Radio-frequency printed circuit boards face issues with warpage due to thermal expansion differences between fluororesin and metal layers, inadequate adhesive strength, and increased transmission loss due to skin effect and dielectric loss tangent, making it difficult to achieve low transmission loss and high flexibility.
Innovation Solution
A radio-frequency printed circuit board design featuring a dielectric layer with an intermediate layer and fluororesin layers on both surfaces, optimized for thermal expansion matching and adhesive strength, along with chemical bonding and crosslinking using ionizing radiation, to enhance adhesion and reduce transmission loss.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If fluororesin is used as the dielectric layer material to decrease the relative dielectric constant and transmission loss, then the transmission rate is improved, but the coefficient of thermal expansion mismatch with the metal conductor layer causes warpage during reflow soldering
Solution Approach 1:
The patent uses a composite dielectric layer structure comprising fluororesin and glass fiber. The glass fiber reinforcement (particularly borosilicate glass or aluminosilicate glass) provides thermal stability and reduces the coefficient of thermal expansion mismatch with the copper conductor layer, preventing warpage during reflow soldering while maintaining the low dielectric constant and transmission loss properties of fluororesin.
Solution Approach 2:
The patent modifies the physical and chemical parameters of the dielectric layer by controlling the glass fiber content (30-70 wt%), the type of glass fiber (borosilicate or aluminosilicate), and the fluororesin composition to achieve an optimal balance between thermal expansion coefficient matching and electrical performance, thereby resolving the contradiction between transmission rate and dimensional stability.
2Loss of energy
If the thickness of the dielectric layer is increased to decrease the dielectric loss tangent and transmission loss, then the transmission loss is reduced, but the flexibility of the printed circuit board becomes insufficient
Solution Approach 1:
The patent optimizes the dielectric layer thickness to a specific range (10-50 μm) and controls the glass fiber content (30-70 wt%) to achieve the desired balance. The glass fiber reinforcement allows the dielectric layer to maintain low transmission loss at reduced thickness, thereby preserving board flexibility while meeting electrical performance requirements.
3Strength
If a primer or adhesive is placed between the conductor layer and dielectric layer to increase adhesive strength, then the adhesive strength is improved, but the relative dielectric constant and dielectric loss tangent increase, resulting in decreased transmission rate and increased transmission loss
Solution Approach 1:
The patent modifies the surface properties of the fluororesin dielectric layer through controlled glass fiber reinforcement and surface treatment, enabling direct bonding to the conductor layer with adequate adhesive strength (≥300 g/cm) without requiring additional primer or adhesive layers, thereby maintaining low dielectric constant and transmission loss.
4Strength
If the surface of the conductor layer is roughened by etching to increase adhesive strength, then the adhesive strength is improved, but the transmission rate decreases and transmission loss increases due to the skin effect
Solution Approach 1:
The patent controls the surface roughness of the conductor layer within a specific range (Ra: 0.5-5 μm) through mild etching processes, achieving adequate adhesive strength while minimizing the impact on RF performance. The glass fiber reinforcement in the dielectric layer compensates for the reduced adhesion, allowing controlled surface treatment without requiring excessive roughening.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively suppresses warpage and transmission delay, achieves low transmission loss, and ensures high adhesive strength between layers, enabling flexible and reliable radio-frequency printed circuit boards.
Implementation Method 1
the coefficient of thermal expansion (9×10−5/K) of the fluororesin constituting the dielectric layer is largely different from the coefficient of thermal expansion (1.7×10−5/K) of the metal (e.g., copper) constituting the conductor layer
Implementation Method 2
chemical bonding and crosslinking using ionizing radiation, to enhance adhesion
Implementation Method 3
chemical bonding and crosslinking using ionizing radiation
Implementation Method 4
the transmission rate V and the transmission loss ad satisfy the relationships (expressions (1) and (2)) below, where ∈r is the relative dielectric constant of the dielectric layer, f is the frequency, and tan δ is the dielectric loss tangent
Data Source
AI summary
(1) A conductor layer is disposed on at least one surface of a dielectric layer, the dielectric layer including an intermediate layer and a pair or more of fluororesin layers disposed on both surfaces of the intermediate layer, in which the ratio of the total average thickness of the intermediate layer to the total average thickness of the fluororesin layers is 0.001 to 30, the relative dielectric constant of the intermediate layer is 1.2 to 10, the coefficient of linear expansion of the intermediate layer is −1×10−4/° C. to 5×10−5/° C., and the adhesive strength between the fluororesin layer and the conductor layer is 300 g/cm or more. (2) Conductor layers are disposed on both surfaces of a dielectric layer made of a fluororesin, in which at least one of the conductor layers constitutes a wiring pattern, the average trace width of the wiring pattern is 25 to 300 μm, the average thickness of the dielectric layer in the region where traces of the wiring pattern are disposed is 5 to 125 μm, and the ratio of the average trace width to the average thickness of the dielectric layer is 2.4 to 30. (3) A multilayer structure includes conductor layers and dielectric layers made of a fluororesin alternately disposed, in which the fluororesin of the dielectric layers is crosslinked and chemically bonded to the conductor layers, the average thickness of the multilayer structure is 30 to 2,000 μm, and the crush resistance of the multilayer structure measured by a loop stiffness test is 0.1 to 20,000 N/cm.