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When choosing a PCB material, it is important to make the right choice for your design because the material affects the overall performance. Understanding how thermal and electrical characteristics affect your design before entering the manufacturing stage can save you time and money while achieving the best results.
PCB material selection: considerations for stacking
PCB material selection: electrical and manufacturing considerations
PCB stacking
The PCB stack structure is to build a multi-layer PCB in a continuous sequence. The laminate is composed of magnetic core, prepreg and copper foil. Generally, the stacking is symmetrical. The board thickness of most products is less than 62 mils.
What material is used for the circuit board?
PCB material selection: electrical and manufacturing considerations
PCB materials: foil, core and prepreg
Use the following 3 items to manufacture printed circuit boards:
Prepreg: B-stage material, which is sticky and allows the bonding of different laminates or foils
Copper foil: used as a conductor in the PCB.
Copper Clad Laminate (Core): Laminated and cured by prepreg and copper foil.
Basic characteristics of dielectric materials
We know that PCB laminates are made of dielectric materials. When choosing a laminate, we need to consider the various characteristics of the dielectric material used. they are:
Thermal performance Electrical characteristics
Glass transition temperature (Tg) Dielectric constant (Dk)
Decomposition temperature (Td) loss tangent or loss factor (Tan δ or Df)
Thermal conductivity (k)
Coefficient of Thermal Expansion (CTE)
Thermal performance:
Glass transition temperature (T g): The glass transition temperature or T g is the temperature range at which the substrate changes from a glassy, rigid state to a softened, deformable state as the polymer chain becomes more mobile. When the material cools down, its characteristics will return to its original state. T g is expressed in degrees Celsius (°C).
Decomposition temperature (T d ): The decomposition temperature or T d is the temperature at which the PCB material undergoes chemical decomposition (the material loses at least 5% of its mass). Like T g, T d is also expressed in degrees Celsius (°C).
Thermal conductivity (K): Thermal conductivity, or k, is the property of a material to conduct heat; low thermal conductivity means low heat transfer, and high conductivity means high heat transfer. The heat transfer rate is measured in watts per meter per degree Celsius (W/M °C).
Coefficient of Thermal Expansion (CTE): The coefficient of thermal expansion or CTE is the expansion rate of the PCB material when heated. CTE is expressed in parts per million (ppm) per heating degree Celsius. When the temperature of the material rises above T g, the CTE also rises. The CTE of the substrate is usually much higher than that of copper, which can cause interconnection problems when the PCB is heated.
Electrical characteristics:
Dielectric constant (E r or D k): Considering the dielectric constant of the material is very important for the consideration of signal integrity and impedance, which is a key factor in high-frequency electrical performance. The Er of most PCB materials is in the range of 2.5 to 4.5.
The values in the data sheet are only valid for the specific (usually 50%) resin content percentage in the material. The actual percentage of resin in the core material or prepreg varies with the composition, so D k will vary. The copper percentage and the thickness of the extruded prepreg will ultimately determine the media height. The dielectric constant generally decreases with increasing frequency.
Loss tangent (tanδ) or loss factor (D f ): Loss tangent or loss factor is the tangent of the phase angle between the resistive current and the reactive current in the dielectric. The dielectric loss increases as the value of D f increases. A low value of D f results in a "fast" substrate, while a large value results in a "slow" substrate. D f increases slightly with frequency; for high-frequency materials with a very low D f value, its change with frequency is very small. The value range is from 0.001 to 0.030.
PCB material selection: basic categories
The basic PCB material categories are:
Normal speed and loss
Moderate speed and loss
High speed and low loss
Very high speed and very low loss (RF/Microwave)
Normal speed and loss: Normal speed material is the most common PCB material-FR-4 series. Their dielectric constant (D k) and frequency response are not very flat, and they have a higher dielectric loss. Therefore, their applicability is limited to a few GHz digital/analog applications. An example of this material is Isola 370HR.
Medium speed and loss: Medium speed materials have a flatter D k vs. frequency response curve, and dielectric loss is about half of normal speed materials. These are suitable for up to ~10GHz. An example of this material is Nelco N7000-2 HT.
High speed and low loss: These PCB manufacturing materials also have a flatter D k and frequency response curve and low dielectric loss. Compared with other materials, they also produce less harmful electrical noise. An example of this material is Isola I-Speed.
