First, the main parameters of the PCB board

1.1 Dielectric constant (Dk) Dieletric Constant

Characterize the dielectric properties of dielectric materials or polarization properties of the physical parameters, its value is equal to the material to be measured as a medium and vacuum as a medium made of the same size capacitor capacitance ratio, the value of the material is also a characterization of the ability to store electricity.

The dielectric constant indicates that the signal line in the transmission energy will have a lot to be stored in the plate, will cause the "signal integrity" of the quality is not good, and the propagation rate of the slowdown, so the material of the dielectric constant of the lower the quality of its signal transmission is better.

1.2 Dissipation Factor (Df) Disspation Factor

That is, for the signal line has been lost to the insulating plate in the energy and still exists in the risk of energy ratio, also known as loss factor, dielectric loss or loss tangent, etc.

1.3 Glass Transition Temperature (Tg)

Polymer will rise and fall with the temperature and the physical changes that occur, when at room temperature usually presents a non-crystalline amorphous form of brittle hard glassy solid, when at high temperatures but will be transformed into a kind of rubbery elastic solids, such as room temperature "glassy state" into "rubbery state This temperature transition from the "glassy state" to the "rubbery state" at room temperature is called the "glass transition temperature".

The coefficient of thermal expansion in the rubbery state will be 3 to 4 times higher than the coefficient of thermal expansion in the glassy state.

1.4 Coefficient of Thermal Expansion (CTE)

Copper-clad plate in the X, Y direction due to the glass cloth by the hold, its CTE value is not large, generally in the 20ppm / ℃ below; and in the Z direction without the hold, its CTE will increase to 55-60ppm / ℃.

1.5 Thermal cracking temperature Td

Also called decomposition temperature or laminate separation temperature, which is defined as the temperature of the material weight loss of 5%, to thermogravimetric analysis of the value of heating, until the material weight loss of 5% of the temperature that is the thermal cracking temperature.

1.6 Heat-resistant cracking time (T260, T288, T300)

It refers to the time when the material is heated to a fixed temperature of 260℃, 288℃ and 300℃ respectively by TMA method, and it is observed that the Z-direction can resist thermal expansion without cracking in this thermal environment.

1.7 Resistance to ion migration CAF performance

PCB and other electrodes due to moisture absorption and condensation and other effects of adsorption of water after adding an electric field, metal ions from a metal electrode to another electrode to move, precipitation of metals and compounds of the phenomenon known as ion migration.

Ion migration will lead to a short circuit between the wires, seriously affecting product performance and reliability.

1.8 Peel strength

Also called tear strength, the adhesion of copper foil to the substrate board, often expressed in terms of the force required to vertically tear up the copper foil per mm width.

1.9 WaterAbsorption

It refers to the degree of moisture absorption of the material, the larger the worse. Water absorption not only reduces the insulating property of the board, but also slows down the signal propagation.

Second, different use of temperature requirements for the plate

2.1 PCB materials commonly used resin type:

A. FR-4 material, epoxy resin type:

B. PI material: polyimide material

C. PTFE material: polytetrafluoroethylene material

D. BT resin material: bismaleimide triazine resin

The resin material itself is used in the temperature range:

Resin type

Epoxy resin

PI resin

PTFE resin

BT resin

Working Temperature

Below 130 degrees

Below 260 degrees

Below 250 degrees

Below 230 degrees Celsius

FR-4 material: epoxy resin, is a thermosetting polymerization material, the maximum operating temperature of 204 ℃; data quoted from the "Electrical and Electronic Insulation Technology Handbook" P. 451; the actual use of temperature by the product's heat-resistant grade: such as epoxy-laminated glass cloth plate (Model: 9320, on the 3242, etc.) heat-resistant grade F (i.e., 155 ℃), copper-coated epoxy resin The heat-resistant grade of glass cloth board (Model: CEPGC-31CEPGC-32F) is B (i.e. 130℃), the data is quoted from the "Newest Commonly Used Electrical Products Catalog", P.2312～2314.

PI material: polyimide, a new type of high temperature resistant thermosetting engineering plastics, due to its ability to maintain high physical and mechanical properties over a wide range of temperatures from -270-400°C, while being able to be used in the air for a long period of time at -240-260°C and It has excellent electrical insulation, abrasion resistance, high temperature radiation resistance and physical and mechanical properties.

PTFE material: polytetrafluoroethylene, abbreviated as teflon, the use of temperature -190 ~ 250 ℃, allowing sudden cold and heat, or hot and cold alternating operation.

BT material: bismaleimide triazine resin, cured and molded with the following characteristics:

l Excellent heat resistance tg: 200～300℃, long-term heat resistance temperature: 160～230℃.

l Low dielectric constant ε = 2.18 ~ 3.15 (1MHz), low dielectric loss tanδ = 1.15 × 10-3 ~ 3.10 × 10-3 (1MHz).

l high resistance to metal ion migration, moisture absorption still maintains excellent insulation.

l Excellent machining characteristics, drug resistance, radioactivity, wear resistance and dimensional stability.

2.2 Temperature changes can lead to PCB performance failure, which is mainly reflected in the following aspects:

A. High temperature leads to delamination

B. High temperatures lead to breakage of copper holes and open circuits

Whether high temperature is easy to lead to board delamination mainly depends on the material of the following parameters indicators:

l Td: the higher the Td, the less likely to be delaminated at high temperatures, the better its heat-resistant properties

l T260 / T288 / T300: the longer the time indicates that its heat resistance is better, the less likely to delamination

Whether the high temperature is easy to lead to the hole copper fracture open circuit mainly depends on the material of the following parameters indicators:

l Z-CTE: the smaller the high temperature, the more resistant to breakage

Tg value: the higher it is, the less likely to break at high temperature.

2.3 CTE of thermal expansion

2.3.1 а1-CTE: coefficient of thermal expansion before Tg.

2.3.2 а2-CTE: coefficient of thermal expansion after Tg.

2.3.3 (50-260 °C)-CTE: average coefficient of thermal expansion between 50 and 260 degrees Celsius

PCB in the X. Y. direction by the clamping of glass fiber cloth, so that the CTE is not large, about 12-15ppm / ℃ or so. But the board thickness in the Z direction without constraints (before Tg) will expand to 55-60ppm / ℃ (after TG, about 5-6 times before TG), and PCB through-holes and solder pads in the copper CTE is about 16-18ppm / ℃.

2.4 Design material selection conditions

Material itself, the use of resin temperature needs to be higher than the PCB board temperature

From the material's CTE characteristics, the material's Tg value must be greater than the PCB use temperature, to ensure that the thermal expansion of the Z-CTE in the Tg before, and need to select the Z-CTE is small enough materials

From the material itself, select the Td is large enough, the larger the better high temperature performance

From the material itself, choose T260 / T288 / T300 large enough material, the larger the better high temperature performance

Third, the requirements of PCB materials under different use frequency

3.1 High-frequency signal transmission requirements for dielectric materials

Signal propagation loss is small (with a low dielectric constant, low dielectric loss factor), high signal transmission speed, in the dielectric properties by the frequency, temperature, humidity changes under the performance of high stability and so on.

It is necessary to take into account the characteristics of the signal propagation loss in high-frequency circuits PCB. 1GHZ and above in the field of conductor loss due to the "skin effect" problem.

In the substrate material, in the PCB manufacturing, in the assembly due to the existence of small deviations (especially in the interlayer thickness, dielectric constant, conductor thickness, conductor width of the four aspects of the deviation), will result in the substrate material characteristic impedance of the non-integration of the reflection, attenuation increases!

3.2 Signal operating frequency of different requirements for different boards

PCB working below 1GHz can use FR4, low cost, multi-layer compression plate process is mature. Such as signal inlet and outlet impedance is low (50 ohms), in the wiring need to strictly consider the characteristic impedance of the transmission line and the coupling between the lines, the disadvantage is that different manufacturers as well as different batches of FR4 board doping is different, the dielectric constant is different (4.2-5.4) and is not stable.

1G above 3GHz below the small-signal microwave transceivers, you can choose modified epoxy resin materials, due to its dielectric constant at 10GHz is more stable, lower cost, multilayer compression plate process with the same FR4.

3GHz above the large-signal microwave circuits such as power amplifiers and low-noise amplifiers is recommended to use a similar RO4350 double-sided sheet, RO4350 dielectric constant is quite stable, low loss factor, good thermal characteristics, processing technology and FR4 equivalent. The cost of the sheet is higher than FR4.

Microwave circuits above 10GHz, such as power amplifiers, low noise amplifiers, up and down converters and other higher requirements for the plate, it is recommended to use PTFE.