Introduction to the selection of materials for PCB design copper-clad laminates

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...

Moisture-induced common circuit board failure problems

Humidity-induced common circuit board failures, resulting in circuit boards in the circuit parameters change caused by circuit board failure, circuit boards in the circuit board in a short-circuit state, resulting in circuit board failure, signal processing or transmission line breakage, resulting in circuit board failure.

Humidity caused by common circuit board failure problems

The definition of humidity that contains more water than the normal state, so the use of circuit boards in humid environments, due to the air contains a relatively large amount of moisture, when the moisture is too large it will be turned into water droplets fall to the circuit board, fall to the circuit board on the water droplets on the circuit board after the board to spread out, it will be dependent on the electronic components of the pins or printed lines.

Due to the current application of electronic components in the circuit board are SOP or SSOP chip components, pins and pins between the distance is very small, especially the SSOP package of integrated circuits and its pins and pins between the distance is very small, when the moisture is converted into water droplets in the SSOP package integrated circuit pins, if the board is in operation at this time, it will be for the integrated circuit between the pins to increase an Invisible resistance (because the water is conductive), or even trigger a short circuit, resulting in a failure of the circuit board in the working state.

If the moisture converted into water droplets on the circuit board between the pins of the electronic components, and at this time the circuit board just did not work or power outage, will not immediately cause harm to the circuit board, but the pins of the electronic components or printed line by the water droplets after the infiltration of the pins of the components will be rusted, and for a long time will be due to the corrosion of the broken foot triggered by the failure of the circuit board, the printed line was infiltrated by the water droplets, especially after the signal After the printed line is infiltrated by water droplets, especially the signal transmission line is relatively small, after being infiltrated for a period of time, there will be moldy breaks in the printed line, resulting in the circuit board in the next put into operation, there can not run the situation.

For the reasons that humidity causes circuit board failure, the outline of three:

1. Resulting in circuit parameters in the circuit board to change the circuit board failure;
2. Triggered by the circuit board in the short circuit state, resulting in circuit board failure;
3. Resulting in circuit board signal processing or transmission lines appear broken, resulting in circuit board failure. Friends can be based on these three major measures to reduce or avoid your jurisdiction over the equipment in the circuit board due to moisture caused by circuit board failure.

Single-sided PCB board prototyping design process

Single-sided PCB board proofing design process:

1. The preparation part

In the beginning of the PCB layout, the first should complete the schematic design, get a correct schematic, which is the basis of single-sided PCB board proofing design. Through the schematic we can get a network table of each device connection properties, in addition, according to the parameters of the device, we can find the relevant component information and establish the package of all components. In addition, we also need the structural department to cooperate with the board frame dimensions and the location of each mounting position, the location of the functional excuses.

2. In the completion of the preparatory work, into the specific operation of the part, but first of all you need to import all the package files and network tables into the establishment of a good frame of the PCB file. The import process may prompt some component package errors, please follow the error tips to eliminate errors.

3. In the completion of the import file operation, the first step to do is to fix the structure of the relevant devices, such as LEDs, buttons, card holders, liquid crystals, infrared emitters and so on. Move these devices to the corresponding mounting position and select Lock in the properties to prevent misuse.

4. In the establishment of the priority devices, we can carry out the general layout, the purpose of the general layout is to determine the location of the various functional modules in the PCB design, the general default:

• In addition to the need to install the device on the surface, all the patch device placed to the side of the plug-in device, generally the bottom;
• The metering unit is placed in the lower left corner to facilitate wire entry;
• MCU placed on the back of the LCD, and to do the lead is short enough;
• The interface part of the PCB placed in the lower right corner, easy to get out of the line;
• The transformer is far away from the transformer and manganese-copper shunt and other devices that are more sensitive to magnetic leakage;
• The need to isolate the circuit to retain sufficient creepage distance;

5. In the completion of the general layout, you can carry out local layout. Completion of each functional module corresponding to the placement of the device. Factors to be considered in the local layout:

• Crystal should be as close as possible to the crystal pin, to achieve the shortest possible alignment;
• Decoupling capacitors should be as close as possible to the power input pin of the IC;
• IC high-speed connection between the device should be as close as possible;
• To consider the convenience of maintenance, the placement of some devices to optimize the location, so as not to cause production difficulties;
• Stay a certain distance between the board edge, the requirements of the edge of the best to do more than 4mm, otherwise in the SMT shop is easy to cause patch patch pickup head accidental damage in the wave soldering caused by the device and the chain collision, can not be a one-time wave soldering to complete the plug-in soldering, the need to arrange for more stations to make up for the soldering;
• Varistors, polyester capacitors, transient suppression diodes and voltage regulators, filter capacitors should be placed in the front of the need to protect the device;
• Pay attention to the distance between the high-voltage and low-voltage signals.

6. In the completion of the local layout, you can connect the components. Connections need to pay attention to the following aspects:

• Understand the current size of each device may flow and the maximum size of the inrush current, can roughly understand the signals carried on the alignment of other signals may have an impact. In order to set the thickness of the alignment
• High-voltage signals to the piezoresistors and polyester capacitors at both ends of the line should try to use a wider line width, so that the protection device can release the overload energy in a timely manner, while also...