The role of the substrate for printed circuit boards is as important as the role of printed circuit boards for electronic devices. According to the property of PCB substrate, it can be divided into organic substrate and inorganic substrate.
The organic substrate is composed of multiple paper layers impregnated with phenolic resin or non-woven or glass cloth layers impregnated with epoxy resin, polyimide, cyanate ester, BT resin, etc.
Inorganic substrates mainly include ceramic and metal materials, such as aluminum, soft iron, and copper. The purpose of these substrates usually depends on heat dissipation needs.
The rigid printed board substrates we commonly use are organic substrates, such as FR-4 epoxy glass fiber cloth substrate, which uses epoxy resin as the adhesive and electronic grade glass fiber cloth as the reinforcing material.
We see that FR-4 uses epoxy resin as a binder. The resin material has an important characteristic parameter: the glass transition temperature Tg, which refers to the temperature transition point at which a material changes from a relatively rigid or "glass" state to a flexible or soft state.
The temperature at which a glassy substance reversibly transforms between the glassy state and the high elastic state. That is to say, if the temperature of the adhesive epoxy resin of the FR-4 substrate is lower than Tg, the material is in a rigid "glass state" at this time. When the temperature is higher than Tg, the material will be soft and flexible like rubber.
Glassy
The state of the resin material at a temperature below Tg is a hard solid, that is, a glass state. Under the action of external force, there is a certain deformation but the deformation is reversible, that is, after the external force disappears, the deformation also disappears, which is the state of most resins.
High elasticity
When the heating temperature of the resin exceeds Tg, the molecular chains in the amorphous state begin to move, and the resin enters a highly elastic state. The resin in this state is similar to the elastomer in the rubber state, but it still has reversible deformation properties.
It is worth noting that after the temperature exceeds the Tg value, the material becomes soft gradually, and as long as the resin does not decompose, when the temperature is cooled below the Tg value, it can still return to the same rigid state as before.
Nitrogen has a Td value called thermal decomposition temperature. When resin materials are heated to a certain high temperature, the resin system begins to decompose. When the chemical bonds in the resin begin to break and volatile components overflow, the resin in the PCB substrate becomes less. The Td point refers to the temperature point at which this process begins. Td is usually defined as the decomposition temperature when 5% of the original mass is lost. But this 5% is very high for multilayer PCBs.
We know that the factors that affect the characteristic impedance of the transmission line on the PCB include the line width, the distance between the trace and the reference plane, the dielectric constant of the sheet, and so on. The amount of resin in the substrate material has a great influence on the dielectric properties, and the volatilization of the resin also has an influence on controlling the distance between the trace and the reference plane.
This Td value needs to be considered for the lead-free soldering process. For example, the temperature range of the traditional tin-lead soldering process is 210~245℃, and the temperature range of the lead-free soldering process is 240~270℃.
Conventional FR4 KB-6160 Tg value is 135℃, (5% weight loss ) Td value is 305℃.
FR4 lead-free KB-6168LE Tg value is 185℃, (5% weight loss) Td value is 359℃.
Studies have shown that the Td value of conventional FR4 plates is above 300℃, while the Td value of leaded soldering fully meets the temperature of 240~270℃, so why is there a lead-free version?
The 5% resin mass volatilization rate is too large for multi-layer PCBs that require impedance control. For tin-lead soldering processes, materials at a temperature of 210 to 245°C will basically not undergo significant thermal decomposition. But ordinary Tg FR-4 substrates has begun to lose 1.5~3% of the resin quality if the temperature reached the range of 240~270℃ for lead-free soldering. This level of decomposition may also affect the long-term reliability of the substrate material or cause delamination or void defects during the welding process, especially in the case of multiple welding processes or rework heating. Therefore, if a lead-free soldering process is used, in addition to the Tg value, the Td value should also be considered.
The performance of the substrate material varies greatly between the Tg value above and below the Tg value. However, the Tg value is generally described as a very accurate temperature value. It does not mean that the substrate becomes very soft when the temperature exceeds this value.
The Tg value of the resin system has two main effects on the performance of the material:
As the board expands due to heat, you can imagine that the spacing of the BGA pads changes during SMT soldering. Moreover, the mechanical stress caused by thermal expansion will cause fine cracks on the PCB traces and pad connections. These cracks may not be found during the final open/short circuit test after the PCB production is completed. While the faults may appear after SMT and other secondary heating, and the worst case was that no obvious faults appeared when SMT was heated. After the product was sent out, in the alternate use environment of cold and heat, the thermal expansion of the board made these small cracks occur randomly, causing board failure.
In addition to the standard Tg and Td values, the thermal performance parameters of the substrate material also have the thermal expansion coefficient CTE. There are CTE in the X/Y/Z axis direction.
The CTE of the Z axis has an important influence on the reliability of the PCB. Since the plated holes penetrate the Z-axis of the PCB, thermal expansion and contraction in the substrate will cause distortion and plastic deformation of the plated holes, and also deform the copper pads on the PCB surface.
The CTE of X/Y axis becomes very important in SMT, especially when chip scale packaging (CSP) and chip direct mounting are used, the importance of CTE is more prominent. At the same time, the CTE of the X/Y axis will also affect the inner adhesion and anti- delamination ability of the copper clad laminate or PCB. Especially for PCBs using lead-free soldering process, the X/Y axis CTE value in each layer is particularly important.
So, does it mean that the higher the Tg value of the substrate, the better the board? In many discussions about the Tg value, it is often believed that a higher Tg value is always beneficial to the substrate, but this is not always the case. It can be determined that for a given resin system, the high-rate expansion of the substrate with high Tg value will start relatively late when heated, while the overall expansion has a lot to do with the type of material. A substrate with a low Tg value may show a smaller overall expansion than a substrate with a high Tg value, which is mainly related to the CTE value of the resin itself, or the addition of inorganic fillers in the resin formulation to reduce the CTE of the substrate.
It should also be noted that for some low-end FR-4 materials, the substrate with a standard Tg value of 140°C has a higher thermal decomposition temperature Td value than the substrate with a standard Tg value of 170°C.
As we mentioned, Td is an important indicator for lead-free soldering. It is generally recommended to choose a larger Td value, and high-end FR-4 often has both high Tg and high Td values. In addition, substrates with high Tg values tend to be more rigid and brittle than substrates with low Tg values.
In general, as the Tg of the substrate increases, the characteristics of the circuit board such as heat resistance, moisture resistance, chemical resistance, and stability will be improved.
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