Why choose high Tg materials in PCB fabrication？In other words, you will be charged for the Tg value of the board. What is the TG value? When must additional fees be paid for high TG value plates? Permit ETE to describe in full the TG-related challenges in the PCB sector.
For a very long time, the Tg value has been the most prevalent criterion for classifying the quality of FR-4 materials, and it is usually considered that the higher the Tg value, the greater the material’s dependability.
Tg135°C, utilization of the sheet: motherboard, consumer electronics, etc.
Tg180°C: utilization of the sheet: CPU motherboard, DDR3 memory substrate, IC package substrate, etc.
The significance of materials for printed circuit boards is equal to that of printed circuit boards for electrical devices. According to their nature, PCB materials can be split into two major systems: organic materials and inorganic materials.
Organic materials consist of paper layers impregnated with phenolic resin or nonwoven or glass cloth layers impregnated with epoxy resin, polyimide, cyanate, BT resin, etc. The use of these materials is determined by the PCB application’s needed physical attributes, such as operating temperature, frequency, or mechanical strength.
Inorganic materials are composed mostly of ceramic and metallic materials, including aluminum, soft iron, and copper. Typically, the utilization of these materials is determined by the demand for heat dissipation.
Ours most often used rigid printed board materials are organic materials, such as FR-4 epoxy glass fiber cloth materials, a class of materials with epoxy resin as the binder and electronic grade glass fiber cloth as the reinforcement material.
Therefore, FR-4 utilizes epoxy resin as a binder, and resin material has an important characteristic parameter: glass transition temperature (Tg), which is the temperature at which material transitions from a relatively stiff or “glassy” state to a meta-stable or “softened” one.
It is the temperature at which a material changes reversibly between its glassy and extremely elastic phases. In other words, if the temperature of the epoxy resin, the binder for FR-4 materials, is below Tg, then the material is in a hard “glassy” state. When the temperature exceeds Tg, the material will become rubbery and stretchy.
Below Tg, the resin substance is in a glassy, hard solid form. The deformation is reversible under external forces, i.e., when the external force departs, so does the deformation; this is the state in which the majority of resins are utilized.
When the resin’s temperature exceeds Tg, the amorphous molecular chains begin to move and the material enters a condition of great elasticity. In this state, the resin resembles an elastomer in its rubber state, but its deformation properties are still reversible.
Note that the material softens gradually as the temperature exceeds the Tg value, and as long as the resin does not break down, it can return to its prior hard state with the same qualities when the temperature falls below the Tg value.
Nitrogen has a Td value, also known as the thermal decomposition temperature; when resin-like substances are heated to a specific high temperature, the resin system begins to break down. TD point refers to the temperature at which the onset of this process occurs. TD is often defined as a mass loss of 5 percent at the decomposition temperature point. However, this percentage is quite large for multilayer PCBs.
We are aware that line width, alignment, reference plane spacing, plate dielectric constant, etc. affect the PCB transmission line characteristic impedance characteristics. The amount of resin on the substrate material has a significant effect on the dielectric characteristics, as does the evaporation of resin to adjust the distance between the alignment and the reference plane.
This Td value must be considered for lead-free soldering processes; for instance, the temperature range of typical tin-lead soldering processes is 210-245°C, whereas the temperature range of lead-free soldering processes is 240-270°C.
Conventional FR4 sheet KB-6160 has a Tg of 135 °C and a Td of 305 °C at 5 percent mass loss.
FR4 sheet without lead. KB-6168LE Tg is 185 °C, Td is 359 °C, and mass loss at 5 percent occurs at 359 °C.
As can be seen, the Td value of a standard FR4 sheet is greater than 300 °C, whereas the temperature range of the leaded soldering process is 240–270 °C. Therefore, the Td value is fully fulfilled, so why do we need the lead-free version?
As stated previously, the 5 percent resin mass volatilization rate for the need to control the impedance of multilayer PCBs appears too high for the tin-lead soldering process. The 210–245 °C temperature material does not exhibit obvious thermal decomposition, and the lead-free soldering temperature range of 240–270 °C has begun to degrade 1.5–3 percent of the resin quality. This loss of resin quality cannot be overlooked, even being less than the 5 percent needed by the IPC standard. This level of breakdown may potentially damage the long-term durability of the substrate or cause delamination faults or voids during the soldering process, particularly if repeated soldering procedures are required or rework heating is performed.
In addition to the Tg value, the Td value should be taken into account if a lead-free soldering technique is employed.
The performance of the substrate material varies significantly above and below the Tg value. Nonetheless, the Tg value is typically reported as a highly exact temperature value, such as Tg135, which does not imply that the substrate becomes floppy when the temperature exceeds 135°C. Rather, when the temperature approaches the Tg value, a progressive change in the material’s characteristics occurs.
The result of thermal expansion
Resin system cure duration
The circuit board is prone to thermal expansion, but SMT soldering BGA pad pitch is not also subject to change? Moreover, the mechanical stress caused by the thermal expansion will cause subtle cracks in the PCB alignment and pad connections. These cracks may not be discovered until the PCB production is complete at the end of the open/short circuit test, and in the SMT and other secondary heating, failures are revealed. The worst-case scenario is the SMT heating when the hidden problems do not appear until after the product has been deployed in a hot-and-cold-alternating use environment. The heat expansion of the board permits the random occurrence of these minute cracks, resulting in device failure.
parameters for the thermal performance of the substrate In addition to the normal Tg and Td values, there is a thermal expansion coefficient, CTE, as well as X/Y and Z-axis directions of CTE.
As the plating hole passes across the Z-axis of the PCB, thermal expansion and contraction in the substrate will cause distortion and plastic deformation of the planting hole, as well as deformation of the PCB’s surface copper solder pads.
The CTE in the X/Y axis becomes crucial in SMT. CTE is particularly significant when chip-level packing (CSP) and chip direct placement are employed. Additionally, the CTE of the X/Y axis influences the internal adhesion and delamination resistance of the copper-clad laminate or PCB. The X/Y-axis CTE value in each layer is very essential for PCBs using lead-free soldering processes.
In several discussions about Tg levels, it is sometimes assumed that larger Tg values are always advantageous for the substrate. This is not always the case. High Tg materials have a slightly slower commencement of material high-rate expansion when exposed to heat for a particular resin system, whereas the overall expansion is largely dependent on the type of material. Low Tg materials may display less overall expansion than high Tg materials, mostly as a result of the CTE of the resin or the inclusion of inorganic fillers in the resin formulation that reduce the substrate’s CTE.
Notably, some low-end FR-4 materials with a standard Tg of 140 oC have a greater thermal decomposition temperature Td than those with a standard Tg of 170°C. Td is a crucial indicator for lead-free soldering, and it is generally advised to use a higher Td value, although premium FR-4 typically has both a high Tg and a high Td value.
Moreover, materials with high Tg values tend to be more rigid and brittle than those with low Tg values, which frequently affects the productivity of the PCB manufacturing process, particularly the drilling process. As the board density increases, the distance between adjacent vias decreases to meet material requirements. TG = 155 board is approximately 20 percent more expensive than TG = 135 board. Due to its extreme hardness, TG must be drilled with a brand-new drill (normally, the drill bit can be honed four times). Pressing time: the standard TG=135 requires only 110 minutes to press. While the medium TG = 1 55 must be pressed for 150 minutes, it must be pressed together.
The board factory side said that one of the reasons is because of the high density of the hole. The normal TG hole spacing cannot be less than 12 MIL, while the TG hole spacing cannot be less than 10 MIL. Because the plate has glass cloth, there will be some strains when drilling, between the two holes you pull a little, I pull a little to form a wick effect, and the TG because of the hardness of the plate is not the
The greater the TG value, the greater the temperature resistance of the board, particularly in the lead-free tin-spraying method, where high TG applications are more prevalent.
The manufacturability of the board factory is taken into account, and if the PCB assembly uses a lead-free soldering technique, the glass transition temperature Tg, decomposition temperature Td, coefficient of thermal expansion CTE, water absorption, and delamination time must also be addressed.
High Tg is employed in lead-free process PCB circuit boards; high Tg refers to strong heat resistance. For the electronics industry, particularly with the rapid development of electronic devices symbolized by the computer, to high functionality, PCB substrate materials with higher heat resistance are required for high multi-layer development. As opposed to SMT, CMT, as the representative of the emergence and growth of high-density placement technology, makes PCBs in small aperture, thin line, thinning, etc., increasingly detachable from the substrate’s high heat resistance support.
