The different of high TG and normal TG

- Nov 03, 2018-

When the temperature of high Tg printing plate increases to a certain area, the base plate will change from "glass state" to "rubber state". The temperature at this time is called the vitrification temperature (Tg) of the plate.That is to say, the Tg is to maintain the rigidity of base material the highest temperature (℃).That is to say, common PCB substrate material not only produces softening, deformation, melting and other phenomena, but also exhibits a sharp decline in mechanical and electrical characteristics. 

Generally, the plate of Tg is above 130 degrees, high Tg is above 170 degrees and medium Tg is above 150 degrees. 

Tg of 170 ℃ or more usually PCB printed circuit board, called high Tg, PCB. 

The Tg of substrate is improved, and the thermal resistance, humidity resistance, chemical resistance and stability of printed board can be improved.The higher the TG value, the better the temperature resistance of the plate, especially in lead-free process, the higher TG is applied. 

High Tg refers to high heat resistance.With the rapid development of the electronics industry, especially the electronic products represented by computers, the PCB substrate material needs higher heat resistance as an important guarantee.The appearance and development of high-density installation technology represented by SMT and CMT have made the PCB more and more inseparable from the support of the high-heat resistance of the substrate in the aspect of small aperture, fine wiring and thin molding.

 

Therefore, the difference between the general fr-4 and the high Tg fr-4 is that under the thermal state, especially under the heat after moisture absorption, the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition and thermal expansion of the materials are different, and the high Tg product is significantly better than the common PCB substrate material. 

The expansion of the core board with good interior graphics is different due to the difference of the graph distribution with the thickness of the core board or material characteristics. When the graph distribution is different with the thickness of the core board or material characteristics, when the graph distribution is more uniform and the material type is consistent, no deformation will occur.When the laminated structure of PCB plate is asymmetric or the graph distribution is not uniform, the CTE of different core plates will be greatly different, which will lead to deformation during the compaction process.The deformation mechanism can be explained by the following principles.

Suppose there are two kinds of CTE large difference of core board through A curing press together, in which A core board CTE is 1.5 x10-5 / ℃, core board length is 1000 mm.In the pressing process, the semi-solidified sheet, which is used as the bond sheet, is bonded together with two core boards through three stages of softening, flowing and filling. 

Figure 1 for ordinary FR - 4 resin under different heating rate in the bottom of the kinematic viscosity curve, in general, materials from 90 ℃ to flow, and achieve above TG for the start of crosslinking curing, and a half before curing curing state for freedom, the free expansion core board and copper foil in a heated state, its deformation can through their worth to the CTE and temperature change. 

To simulate pressing conditions, temperature from 30 ℃ to 180 ℃,

At this time, the deformation of the two core plates are respectively

Delta LA = (180 ℃ ~ 30 ℃) x1.5 x10-5 m / ℃ X1000mm = 2.25 mm

Delta LB = (180 ℃ ~ 30 ℃) X2.5 X10-5 m / ℃ X1000mm = 3.75 mm

At this time, because the semi-solidification is still in the free state, the two core plates are one long and one short, non-interference, and have not yet deformed. 

, as shown in figure 2, and press fit will remain at high temperatures for A period of time, until A cure completely cure, resin to curing condition, at this time can't flow, two kinds of core board together. When the temperature drops, without interlayer resin bound, core board will be back to the initial length, will not produce deformation, but it's actually two core board at high temperature has been cured resin bond, not voluntary contraction in the cooling process, in which A core board contraction should be 3.75 mm, actually when contraction is greater than 2.25 mm is hampered by A core board, to achieve the force balance between the two core board, B core board can't shrink to 3.75 mm, and A coreThe contraction of the plate will be greater than 2.25mm, so that the whole plate will curve towards the B core plate, as shown in figure 2.