List of different High Tg PCB Material
Normal Fr4 : 130oC
Mid-Tg Fr4 : 150oC
High Tg : 170oC
High Tg+ : 180oC
High TG++ : 260oC
High TG+++: 300oC
In the recent years the demand of high T g printed circuit board has been increased. The Tg refers to glass transition temperature and it is indeed an important property of any epoxy . It is the temperature where the polymer transitions from a hard grassy material to a soft, rubbery material. The value of Tg is likely to have adverse effect on its ability to function. Standard printed circuit boards have glass transition temperature in the value of 140oC and it can sustain a temperature of 110oC. In another way it can be said that it cannot sustain high temperature processes automobile, industrial processes, high temperature electric, electronic device. In these processes FR-4 provide best solution.
Therefore, we can define a High Tg Printed circuit board is the board which has glass transition temperature higher than 150 degree Celsius. In applications where printed circuit boards are experiencing a thermal load 25 degree Celsius below the transition temperature. One feature of the high Tg printed circuit boards needed the lead free solder to flow. The T g value of the printed circuit board material sets the upper boundary, the which the matrix of the resin decomposes and subsequent melting occurs. Thus Tg is not the maximum value of operational temperature, but it is the endurance limit of the printed circuit board material in terms of temperature.
Fr-4 is a flame retardant fiberglass epoxy material which is reinforced. FR-4 provides good resistance to heat than a standard printed circuit board. There is difference between FR-4 and high glass transition temperature FR-4, in hot situation, especially in case of heat absorption with moisture, the high Tg pinted circuit board perform better than the general FR-4 in the aspect of mechanical strength, stability of dimensions and thermal decomposition. There are four types of FR-4 circuit based on number of copper trace layers found in material.
1) Single sided printed circuit board
2) Double sided printed circuit board
3) Four or more layer printed circuit board
Ordinary printed circuit board materials at elevated temperature not only will be softened, deformed but mechanical and conductive properties are also influenced negatively in this kind of situations. In another word it can be said that glass transition temperature defines the stability of printed circuit board dimension. The characteristics of high Tg materials are enlisted below
- Very high resistance to heat.
- High value of thermal stress resistance.
- Good mechanical properties.
- High temperature durability.
- Long delamination durability.
Applications of High Temperature PCB: If the electronic product has higher power density where heat generation would have that propensity to disturb heat sink or other parts of the product, High Tg printed circuit boards would be the best solution. Link to the reduction of heat generation of printed circuit boards may affect the weight, cost, power supplied and the ultimate size of the product. Therefore, in other aspects to the implementation of high Tg printed circuit boards is a cost-effective and pragmatic solution. In applications where printed circuit board is subjected to high temperature, it result into damaging the dielectrics and conducting elements, generation of mechanical stress due differences in thermal strain at different points. And it could lead to permanent failure.
By and large if we look at the application of high Tg printed circuit board it is important when we are seeking protection of board from high temperature. In any such application where heat generation is higher, one task of the designer is to dissipate the heat. There are three methods of heat extraction viz. conduction, convection and radiation.
Conduction heat transfer can be done by providing a heat sink in direct contact with the hat generation point, which allows the heat flow away from the source point directly akin to the electric current.
Convection mechanism is related to the heat transfer by mean of fluid like air and water. Convection can be of two types, either free or forced. Free convection is termed as a natural process. In case of force convection a pump or fan is used in the process to increase the speed of the fluid. In most of the electronic products the heat dissipation technique is forced convection by mean of implementing a fan with it.
Radiation phenomenon is possible only if there is generation of heat at very high temperature. Although most of the electronic devices do produce electromagnetic radiation but the amount is very less comparatively. The generated radiation can be eliminated by providing reflective surfaces within the product, no other special arrangements are required.
Here we can see, more we will try to deal with the heat removal mechanism it will affect the design of the printed circuit board. Limiting the power density of the products will reduce it features and market value at the same time. Inclusion of more number of heat sinks would lead to increase in weight and size of the printed circuit board and the cost the board as well. Even if the designer is aware of numerous method of heat management in such cases but still it will affect performance of the printed circuit board in some other aspect.
Therefore, to get rid off from all such problems the best way is to implement the high Tg printed circuit board in such high temperature applications. If working temperature of the product is higher than normal (130-140C), then have to use high Tg material which is > 170C, and popular PCB high value are 170C, 175C, and 180C. Normally the PCB Tg value should be at least 10-20C higher than working temperature of product.
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