Changes in temperature could have a significant effect on the quality, reliability, and operation of PCBAs. Increase in temperature leads to the expansion of the materials, however, the main materials which a printed circuit board is composed of feature disparate coefficients of thermal expansion.
This leads to mechanical stress, which may create undetected micro-cracks during the electrical tests (short/open tests) which is performed when the manufacturing is coming to a close. The best case scenario is that the micro-cracks would result in failures that are detected once brazing is completed. While the worst case scenario is that this could lead to different failures of your finished product.
In 2002, when the RoHS directives were introduced, it required the utilization of lead free alloys for the purpose of soldering. The removal of lead leads to an increase in melting temperature; therefore PCBs are subject to greater temperatures during the soldering process (wave and reflow). Regarding the reflow process chosen (double, single, etc.), it is very important to make use of a printed circuit board having the appropriate mechanical characteristics, most especially one having a suitable temperature for glass transition.
What is Temperature of Glass Transition (Tg)?
Tg can be described as a mechanical property which designates the temperature of glass transition. This is the temperature whereby the base material (glass or polymer) decides to shift from its rigid, solid, and glassy state to a new rubbery state. Whenever the Tg has been exceeded, then materials will not melt, but will have to pass through structural changes in order to be rubbery.
Measuring this temperature precisely is very difficult to achieve since so many factors usually come into play. These include the molecular structure of the material. This is why different materials usually come with their own temperatures for glass transition. However, it is possible for two materials to share one pcb Tg value even though their technical characteristics are different. The viscosity of a material increases whenever it is heated. Also, after cooling, they easily undergo breaking or cracking.
What Does PCB Tg Mean?
PCB Tg temperature of a polymer system can be described as the temperature whereby a material changes from its relatively glassy and stiff state to one that is softened or pliable. This thermodynamic change in material can be reversed so far the polymer system isn’t degraded. Explaining further, whenever that material gets a heat that is higher than the Tg and then made to cool to the Tg, it will return to the stiffer state and with its initial properties.
Most base materials usually have organic contents in them. At least, the PCB base hydrocarbons, binders, have lower stability of temperature compared to the inorganic constituents of the composite materials.
Chemists spent so many great efforts in improving the polymers’ thermal properties, making them good as binders. Moreover, these properties remained at a distance from the thermal stabilities of metal foil, reinforcing glass fibers, and ceramic fillers.
Furthermore, they are great to use in different lead-free technologies, initially, a slight increase in the soldering temperature, in fact, resulted in being significantly higher compared to the stability of temperature of the PCB binders’ polymers.
What are the Major Properties of the PCB Tg Material?
A dielectric PCB substrates’ main property is its thermal expansion when in a transverse direction. This means across reinforcing layers. This holds especially true for the HDI boards that are distinguished by the utilization of the thin plated holes present in the thick board base.
Furthermore, the difference in the thermal expansion of the metal and base in the holes can result in a great decrease in the interconnectors’ reliability. Moreover, if this material is heated to any temperature that is much greater than the glass transition temperature, then there may be irreversible changes in the properties. This temperature whereby this happens varies with the material type is relevant to the polymer’s degradation
What are the Classes of the PCB Tg?
High Tg PCB
All the time, we stress that higher Tg value PCB are usually better. However, this isn’t usually the case. Although it holds true that higher Tg value PCB would lead to delaying the start of great thermal expansion for any specific polymer system, the expansion as a whole might still be different for each material. Materials having lower glass transition temperature might exhibit lower net expansion compared to materials having higher glass transition temperature.
PCB high Tg results when there is a rise in temperature to a specific range leading to a transition of the substrate from its initial glassy and rigid state to a state that is more rubbery. The temperature at which this happens is referred to as the temperature for glass transition or glass transition temperature of the circuit board.
We can explain further by saying it is the highest possible temperature in ℃, whereby the substrate will maintain its rigidity. This means that ordinary materials of the PCB substrate usually exhibits melting, deformation, softening, as well as a sharp drop in the electrical and mechanical properties, when under very high temperatures.
In general, the material’s Tg is higher than 130℃ whereas the high Tg FR4 usually stands at a PCB Tg value greater than 170℃. Medium Tg usually stands higher than FR4 Tg150 PCB. Tg170 PCB materials having a Tg of at least 170℃ are called high Tg FR4 PCBs.
An increase in the substrate’s Tg helps in enhancing the moisture resistance, heat resistance, chemical resistance, as well as the stability of the PCB. Furthermore, higher values of Tg indicates that the material is exhibiting a greater temperature resistance, most especially in the lead-free processes whereby there is a prevalence of PCB high Tg applications.
Main Features of the High Tg PCB
- Great resistance to any thermal stress
- Dimensional stability
- The mechanical performance is improved
- High plated through hole reliability
- High resistance to heat
What are the Applications of the High Tg PCBs?
Communication Equipment
Most times, high Tg FR4 PCB is used in communication equipment like network equipment, fiber optic devices for communication, and wireless base stations. Usually, these devices require great electrical reliability and performance in high-frequency and high-temperature environments.
Industrial Control Equipment
Most times, high Tg PCBs are utilized in robotics, automation systems, and industrial control equipment. Typically, these devices operate in high humidity, high temperature, as well as high vibration environments that require great mechanical strength and thermal stability.
Automotive Electronics
High Tg PCBs are utilized in sensors, on-board computers, dashboards, as well as other critical systems. A car’s interior usually experiences significant fluctuations in temperature, so the materials of the circuit board that can withstand high thermal stress and temperatures are required.
Aerospace
This industry demands a high temperature resistance and high reliability from electronic devices. The high FR 4 Tg PCBs are used extensively in satellites, aircraft, as well as navigation equipment for withstanding extreme changes in temperature as well as harsh conditions for working.
Medical Devices
There is a need for medical equipment to function under high-temperature disinfection and sterilization conditions. High Tg PCBs are useful in missile control, radar systems, tactical equipment, military communications, as well as other fields.
What Benefits does High Tg PCB Offer?
High temperature durability
Usually, high Tg PCBs have great resistance to very high temperatures as well as maintain a great performance regardless of extreme thermal stress. They don’t deform or warp as a result of heat and they stay sound structurally.
Great Mechanical Strength
The high Tg PCBs are known to be very resilient and have the ability to withstand high stress and pressure. Inherently, they are strong and hard, and with each additional layer, there is an increase in their durability.
Better reliability
High Tg PCBs are much more dependable compared to the conventional PCBs. Also, they can perform optimally even in harsh and extreme temperatures. This ensures constant electrical performance even during extremely harsh conditions.
Great Power Density
Normal PCBs can’t work with high power densities due to the components that generate much heat. Moreover, the high Tg PCBs have the ability to tolerate conditions of high temperature. This allows them to dissipate heat very effectively. With these, you can place more components on the board and not be worried about overheating.
What are the Properties of high Tg PCBs?
With the quick development of our electronics industry, the PCB high Tg materials are widely utilized in communication equipment, computers, precise instruments as well as apparatus and so on.
Towards high functionality, the high multilayer development, the PCB substrate material requires higher resistance to heat as a prerequisite. Also, the boards are more inseparable from the substrate’s high heat resistance, in case there’s small aperture, thinning, and fine wiring due to the emergence as well as development of a high density mounting technology, which is represented by CMT and SMT.
Therefore the difference between high FR4 Tg and the general FR-4 is the adhesion, mechanical strength, water absorption, thermal decomposition, dimensional stability in hot states, most especially after the absorption of water, which are the differences in several conditions like thermal expansion, and it is clear that the high Tg PCB is much better compared to the normal PCB substrate materials. Therefore there’s a huge demand on the high Tg PCBs recently, i.e. Tg150 FR4 despite that the price is higher compared to the ordinary ones.
In addition, the High Tg material is well-known in the LED lighting industry. This is due to the fact that LED’s heat dissipation is higher compared to the usual electronic components. However, this same FR-4 board structure is less expensive than the metal core PCBs.
The following are the properties of high Tg materials:
- Its thermal expansion is low
- Resists high temperatures
- Great mechanical properties
- Its PTH reliability is excellent
- It has a high thermal stress resistance value
When Do I Need High Tg PCBs?
If your PCB will not be able to bear a thermal load that is not more than 25℃ less than the Tg, then you have to get a high Tg PCB to work with your applications. Also, if your own product is running at a range of 130℃ or more, then you will need the high Tg PCBs to ensure safety.
The major reason for using the hightg PCBs is due to the movement into the RoHS PCB. Therefore, there are other PCB industries that are leaning towards the use of high Tg materials.
What is PCB Middle Tg Material?
Substrates usually vary in terms of their material composition, insulating properties, and flame resistance. There are common raw materials such as glass cloth, bakelite, and other plastic boards.
The process of manufacturing a copper foil board involves pressing the copper foil with an insulating prepreg. This prepreg comprises epoxy resin and glass cloth. The base material for a rigid substrate is a non-flexible insulator. Rigid substrates have hard base material.
The base material of flexible substrates is made of flexible insulators. Most PCBs are made of rigid substrates. You can classify rigid substrate based on the materials used for them and their resin content. The base material can comprise paper and glass cloth while epoxy and phenol are best used as the resin.
What are the Applications of a Middle Tg PCB?
Middle Tg PCBs have proved to be very useful in many applications. Single sided substrates are commonly used for paper phenolic substrates. Also, this PCB is suitable for consumer electronics. You will find them in phones, keyboards, radio cassette players, game consoles, and stereos.
Middle Tg PCB are applicable in silver through-hole substrates in which copper plating can’t form through-holes. However, through-holes can be formed by filling the holes with silver paste.
What Thermal Analysis Techniques are available for PCB Middle Tg?
A crucial aspect electronic manufacturers usually consider when using circuit boards is thermal performance. A PCB will perform well if the material used in fabricating it has great thermal reliability. This makes it very important to always evaluate the thermal performance of a circuit board material.
When it comes to the PCB Middle Tg, there are several techniques for thermal analysis.
Thermomechanical analysis
It evaluates the extent of the changes in a material’s physical properties in response to varying temperature. The thermomechanical analysis (TMA) measures the changes in the physical appearance of PCB Middle Tg in response to temperature changes. TMA can also evaluate some properties like volume, density, and physical dimensions of a material.
It determines the changes in these properties in response to force and temperature. Furthermore, TMA can measure the dimensional changes of Middle Tg PCB. The glass transition of a material can be evaluated through this analysis.
Thermogravimetric analysis
Thermogravimetric analysis measures the impact of temperature on PCB Middle Tg material’s weight. TGA is a common analysis technique used in determining the relationship between weight loss and temperature. The engineer carrying out this analysis weighs the material at room temperature first. Then the analysis checks how the material changes when exposed to a particular amount of heat. This is usually carried out in a controlled environment.
Dynamic mechanical analysis
DMA helps to determine the Viscoelasticity of PCB Middle Tg. Viscoelasticity is the combination of viscosity and elasticity. This analysis also determines this materials’ resistance to deformation when exposed to vibration. DMA evaluates the extent at which a PCB material stretches and bends when it is in motion. You can use this analysis to evaluate Tg since temperature changes can impact a material’s thermal property.
Benefits of Middle Tg PCB Material
PCB Middle Tg material has many features which contribute to the performance of a circuit board.
Anti-CAF property
The PCB middle Tg material is resistant to Conductive Anodic Filament (CAF). This makes it suitable for the fabrication of circuit boards. PCB manufacturers are concerned about CAF during fabrication. CAF always occurs between adjacent PCB vias.
Environmentally friendly
One of the amazing benefits of PCB Middle Tg material is that it contains no phosphorus and halogen. This makes it safe for the environment.
UL-94 flammability
This PCB material is UL-94 V-0. It adheres to the PCB flammability standards. The UL-94 flammability is a popular standard in the PCB industry. It classifies how PCB materials burn in different scenarios.
Excellent thermal performance
PCB Middle Tg material features great thermal properties which contribute to its thermal performance. For instance, this material has low Z-axis CTE and middle Tg. Materials with middle glass transition temperature are known to perform exceptionally well. These PCB materials are ideal for fabricating rigid circuit boards and other high-performance boards.
Conclusion
High Tg in PCB i.e. PCB tg150, fr 4 high Tg, Tg 180 pcb results when there is a rise in temperature to a specific range leading to a transition of the substrate from its initial glassy and rigid state to a state that is more rubbery. The temperature at which this happens is referred to as the temperature for glass transition or glass transition temperature of the circuit board. Middle Tg of PCBs has proved to be very useful in many applications. Single sided substrates are commonly used for paper phenolic substrates. Also, this PCB is suitable for consumer electronics.
