The glass transition temperature (Tg), decomposition temperature (denoted as Td), and coefficient of thermal expansion (denoted as CTE) are the three thermal parameters that are most crucial for PCB laminates.
However, for this article, we will be focusing on TD PCB.
What is Thermal Decomposition?
Td is also called thermal decomposition temperature. This is the temperature whereby a Circuit board material chemically decomposes (losing about 5% of its mass) is known as the decomposition temperature. Td is measured in degrees Celsius, just like glass transition temperature Tg. When constructing PCBs, the Td of a dielectric material is a crucial limit because if it reaches or exceeds it, changes to the properties are irreversible. Compare this to the glass transition temperature (Tg), below which a material’s characteristics will revert to their initial states.
Select a material that allows you to work at temperatures that are higher than the Tg but significantly lower than Td. Make sure the Td is greater than this range because solder temperatures in PCB assembly often vary between 200 °C and 250°C (fortunately, most materials feature a Td of at least 320°C).
More on Thermal Decomposition temperature
A Circuit board substrate will disintegrate whenever exposed to different temperatures that are higher than a particular point. The Circuit board loses at least 5 percent of its total mass during the procedure. Decomposition temperature, expressed in °C, is the range of temperatures where this process takes place. When a PCB material is taken away from its temperature of glass transition, all effects of the glass transition temperature Tg transformation reverses itself; however, impacts of the substrate’s decomposition temperature are permanent. There is a resin system too.
Use a Circuit board material which can withstand the temperature range of your work environment as this is the greatest option.
A large TD PCB Td value will need a TD of at least 340°C in order to perform reliably and under difficult circumstances. When a material is taken out of its temperature for glass transition, the consequences of the Tg transformation will self-reverse; whereas, the consequences of the substrate’s decomposition temperature are irreversible. Use a Circuit board material which can withstand the range of temperature of the working environment as a result, as this is the greatest option. Typically, a PCB substrate must be soldered at a temperature of 200° C to 250° C. The Td ought to be higher and the Tg would ideally fall within that range.
How to Choose a TD PCB Td
The Td of a Circuit board laminate material is significant when putting together PCBs because once the material exceeds or reaches its Td, modifications to its qualities are irreversible. When a material cools just below Tg range, in contrast, its characteristics will revert to their former states.
The Circuit board substrate will disintegrate when subjected to temperatures that are higher than a particular point. The Circuit board loses at least 5% of its total mass during the process. Decomposition temperature refers to the range of temperatures where this process takes place (Td). Select a Circuit board base material that allows you to operate in a range of temperature that is higher compared to the glass transition temperature Tg but significantly lower than Td.
Ensure Td is greater than this range because the majority of solder temps during circuit board assembly are within the 200–250 degree Celsius range. It’s fortunate that the majority of materials own a Td above 320 degrees Celsius.
Thermal PCB Parameters
The glass transition temperature (Tg), the delamination time (Td), as well as the thermal expansion coefficient are the three thermal characteristics that are most crucial for PCB laminates. These numbers are crucial because the PCB’s thermal gyrations during rework and assembly may have an impact on the board’s life itself, regardless of the temperature of the environment that the board would have to be deployed in. Because thicker designs experience larger temperature variations when fabrication is ongoing, design parameters, in particular board thickness, are important to take into account.
Additionally, we must remember that stresses of Z-directional temperature are cumulative, which means there are additional chances for failure. Properties here include thermal expansion, dielectric constant, thermal conductivity, surface resistivity, mechanical stress, thermal shock, etc.
Thermal parameters i.e. thermal expansion have been affected by the Restriction regarding the European Union’s utilization of Hazardous Substances directive because the lead-free solder that is now required does have a greater melting point compared to the tin-lead materials and as a result, temperatures throughout rework or assembly can frequently reach a value >260°C. Utilizing mid-Tg materials allows manufacturers to use thinner boards, which may be crucial in massive yields and save at least 25% on the material costs.
Why is it Important to Monitor the Temperature of PCB?
Since high temperatures may change a PCB’s structure, reduce its performance, and even make it fail, it is crucial to monitor PCB temperature.
Because the effects of high temperatures need not stay localized, Measurement of PCB temperature is also essential. They can swiftly spread to certain other PCB components, resulting in a series of errors and damage. The following sorts of damage can result from excessive heat inside a PCB:
Structural integrity loss
A PCB’s integrity can be harmed by excessive heat. A PCB’s layers are very sensitive to temperature changes, and they contract and expand when it gets too cold or hot. Warping in its widths, lengths, as well as thicknesses of certain PCBs layers can be caused by excessive heat.
Circuit line disruption
Circuit damage can also be caused by excessive heat. When circuit wires become too hot, they enlarge and take on new shapes. Once this occurs, frequency shift, distortions, as well as straight losses might affect the circuits. Additionally, its conductor impedance, which is typically 50 ohms, might vary. Particularly tiny, sensitive components in millimeter-wave and microwave circuits are prone to damage when they expand or flex under extreme temps.
Incompatible expansion rates of the material
Different materials stretch at various speeds, which exacerbates the negative effects mentioned above. Dielectric layers as well as conductive metal layers are the two primary layer types found on a PCB. They respond to heat differently, expanding in different ways because they comprise various elements.
Consequently, as the various kinds of layers separate, an overheating Circuit board may sustain additional damage and moisture absorption.
Oxidation
PCB components oxidizing at a high glass flow temperature is a concern. If a protecting laminate covering is not present, exposed dialectic materials present in printed circuit boards (PCBs) is not protected against oxidation. In that situation, after being exposed to intense heat, the material can rust. Transmission line failure frequently leads to increased dissipation factor.
What Causes High PCBs to Heat Up a Lot?
Component Failure Leading to Dissipation
Another frequent reason for increased heat in the circuit board is when a component malfunctions and then dissipates, not generating the normal power required. In order to make up for this, the components close by must provide more power. Overheating is a concern associated with producing greater power.
Through-Hole Interference
The PCB’s power-supplying components are through-hole as well as heat-sink components. They produce heat and release it onto the atmosphere. The remaining components would produce more heat even more than normal to make up for improperly soldered heat sinks or through-hole interference from another PCB component. Additionally, there is a danger of overheating in this case.
Surface-Mount Device Distance
Similar to how through-hole components are connected to the Circuit board, surface-mount devices do the same. They provide a more streamlined current flow via the heat-sink and through-hole components. However, the SMDs and through-hole components need to be spaced properly from each other.
This current needs to travel further if they’re too distant away. These receiving components may overheat due to the additional time required for current to move. Other parts could start to overheat as a result of that.
High-Frequency Circuits
High-frequency circuit applications are more prone to experience high temperatures than other types of applications. The cause is that increased electricity production inevitably results in more heat.
For instance, radio-frequency circuits, are a quickly expanding area of PCB engineering. Despite their complexity, these circuits feature a wide range of practical uses, including cellphones as well as wireless security used in industrial and medical equipment. Special design methods are required for these kinds of PCBs because high-frequency circuits can generate a lot of heat.
Lead-free Solder
The Circuit board industry is evolving in the direction of limiting harmful substances (RoHS). Lead-free solder, which is used on RoHS printed circuit boards, needs high temperatures in order to flow smoothly.
Conclusion
Temperature affects performance, reliability, and safety significantly printed circuit boards (PCBs). High temperatures have the potential to cause malfunctions, moisture absorption, and long-lasting harm.
A PCB’s operations can become heated under a variety of conditions. A PCB-mounted component could generate a lot of heat. This could be an outside factor, such as a different part of a complicated system like an aeronautical system or any medical application.
