Differential Thermal Analysis and PCB Substrate Glass Transitions
This dilatometer is used to measure thermal expansion coefficients for liquids and gases
Thermal expansion is one of those pervasive physical phenomena that silently happens all around us, yet we hardly notice unless we take precise measurements. Whether you measure it or not, components in your PCB will dissipate heat and pass that heat to your conductors and substrate. Your PCB substrate and conductive structures in your PCB will expand as the board operates and heats up to high temperature.
Thermal expansion is one of many physical properties quoted on datasheets for PCB substrate materials, but this value is often an afterthought for many designers. If your board will operate in a high temperature environment, you will need to pay attention to the thermal expansion coefficient (CTE) of your PCB substrate and conductors in order to avoid a glass transition. In some cases, it will make more sense to choose a substrate material with a higher glass transition temperature to prevent undue stress on thin conductors and, hopefully, avoid board failure. Differential thermal analysis is one technique that can be used to measure these and other physical properties.
What is Differential Thermal Analysis?
Differential thermal analysis is a technique for measuring precise temperature changes in a material and their relationship to thermodynamic properties. Exothermic or endothermic changes in a sample can be detected by examining the temperature difference between the material under test and the inert reference material. Effects such as phase changes, glass transitions, crystallization, and polymerization.
A differential thermal analysis measurement requires comparing two temperature measurements between a test material and some reference. The two materials are mounted in a heated chamber, typically in an inert atmosphere, and the temperature of the chamber is increased over a specified range. The temperature of each material can be gathered with thermocouples. During the test, the reference material should not exhibit any phase changes, crystallization changes, or other thermodynamic changes over the relevant temperature range.
The measured temperature difference between the test and reference materials is plotted against time or the furnace temperature, called a DTA curve. Peaks and valleys will appear in the DTA curve, and these peaks and valleys can be correlated to some thermodynamic change in the test material. The area beneath the DTA curve during the transition is equal to the enthalpy change in the material. This allows a phase change or crystallization/polymerization transition to be easily identified in the DTA curve. In the case where the heating rate is very fast, the thermal conductivity and/or specific heat of the test material can also be extracted from a DTA curve.
Differential thermal analysis setup (left) and DTA curve (right)
The glass transition temperature (Tg) in a PCB substrate or other material can also be identified in a DTA curve as an endothermic phenomenon (i.e., a dip in the DTA curve). A glass transition will appear in a solid composite/polymeric material (i.e., before the melting point). In the solid phase, crystallization or crystal phase transitions will appear as exothermic peaks in a DTA curve, allowing them to be easily distinguished from a glass transition. Above the melting point, polymer cross linking appears also as an exothermic transition. Understanding how to identify these different transitions provides a simple way to identify a glass transition in a PCB substrate without taking volumetric measurements.
Differential thermal analysis curve
Why Measure CTE and Tg Values?
As your board operates and heats up to high temperature, the substrate and copper will expand, just like any other material. PCB substrates and copper expand at different rates, i.e., they have different CTE values. As an example, FR4 has a CTE value of ~70 ppm/K along the vertical direction (perpendicular to the board surface), while copper only expands at ~16 ppm/K. As the board expands, the substrate places more stress on copper structures as the temperature increases. Once the substrate temperature increases above the glass transition temperature, the CTE value for the substrate increases further, which places even more stress on conductors as the substrate temperature increases.
In boards with thick conductors and low aspect ratio plated vias, thermal expansion is not such a problem until extreme temperatures above the glass transition point are reached. However, thermal expansion is known to lead to fracture at via necks in HDI boards, and in via barrels when the via aspect ratio is very high (~8 or more).
Vias on high density lines can fracture at high temperature due to a large mismatch in CTE values
When choosing a substrate material for boards in hot environments, it is better to use a board with a sufficiently large Tg value as you want to avoid a glass transition. This is quite important when high power components are used in these boards. The ideal PCB substrate will also have a CTE value that matches the CTE value for copper. However, these two goals cannot always be reconciled, and a designer must consider their board’s intended environment when selecting a PCB substrate.
After you perform differential thermal analysis for your PCB substrates, you can create your new design with the right PCB layout and design software. Allegro PCB Designer and Cadence’s full suite of design tools can help you create your next layout on any substrate and prepare your board for manufacturing. You’ll also have access to a set of tools for MCAD design, electrical simulations, and data management.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.