PCB CTE Values and Why CTE Mismatch Should Be Avoided
All materials in a PCB have some material properties that are related to temperature. One important thermomechanical property of PCB materials is the CTE value, or the rate of change in a material’s volume with temperature. At excessive temperature changes, the system can experience excessive deformation, which should create a mechanical reliability concern for a designer.
To minimize potential mechanical failures associated with thermal excursions, a designer should select materials such that any CTE mismatch between materials is minimized. The mismatch between CTE values can never be set to zero, but it can be minimized to some extent. Keep reading to learn more about CTE values what materials could be more desirable for thermal reliability.
What Are PCB CTE Values?
All materials have an important thermal property known as the coefficient of thermal expansion, or CTE. This refers to the rate of volume expansion a material will experience for a given temperature change. It is typically measured in parts per million per degree, i.e., for every increase of 1 °C, the material will expand or contract by 1/10,000th of 1%. Other material properties like dielectric strength and tensile modulus are also related to temperature.
Typical PCB CTE Values
Some of the important CTE values for various materials are found below.
X: ~15 ppm/°C
Y: ~15 ppm/°C
Z: ~70 ppm/°C
X: Varies (~16 ppm/°C on average)
Y: Varies (~16 ppm/°C on average)
Z: Varies (~16 ppm/°C on average)
~50 ppm/°C, depends on curing conditions
The values above are generalizations; make sure to check materials datasheets before selecting materials if the board might experience thermal cycling to extreme temperatures. In addition, more exotic materials like ceramics can have CTE values all over the map. Because of its strong thermal conductivity, aluminum nitride is a valuable material, but its CTE value is very small (4.3 to 5.8 ppm/°C) and will not match well to copper.
Glass Transition Temperature
All PCBs have another important material property related to CTE, which is the glass transition temperature, or Tg. Standard FR4-grade laminates have Tg values in the neighborhood of 130 °C, although more expensive high-Tg laminates will have values reaching 170-180 °C. Other more specialized materials like Rogers or similar PTFE laminates can have much higher Tg values, although the specific value depends on the composition of the base materials in the laminate.
What Happens When There is a CTE Mismatch?
When designing a PCB, you want it to be as reliable as possible, but there are several reliability problems that can arise from a CTE mismatch. Due to differences in CTE values in PCB materials, stress will arise and concentrate in the region between two mismatched materials. Generally, a single thermal excursion does not cause problems unless that excursion is extreme. However, repeated thermal cycling can lead to mechanical failures related to volumetric expansion.
Solder fatigue is a key concern in high-reliability electronics that could experience excessive vibration or temperature changes. One of the main causes of solder fatigue is a mismatch in the CTE values for the solder material and the copper to which it is soldered. The other main mechanical factor leading to solder fatigue is vibration. Together, these two factors can lead to mechanical fatigue in a solder joint.
There are some PCB manufacturing processes that will be affected by changes in volume and CTE mismatches. Solder bridging is one problem that can arise when soldering BGA packages. During reflow soldering, wire-bond-molded BGA packages can expand at the corners due to a variation in CTE between different materials on the package. This causes the ball of molten solder to deform, and the result could be bridging between adjacent balls, resulting in an electrical short.
This BGA package can experience expansion if reflow profiles are not configured properly.
Thermal Stress in High Aspect-Ratio Vias
When a via's aspect ratio is higher, the copper coating along the via wall can be thinner, making the center more vulnerable to thermal stress cracking. This means thicker plating is required to reduce stress concentration when the board temperature changes. On HDI circuit boards, persistent stress buildup due to repeated thermal cycling (temperatures changing from high to low and vice versa) is known to cause fractures at via necks and layer interfaces in stacked blind-buried or buried-buried vias.
Delamination and PCB Warping
If the CTE mismatch between copper and the laminate is excessive, a high temperature rise can create enough stress to cause delamination between layers and begin to deform the PCB. Copper and FR4 circuit boards are more susceptible to this form of damage resulting from high temperature changes and CTE mismatch. Laminates with higher resin content can have higher CTE mismatches with respect to copper. Additionally, thicker copper layers create more stress for a given temperature change.
Once you’ve selected the materials you need to build a reliable PCBA, make sure you use OrCAD from Cadence to specify your design requirements and create your PCB layout. OrCAD includes the industry’s best PCB design and analysis software. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.
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