Book Excerpt: The Printed Circuit Designer’s Guide to... Thermal Management with Insulated Metal Substrates, Vol. 2, Chapter 4

Estimated reading time: 1 minute

Chapter 4: Selecting IMS for High-Temperature Applications

The UL 746 test series offers a prediction indication on the lifetime of polymeric materials. In previous years, 100,000 hours has been used as an indication. New approvals are done with a calculation of 60,000 hours, which leads to some confusion when comparing figures stated in older technical datasheets against test results with new materials.

In practical terms, a MOT 130 material tested under the old regime would be required to retain at least half of the electrical and mechanical initial values after 100,000 hours. Using the new 60,000-hour tests of the UL, the same MOT 130 material could be approved with a new MOT of 140, a difference of about 10°C. Note that the technical datasheet for any given material may not specify whether the MOT is expressed in terms of the 60,000-hour or 100,000-hour tests.

Direct-bonded copper (DBC) with a ceramic base is a commonly used assembly technology. While advantages include excellent solder crack compensation due to similar CTE values between component and substrate, the construction is fragile and therefore places limits on size and exposure to vibration. Small assemblies in basic shapes such as a rectangle can be done, although larger, more complex shapes become difficult.

The latest IMS technologies can overcome these drawbacks. With the support provided by the aluminum base, the assembly can have almost any shape without compromising reliability. New materials like Ventec’s VT-4B5H, which has MOT of 155°C (according to UL 746E) and thermal conductivity of 4.2 W/mK (according to ISO 22007-2), can address a large variety of high-temperature applications such as power supplies, power converters, rectifiers, and motor drives for heavy industrial applications. Others include battery charging for electric vehicles, where high voltages are used for fast charging, as well as automotive ECUs that must withstand high temperatures such as in under-the-hood environments, and electric power steering (EPS).

Further development is increasing thermal conductivity towards 7 W/mK (according to ISO). In addition to enhancing high-temperature performance, this should also meet requirements such as high breakdown voltages and large creepage distances. Note also that increased thermal performance allows the usage of thicker dielectrics.

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