SCR thermal path and heat sink material: assertion–reason Assertion (A): A typical overall thermal resistance from device source to heat sink for a power SCR can be about 0.3 °C/W (order of magnitude value with an adequate heat sink). Reason (R): Heat sinks for thyristors are generally made of aluminium, which has high thermal conductivity and low mass.

Introduction / Context:
Thermal management of thyristors (SCRs) is critical to reliability. The thermal resistance chain includes junction-to-case, case-to-sink, and sink-to-ambient. Designers often quote order-of-magnitude values when sizing heat sinks and airflow. Aluminium is a standard heat sink material due to its excellent thermal properties per cost and weight.

Given Data / Assumptions:

• Power SCR mounted on an appropriately sized aluminium heat sink with good interface (grease/insulator).
• “About 0.3 °C/W” treated as an indicative value achievable with a robust sink, not a universal constant.
• Aluminium heat sinks are commonplace in power converters.

Concept / Approach:

The assertion states an order-of-magnitude thermal resistance figure. Such a value is plausible for sizeable sinks and moderate airflow. The reason correctly states that aluminium is widely used, but the actual thermal resistance achieved depends on sink geometry, airflow, mounting, and interface materials—not merely the choice of aluminium. Therefore, while both statements are true, R does not fully explain the specific numerical value in A.

Step-by-Step Solution:

Verification / Alternative check:

Catalogues show aluminium sinks achieving 0.2–1.0 °C/W depending on size and airflow; junction/case/interface add to the total, reinforcing the “about” nature of A.

Why Other Options Are Wrong:

(a) Overstates the causal link; aluminium alone does not set 0.3 °C/W. (c) claims R is wrong, but it is correct. (d) would dismiss the plausible magnitude in A. (e) rejects both, which is not justified.

Common Pitfalls:

Treating thermal resistance numbers as constants; ignoring interface quality and airflow; assuming copper sinks are always necessary (aluminium is standard and economical).

Final Answer:

Both A and R are correct but R is not the correct explanation of A