With the heat sink attached, calculate the permissible junction temperature of the semiconductor device by using junction-to-ambient thermal resistance (Rth(j-cf) ＋ Rth(cf-a)).
( Tj － Ta ) = ( Rth(j-cf) ＋ Rth(cf-a) ) × P
Select heat sink of thermal resistance so that the junction temperature of the semiconductor device is within the allowable range of value.
Rth(cf-a) ＝ ((Tj － Ta) / P) － Rth(j-cf)
For example, Tj ＝ 120 [deg.C], Ta = 50 [deg.C], P ＝ 10 [W], Rth(j-cf) = 0.5 [deg.C/W] (When the contact resistance between a semiconductor device and a heat sink is disregarded)
Rth(cf-a) ＝ 6.5 [deg.C/W]
To use a heat sink of a thermal resistance value lower than 6.5 deg.C/W, you can obtain the result to be expected.