the crucial thing is to get the transistor flange flat as well. The
majority I've handled are not anywhere near flat, and always bowed
upwards in the centre (something that hasn't improved since the
package style first appeared ~30 years ago).
Could it be that the bow is there to prevent the substrate from
cracking when someone over torques the bolts on a not-so-flat heat
sink ?
I suspect it's not deliberate, but a 'safe side' consequence. Bare
flanges we had were normally very flat, flanges with the carrier pills
fitted usually bowed - whether the pills were brazed on by the package
manufacturer or soldered in place locally. We had limited success with
getting flanges pre distorted prior to pill attach to try and get a flat
final result.
Steve
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