What is a "high failure"...

In the thermal managing of components I have found great reliability in the
three step method of solder, test and bond.  The bond procedure I like best
is pneumatic injection from the farside through multiple plated through
holes.  One hole is used to inject a thermal compound (conductive or non)
while "perimeter" hole or two act as inspection/verification holes.  This
configuration lends itself to the age old question of when is there enough
"thermal contact".  Air bubbles can be a killer, I found the inject was most
reliable in a repeated process.  It also ensures the best configuration for
solder joint success.
I have been involved in the leadless lead frame packages that thermally
manage by soldering the under belly via reflow.  Works good in high volume,
but getting there is painful.  Rework is nearly impossible (the lands and the
"underbelly land" cannot be seen, let alone touched by an iron).  Solder
volume and proper design of stencil is difficult and needs to be exact.
Surface finish... well it goes on, I'm sure you know.
I have had woes with "sticky pads" and their tolerances and absorption of
cleaners.  I like the compound.

Good luck with it.
Brad