Hi, This Not so easy as one wuold think. Has the thermal pad of yours vias? How many? What's the via's dia? What is the pcb thickness? If the the pcb has a "ground layer" connected (vias) to the pcb thermal pad, it could sometimes be a little bit tricky to solder. There are cases that a non via is needed, but still the heat transfer has to work. There are some calculation to be done of how the optimum stencil aperture shold be done. Sometimes; a lot of circles, 4 smaller qsuares, one resized rectangle etc. This vary a lot between different packages and most important; how much effect they will be used at. Remember that solder paste will shrink during reflow while it becomes solder - greatly and will fill up the via's somewhat.
All of this whitout to have the surrounding component "pads" to be lifted during reflow. That is the trick! Some manufactories state that at least 80% of the thermal pad must be solder with no more than 10% of voids to be able to get a proper heat transfer to the ground plane. We have produce the "so called" MLF, QFN, LLP and even TSSOP, QFP, SOP's with a thermal pad for some time now and it is not so easy. Some of the component manufactories have recommendations, but remember - there are often another company who actually does the encapsulation and they should provide you with their best recommendation along with the pcba designers input of how much heat sink is needed. Less is better for You.
We have in the past done several cross sections cut's on theese packages and temp cycling together with our customer, to see what the life time is, because of the "long life time working needs" of their pcba:s (Telecom) and we find out some interesting things, but this forum/thread is to short to expand this info and I am not sure if can, even of I am willing to share... The X-ray inspection is sometime not so easy to do, but is recommended.
BTW. Don't worry about the "copper like" toe-end of the surrounding therminals on the QFN-24 (or any QFN for that matter). You will almost Not likely be able to solder theese bastards 100%. And that is OK! We don't talk toe-fill on QFN packages!! Theese packagages can not and should Not be treaten as a regular leaded package as far as soldering. The inspection people of Yours should also be trained of how to do a proper solder acceptance of theese comps.
/Sincerly Ps. Sorry for my poor english & grammar. Ds.
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