Never! Sorry, Steve But I would expect the same approach, which mean; either 4-small squares with rounded corners or if the pitch are even smaller, do rounded apertures, but still make several of them.
The reason? Yes, You guessed right: Less solder and smaller smaller solder joints => less total area of voids, which mean a much more strength in the joint and much easier to solder such a cooling pad grounded via vias, with this approach!
The same goes for big cooling pads of let say Power transistors without any via etc. Always remember: There are/is not only the thickness of the solder joint, but the contact that the terminal-pad are making to the each other who makes a connection to a electrical & mechanical strength.
Remember to make sure that the total amount of paste is less than the area on the solder pad => less total solder joint than the pad. Something around 70-80 % solder in the joint.
That will give you the best solder and easiest way also to perform. Now there are some people who investigate this and they probably will come up with some figures about this and that's ok.
I will also show you some x-ray pictures taken through the layers, but there is one picture the SMTnet didn't let me to upload, and that is the 40mil.tif Never the less because of the size of the pictures you got an Idea how difficult it is to fulfilled the req. of getting the heat transfer to the ground pad via vias? On other words, get the vias filled to at least 50%.n That is enough!
These pictures shows only the vias filled with soldeer and how much, through different layers. I am sorry to say that I could not up-load the 40mil picture.
First, this QFN 36 + 1 and 0.5mm pad pitch, well...
Best Regards, Ps. I'm sorry for my poor English & grammar. Ds.
Ahh, an correction made 2010-02-04 I re-send my pictures, so here it is. Still > 65 % solder fill in the vias and > 50% is req. The pcb is 1.6 mm thick. In opposite order, Mika
Attachments:
reply »