| i use 70 second for the small chip about 25x25, max 90 for the bigger chips about 35x35. avoid to go furter, too much high TAL will do the solder joint brittle.
i alaways take off the unleaded solder balls and put a new set of leaded ones. But just to avoid problem with unleaded soldering process. Making a good and reliable solder joint with unleaded balls is a very hard challenge. if you intend to use the original unleaded balls you have to bake the chips before the installation, if you fail to do so you will certainly have some internal delamination due to the higher temp. The moisture trapped inside the new chip will damage the chip itself, even if it appear perfect after the soldering process.
from the other hand, changing the original solder balls will add a thermal cycle to the chip, increasing the chance to damage it or to make it unreliable in the future.
bga and mcm rework it is a very hard thing...
good result is the combination of many many factors |