So what is the difference in using the stencil vs just using the hot iron? Thanks for the info
There’s too many reasons to list and i think your questions would have better and more detailed answers served by searching for dedicated articles and videos on the subject.
But short answer and most important reason is ball size consistency, if one (or more) of the balls was marginally smaller and the opposing pad/plane is high thermal mass the IC will be sucked down to the lowest common denominator causing bridges/shorts… Or the ball will simply float above the pad and not make proper contact.
The reason for using solder paste and stencils is because as the IC’s are small it’s easier but more importantly it puts the IC under far less stress as the pads on the IC don’t need to be wicked (just lightly tinned with an iron prior) which reduces the risk of damaging the coating arouns pads… which could again cause bridging/shorts if damaged / stripped back.