BGA Underfill Alternative
Back when RoHS was just an idea, one of the big concerns was the fear of mechanical reliability with lead-free BGAs. A lot of study went into the solder mixes and it's now history, but there is still some concern. Especially in hi-shock prone applications. Sometimes I recommend that the engineer go along with the PCB and just hold the BGA on the board with finger pressure. But that's not terribly comfortable if the board is going into space or down in a tunnel or something like that.
More down to earth, underfill is one of the recommended additions to improve the mechanical strength. We recently helped to qualify a new type of BGA underfill. It's actually a little pad that comes in standard SMT tape and reel and is placed with standard SMT machines. No need to deal with messy goo. It's placed on the board around the BGA pad area and the BGA is placed on top of it. During reflow, it all sticks together and works like underfill. Cool!