Slightly remedial question, perhaps, but I've never used BGA devices, and we're only just moving from hand assembly to automated for our stuff:
How does the solder melt in BGA placement? Does the board have to be heated sufficiently to melt it, or the chip, or both? I can't see how either could be done without damaging stuff - delaminating the board or buggering the IC, but I guess it is done millions of times around the world every day so someone's obviously figured it out ;-P
-- Nobby Anderson
In temperature controlled oven.
Both, I guess. Sometimes, it helps to direct some hot air under the chip, but not too much to blow the solder balls away. Designing BGA re-oven is an art by itself.
The "solder balls" on a BGA so not melt. They are made of a solder with a higher melting temperature than 60/40 solder.
The solder pads on the board have dabs of solder on them. The BGA is placed on these blobs and has to align more or less perfectly. The whole thing is run though a temperature profile which will melt the dabs of solder but not the balls on the BGA. The temp ramps up, stays a while and then ramps down. The BGA then self aligns itself with the solder pads as it floats on the molten solder.
You really have to control the temperature carefully. Otherwise if the balls melt, bah bye!
Al
It depends on the BGA. One BGA I worked with contains just regular solder, so much solder that you can't have additional solder on the board. They do melt to form a flat contact surface.
The BGAs I have worked with - and those I have only considered - are all meant to be soldered like this, i.e. all balls do get liquid during reflow. This also does the final placement adjustment (surface tension), or, well, misplacement (I once had that... :-). I have posted some experience about vias inside the pads, they can be looked up, I guess.
To those without experience and avoiding the BGAs fearing trouble: not really such a pain, I have had more soldering issues with 0.5mm spaced TQFPs than with 256 or 352 balled 1.27 mm pitch BGAs. I have not done 0.8mm BGAs yet, though, not to speak about 0.5mm ones.
For prototyping you can use just about any kitchen oven provided you are armed with an IR thermometer to monitor the temperature. I have done it dozens of times. If the 220 C are reached within 5-6 minutes (mostly due to IR from the top), things should be fine. Be careful when taking the board out of the oven, everything is still liquid. I use a wooden plate to which I fix the preassembled board; once I lightly hit the plate in an obstacle while taking it out and oh boy what a mess of slid BGAs, TSSOPs and stuff I had! Took a complete reflow and remake of all balls...
Dimiter
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