Most everyone's business relies on optimizing process management so the most
reliable product is effected. If not, what's the point.

For plastic BGA's, coplanarity always was and still is an issue as is CTE in
some applications. Therefore, whatever contributes to a reliable solder
joint must be considered.

I am a proponent of applying only flux to effect reflow under specified
conditions. Short of that, I prefer using "hard" balls in all four corners
and reflowing using solder paste thus creating colums that are very
forgiving as compliant in light of coplanarity and CTE issues. When not
using hard balls, a little drop of epoxy, carefully thought out and applied,
does the same job. Not everyone enjoys this concept.

Now, we move on to CBGA's requiring paste applications though CTE issues now
become equally critical but in reverse in some cases. CCGA's are about as
good as you can get because all aforementioned issues are resolved but for
one. Where do you get these but from IBM.

uBGA's offer considerable advantages on both counts concering coplanarity
and CTE matching characteristics. Sure are small though.

Ah yes, about solder paste: How can you not use it but in low volume,
critical applications where flux will do?

Many have stated the obvious as using appropriate design rules. These may
consist of using 10% smaller pads than balls then designing stencil
apertures to overprint paste 20% - and on it goes notwithstanding aperture
shapes as square, diamond, or round. I get along with round very well.

Not to get carried away, but you know I will, but considerations for the
type connection using dogbones is another factor. Don't do right angles. Go
right to the connection and don't forget rework/repair/modification
requirements. Now it's time to say and on it goes.

MoonMan

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