In a message dated 05/02/0 11:01:28, [log in to unmask] writes:
>Ed Popielarski had a good idea of putting hi-temp solder balls in the
>4-corners of the device. These balls would have to be a smaller diameter so
>that you would get the collapse that you need for all the other balls, but
>would prevent the device from collapsing too far and "smushing" them flat.
>Problem I have with that is, why in the %$#@*! should have to jump through
>all these hoops to use these parts? Don't ya' think that TI would have
>discovered the issue before they released these parts for production?
>I know I'm preaching to the choir...just venting I guess....
>-Steve Gregory

Hi Steve, Paul, Phil, and All,
Actually, using high-melt solder balls as shims is not new, both Bell Labs
and TI (!?) have used this idea in the past. There is no need for these balls
to have a lower diameter--there is no advantage of having the other balls
"collapse"--controlled or otherwise.  The higher stand-off will give you
better reliability (roughly the suare of the diameter ratio), and the SJ
goemetry is more column rather than 'smushed' ball shape--also a plus
reliability-wise.
Now to the question of why TI does not put 4 such balls at the corners; how
many component manufacturers do you know that do not work under the
'through-it-over-the-wall principle? Phil mentions "the disastrous leadless
ceramic chip carriers"; I like to add the Alloy 42 TSOPs.
Werner Engelmaier

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