Hi Lou,

        We've had success measuring the solder ball height relative to each
other solder ball on the BGA.  Some math required in the system, but follows
the Jedec method.  The work was contracted to a test house where they used a
non-contact laser lead scanner.  Using a precision height indicator, we
checked the ball height from the substrate.  End result, we met the
coplanarity requirement with a little cupping due to the substrate.  Corner
points were higher than the center points.  Result of bi-material stackup,
heatsink and substrate.

        Hope that helps.

Glenn

Lou Hart <[log in to unmask]> Wrote:
|
| TechNetters, we've been working here with some large BGA and CGA packages.
One had over 800
| columns, for example.  What tools are available to measure the coplanarity
of the solder balls or
| columns on such devices?  And in the case of solder balls, which I presume
get relowed, how critical
| is the planarity of the chip-carrying substrate?  My thought is that if the
chip carrier is bowed, a
| solder ball attached to the center could be coplanar with the others, but
represent a lot of solder
| that could lead to shorting.
|
| And regarding the PCB itself, how do we measure its flatness with a
precision and accuracy to assure
| it is OK to use with a BGA?  The IPC test method I'm familiar with seemes
oriented to measure bow
| and twist in the range of 0.1% and up, which may not be good enough for BGA
work.
|
| Any comments would be appreciated.
|
| Lou Hart
| Quality Assurance Manager
| Compunetix  412-858-6184

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