Dear All,

We're looking at using an LGA interconnect for one of our products.  It
will be a module, pre-manufactured on a FR4 or FR5 substrate (~1mm thick)
up to 40mm x 40mm in size and weighing approximately 10g.

The intention is for the OEM customer to SMT this module onto their
application board which is also FR4/prepreg based board.

We are considering about ~200 pads, with a square pad size of 0.8mm x
0.8mm.  The soldering is NSMD defined.  The paste mask we're looking at is
0.6mm^2 to 0.7mm^2.  The pitch of the pads is 1.4mm in both axes.

Would anyone here know of any design guideline docs (IPC?), and/or studies
done on the reliability/manufacturing of LGA based interconnects?.

Some areas that we're concerned with include:

1. Thermal expansion/stress with possibly different PCB substrates across
40mm. (relatively large).. - Are CGA's any better in this regard?, can
CGA's come in very low profile (<0.3mm?)

2. Reliability with application board flex/twist and cracking of pads?,
Possible board warpage problems?

3. Resistance to shock? - has anyone used underfill for LGA's?.

4. SMT machine placement accuracy? - Would anyone have any comments on any
expected problems with the placement accuracy of larger "parts", our module
would be up to 40mm x 40mm and SMT'd?

Any comments or suggestions would be very much appreciated, wishing you all
a Merry Christmas and the best in 2003!.

Seppo.

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