MCM footprint?

[Sam Silverstein <silver@xxxxxxxxx>]

Is there any documentation on recommended PCB design for the Etrax MCM 
beyond the
comment "Common design rules should be applied"?

We have been having problems soldering our current prototype board, 
which has 6 layers
(3 signal, 2 power, 1 ground) and has 2 BGA components (the ETRAX MCM 
4+16 plus
a smaller Xilinx FPGA). When all of the solder balls are melted, they 
are often unable to support the
weight of the MCM, and a rather messy collapse occurs. If we shorten the 
time of the reflow phase to avoid collapse, the large number of 
non-connected (NC) pads leads to uneven results. We are now 
reconsidering a redesign to make assembly more reliable.

So here are my questions for other board designers:

1. What pad dimensions do you use (land size, solder mask)? The ones on 
the MCM demonstrator board seems to have a diameter of 0.75mm (?) Our 
current ones are a bit larger than that.

2. How do you deal with non-connected pads? Have they been an issue in 
other designs?

3. We use solid ground and power planes under our MCM. Does this have 
any thermal consequences?

4. Has anyone else developed a DRS22/24 reflow profile for the MCM, and 
would be willing to share?

Thanks,
Sam