Techies, need a little help here!

In preparation for assembling some development boards with an 1152 PBGA, I purchased some Practical Components dummy boards & devices double sided ENIG and 1156 PBGA'. I took the opportunity to get 63/37SnPb and leadfree (SAC 305) BGA's which are identical in sized. Alloy comfirmed by XRF.

The evaluation boards (PCB012) amongst others had footprints for 2 off 1156 devices, so populated 1 off each component per board. 

I asked TSL (UK) to provide a stencil with their recommendations and ended up with a 0.005" S/S laser cut with 0.0005" aperture reduction.

The paste used was our standard multicore 62/36/2 SnPbAg

The reflow profile peaked @ 232C with time above liquidus for, 1) SAC - 29 secs 2) SnPb - 74 secs.

Now comes the fun bit:

Post reflow the SAC BGA had a stand off of 0.4mm in the centre (sides) but less in the corners ie. curled down, and all 4 corners with solder bridges.
The 63/37 BGA had a stand off of ~ 0.5mm but this time at the 4 corners with slightly less in the middle (I crudely used feeler gauges!)

So it would appear that one BGA curled up and the other curled down....with different amounts of collapse.........hmm?

Anyone provide an explanation of this phenomenon?

CIA,
        Iain.



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