Hello Iain,

Story sounds familiar here, although not so catastrophic...
BGA material change might be the explanation: lead-free BGAs are 
designed to withstand hotter reflow profiles and use more thermally 
resistant materials like molding compounds and die attach glue. This has 
a direct impact on the warpage orientation and value.
Note also that your specific daisy-chained component has a small die 
size compared to the BGA dimensions, which is un favourable in terms of 
warpage (check with X-Ray).

Question: Was your refow profile a SnPb, SAC or an hybrid one?

Hope this helps.

A. Grivon

Braddock, Iain a écrit :
> 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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