Hi Richard & All,
Actually, the higher occurrances of hole wall separation and trace separation 
are not so much caused by more lateral stresses, but the different thermal 
and mechanical properties of SAC solders.
To understand what is happening, one really needs to understand what the 
whole damage process causing hole wall separation [sometimes also called resin 
recession] and inner-layer separation [also called post separation or trace 
separation.
(1) On heating the resin expands—because of the glass fiber reinforcement it 
is prevented to expand in x/y and thus expands in z at nearly 3-times its 
thermal expansion without the glass reinforcement]; there is an exception to this—
the resin also tries to expand into the volume occupied by the PTH, and is of 
course, to some degree at least, prevented from doing so by the Cu barrel 
plating. 
Thin Cu plating and/or large separation between inner-layer land [serving as 
anchors] encourage larger bulging inwards of the Cu with plastic deformation 
of the Cu.
(2) On cooling, the resin contracts to its former volume—if the Cu is 
plastically deformed to a significant degree, it cannot do so: the result=>hole wall 
separation, and near the surface in connection with land rotation inner-layer 
separation.
(3) For SnPb solders the Solidus is 183C; thus, Solidus - Tg (183-170C)~13C.
For SAC solders the solidus is 217C; thus, Solidus - Tg (217-170C)~47C.
The consequence is that the delta-T after solder solidification is about 
4-times larger for SAC-solders, over which the high above Tg-thermal contraction 
acts—and this also holds the Cu back to its former position: the result =>more 
hole wall separation, and inner-layer separation.

Pretty straight-forward when you think about.

Werner

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