Thank you for this excellent summary, Werner. You have educated us all.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier
Sent: Wednesday, August 02, 2006 5:23 PM
To: [log in to unmask]
Subject: Re: [TN] Barrel to Laminate Adhesion loss
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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