So, this rotation is not the pad turning like a disk on the X-Y plane of the
board. Rather, it would be a rotation viewed from through a cross section on
the Z axis of the board with the PTH being the center of rotation. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner Engelmaier /*
Sent: Friday, February 22, 2008 5:01 PM
To: [log in to unmask]
Subject: Re: [TN] Inner Layer Separation / Land Rotation

Hi Kevin,
Above Tg the PCB resin becomes quasi-liquid, also still incompressible.
Thus, 
it will not expand in the z-direction like a plug, but squish [is this a 
technical term?] further out between PTHs while being held back by the PTHs,
which 
are quite strong and expand rather less than the PCB
[CTE(PCB):CTE(Cu)=350:18 
ppm/C]. Thus, there is much more movement at the pad periphery than where
the 
pad is attached to the barrel. Result: land rotation that is big enough to 
cause Cu plastic deformation and separation of the pad from the PCB resin as
the 
resin returns to its initial volume on cooling.



Werner



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