To Greg Kaskey and All

Greg,  I think you have mis-quoted (or mis-interpreted) my design 
manual on thermals, relative to multiple tie points on planes.

First, a good design rule is that every hole should have its own 
individual land and always connect the land with a trace that is less 
than 50% of the diameter of the land (except thermal connections).  
This holds true for inner and outer layers.

Reason:   On inner layers you don't want a large mass of copper 
working on the plated through holes.  I don't think anyone will argue 
that there is a definite mis-match of CTE's with glass fabric, copper 
and epoxy.  Glass fabric moves very little with temperature, copper 
moves much more and epoxy moves about 4 times more than the epoxy.  
These are all held together in a laminated form in the "X" and "Y" 
plane, with a total movement of about 15 PPM/degree C.   When inner 
layer lands are isolated from the plane, or connected with spokes, 
there is little influence of the large copper plane on those poor 
little plated through hole via connections.

All of the comments under this subject mentions thermals/thermal 
relief.  This term all by itself has a great meaning.  Thermal 
reliefs mean you don't intend the connection to sink heat away from 
the hole area.  Thermals imply the opposite.  For instance, when you 
design "heat pipes" under hot components, you are trying to sink the 
heat away from the component through the plane.  There is no concern 
for electrical integrity here.  However, when you have any concern 
for reliable electrical connections, the opposite is true and you 
need spokes.  

Further, during any soldering process you don't want heat sinked away 
from the hole area or you may have holes half filled with solder or 
cold solder connections.  During wave soldering, this would be a 
common occurance.  During SMT reflow, the temperature of the profile 
may be increased or the duration in the oven increased.  In either 
case, not a good "fix".

Outer Layers:  There are different reasons for relieving the lands on 
outer layers.  The "Z" axis expansion of the materials is significant 
due to the unrestrained epoxy.  After any soldering operation, the 
lands tend to lift around the outside peripheral of the land.  This 
is because your lands are small relative to the hole diameter and 
there is no large copper mass restricting the lifting of the lands.  
If there is a large copper area around the holes the copper will 
still lift due to the expansion of the epoxy, but with a high 
probability of cracking the copper at the knee of the hole.
Look at the outside surface of any of your boards and see if your 
holes all have their own individual lands (including power strips).  
If they don't, there is a good probability that you have designed in 
possible barrel cracks and latent failures.

Many of you have been discussing via holes in surface mount lands.  
Again, you have more copper on one side of the hole than the other.  
Land lifting is influenced more on one side of the land than the 
other.  You probably are getting away with this because these lands 
are so small.  However, an investigation is in order.

The above information is based on thousands of cross sections.  If 
you still do not agree, its your turn for the investigation. And by 
the way, pink ring can be a serious problem, depending on how bad it 
is.  Your comments came over the DesignerCouncil forum.  I am going 
to return this over the Technet forum also to see if we get a 
response from your vendors and assembly people.

Regards - Norm Einarson, PRINTED CIRCUIT TECHNOLOGY


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