I am no expert on this subject, but having been faced with the need to
define the dimensions of the thermal ties for a complex motherboard
recently, I have some thoughts on the subject that may be of interest.

It seems to me somewhat confusing why the size of the thermal ties
(sometimes called spokes, but referred to as webs in IPC-D-275) depends
on the calculation of the minimum land size.  Perhaps this deals with how
much heat is available during wave soldering in the area of the hole,
which may be related to the size of the pin in the hole.  But, while I
can clearly see that the minimum land size must be dependent on the
manufacturing level and class, I don't know why the thermal webs are
dependent on this.  Just because a manufacturer is able to hold to
tighter tolerances on annular rings and does not require such a large
standard fabrication allowance does not seem to have any bearing on the
required web size.  I also noted that the annular ring size used in the
example in IPC-D-275 does not match any entry in the associated table
(5-11).  I assumed this was an oversight and used the appropriate value
in the table.

Putting these questions aside, I used the equation given to calculate the
required web size.

Next, I assumed that the purpose for IPC-D-275 reducing the web size if
the land is larger than that calculated as the minimum, is that the
larger land will conduct more heat and require more thermal resistance in
the web.  This requires a thinner web.

Then I had to consider how many planes were tied to each pin in question.
 I am not quite sure why the limitation in IPC-D-275 is not related to
the minimum land size (or hole size) AND the number of planes.  But,
again, following the stated limitation, I found several cases where this
item was the limiting factor for the web size.  (The statement is as
follows:  "Total cumulative copper web for all layers in any
plated-through hole should not exceed 4.06 mm [0.160 in] for 1 oz copper
or 2.03 mm [0.080 in] for 2 oz copper.")  For example, in the case of
having 4 planes tied to a Ground, and using 2 oz copper, this requires
webs no larger than 0.080/4/4 = 0.005 in when 4 webs/pad are used.

Then I had to consider the current requirement of each pin.
 Unfortunately, I have not been able to get a good answer to how wide
each web must be to carry the required current.  You can use the standard
charts (such as found in MIL-STD-275E) used for "long" traces, but this
does not appear to be correct in the case of thermal webs for the
following reason:  webs are extremely short (say 0.010 in) and are
surrounded by large heat sinks on either end - this should limit the
heating in the line caused by the current flow.  However, I found that if
connector pinouts are chosen properly, meaning that adequate power and
ground pins are allocated, the line widths as calculated in MIL-STD-275E
are not that difficult to meet anyway.  For example: for a case of 10
Amps using 6 pins and using 4 webs per pin - this is 10/6/4 = 0.42
Amps/web which requires approximately 0.006 in. web using 2 oz. copper.

The final consideration is the thermal clearance area.  Several
individuals have stated that this area cannot be less than 0.010 in.
based on IPC-D-275 paragraph 5.3.2.4.  The problem here is that when
using many of the modern high density connectors, even when using the
tighter manufacturing levels a 0.010 clearance basically removes all
copper through the connector.  This means that if you have a group of
power pins on a high density connector (such as on a power supply), the
copper path into the inner pins is removed (except for that going through
the thermal webs and actual plated holes and onto the next pin).  While
this may generally be acceptable, there can be cases where the current
requirements dictate more copper.  One of our vendors, Tyco, has stated
that they do not have a problem with a 0.005 in. thermal clearance, even
on our 2 oz. planes.  From a thermal standpoint, it would appear that the
width of the thermal clearance has little bearing.

Hope these thoughts help rather than confuse.

John Urry
L3 Communications
Salt Lake City, UT