JoAnn,
Here's the word from the IPC Designer's Council.
-----Original Message-----
Jay,
Thermal connections to a plane are described in IPC-D-275 paragraph 5.3.2.3
Thermal Relief in Conductor Planes. Basically it starts with the proper
sizing of the component hole. Once you have the maximum size hole, you must
calculate the minimum size land for that hole (IPC-D-275 para 5.3.2.1). You
use the minimum land size to calculate the number and size of the thermal
connections to the ground. The standard states that the total width of all
web connections, to the plane, are equal to 60% of the minimum land size.
The example in the standard:
LAND SIZE CALCULATION
Maximum hole size = 1.0 [0.040]
Annular ring = 2 x 0.13 [0.005]
= 0.26 [0.010]
Fabrication allowance = 0.25 [0.010]
Minimum land size = 1.0 [0.040] + 0.26 [0.010] + 0.25 [0.010] = 1.51
[0.060] dia.
THERMAL RELIEF CALCULATION
Total thermal width = 60% of land size
= 0.6 x 1.51 [0.060]
= 0.91 [0.036
ORIGINAL WEB SIZE CALCULATION
2-web width = one-half of total thermal width
= 0.5 x 0.91 [0.036]
= 0.45 [0.018]
3-web width = one-third of total thermal width
= 0.33 x 0.91 [0.036]
= 0.30 [0.012]
4-web width = one-fourth total thermal width
= 0.25 x 0.91 [0.036]
= 0.23 [0.009]
If the land size used, in the design is greater than the minimum value
calculated above, then the percentage difference between the land diameters
must be subtracted from the total web width calculation. Remember, that the
more copper that you have, the more heat sinking occurs. There is also a
qualifier for multiple plane connections in a single pad stack.
The next consideration is the clearance area between the land and the
copper plane. Paragraph 5.3.2.4 Clearance Areas in Planes, points to an
illustration that clearly states that the clearance area should not be less
than 0.25 [0.010]. Less than this amount results in inspection and process
difficulties at fabrication. The length of the thermal connection has a
lessened affect on thermal resistance than does the number of parallel
connections. The principle of thermal resistance is usually invalidated
when one states that all thermal connections contain X number of webs,
0.015 wide. Thermal connections are more than just a picture, but a true
calculation, as I have shown.
Thermal isolation is usually associated with through hole component
connections. They are primarily used to prevent cold solder joints during
wave soldering. However, there was some discussion, a while back, about the
necessity of thermal connections for vias and SMT component connections.
The opinion was that if an SMT device is connected to a plane, that you
should still use a thermal connection, since a non-thermal connection will
sink off too much heat during a component removal operation. The thermal
connection has little affect at component on-sertion.
Hopefully this clarifies your question.
Regards,
Gary Ferrari
Executive Director
IPC Designers Council
Jay DeKing
EMA Mid-Atlantic, Inc.
(716)334-2970 voice
(716)334-3481 fax
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