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 sizng 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. Ther 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