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I've come across a design where each pad was connected using 4 'bridges' to the GND copper layar. What stands behind these 'bridges'? Why not make a full copper layer with only solder-mask defining the pads?

enter image description here

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No, they are not bridges, they are pads with thermal relief.

A typical pad on a printed circuit board is only connected to a few narrow tracks. A pad directly connected to the copper pour is difficult to solder since the heat quickly leaks away from the pad into the copper pour due to high thermal conductivity of copper. A thermal connection restricts the heat flow, making the pad easier to solder.

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  • \$\begingroup\$ I agree, they are "thermal conductors," designed/intended to remove heat away from the socket. \$\endgroup\$ – Guill Nov 29 '14 at 2:36
  • \$\begingroup\$ Another thing to be aware of is that sometimes you don't want thermal relief. One example is small SMT connectors. Those are all too easy to tear off the PCB if you're using thermals. Another is (perhaps obviously!) power components. You WANT to conduct the heat from the pad to the copper. \$\endgroup\$ – Barleyman Oct 19 '16 at 15:00
  • \$\begingroup\$ Why they don't use one thermal relief path, instead of four? Is that a matter of stability? \$\endgroup\$ – Michel Keijzers Jun 20 at 14:51
  • \$\begingroup\$ Actually, you can use any number of thermal relief path you want. If you want a low impedance and rigid signal path, no thermal relief is best. But there will be more heat leaks away when soldering, So, you should make a decision between the two. \$\endgroup\$ – diverger Jun 23 at 2:12
  • \$\begingroup\$ BTW, but only one fat path may not 'rigid' as separating them into several narrow paths around the pad. \$\endgroup\$ – diverger Jun 23 at 2:20
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The "bridge" that you called are also called the "spoke". The below image is a thermal pattern with 4 spokes used.

enter image description here

If you hand solder a through hole chip ground pin directly connected to ground plane you will discover a problem; soldering becomes very difficult since the soldering iron heat is all sink by the via and ground plane. This problem becomes more serious with heavier copper plane such as two once or more, obviously it’s also depend on the area of the plane.

To resolve this issue thermal pattern are used in between via barrel and the copper pour; the thermal pattern reduces the total width of copper connected to the copper pour, reducing the thermal conductivity; thus reducing the thermal sink problem.

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  • \$\begingroup\$ Why do they use not just one spoke, instead of four? Or is that a matter of 'stability' ? \$\endgroup\$ – Michel Keijzers Jun 20 at 14:50

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