Display Your Know How: Thermal Relief

Hi this is fairly easy to do within Orcad / Allegro. If using Allegro 17.2 and above you can change the pin properties so that the thermal relief is not needed, To do this select pins in the find filter then right click on the pin and choose property edit. Next assign the property of Dyn_Thermal_Con_Type and choose "none" for the layer you wish to remove the thermal from. Typically for thermal reliefs as in the picture there is a net associated with the pin so the net will still need to be connected on another layer.

This only works for dynamic planes. If using Static planes there wont be dynamic thermals connecting to the pin.

Best regards.

Thanks for pointing this out excellon1 ! Yes it is easy to do manipulate the thermal relief property of pins in OrCAD and Allegro. But the question we would like to focus on is: should we?

For electrical and thermal performance, the thermal type "Full_contact" is superior. This begs the question, why do we use thermal reliefs at all? Why not simply use full contact for shapes at all our pins?

And what PCB design choices can we make to achieve this (if required) without compromising the automated assembly process; any suggestions?

We use thermal reliefs to make it easier for the guy that has to rework the pcb.

Full contact would not be superior for source thermals. If using full contact a use case exists that heat will be drawn away from the pin if using wave soldering as part of the assembly process. To put it another way if the pin is the sink and it is connected directly to a moderate to large ground plane then that sink will suck heat away from the pin during soldering that can lead to poor soldering.

For things like SMD based connectors it is possible to embed small vias within the pads so as to act as the connection to ground. Usually this is done on multi-layer boards. This very much depends on the connector / use case etc. Follow the data sheet !. In this particular case the vias would be direct connect to the planes. The vias basically provide a low inductance path to ground but have minimal effect on the soldering process.

The best repeatable use case for Full_Contact use would be for vias.

Best Regards.

Hi digital1, your comment points exactly to the reason why thermal reliefs are required for pins. 

We propose that thermal reliefs benefit soldering only; not electrical performance.

The thermal benefit they bring to through-holes is to actually reduce thermal conductivity at each pin. Electrical current flow is also hindered by these narrow channels which can be important for high current circuits. 

If anyone can think of benefits of thermal reliefs other than for assembly, please let us know?