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.

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

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?

Also for repair: is pin is full contact on all layer you have to heat up too much in order to remove solder and replace component: this can destroy the area.

A full contact on several layers brings to heat up too much pcb in order to remove/replace a component and bring to pcb destruction localy

HI JCTEYSSIER0, thank you also for your inputs! It seems that these reasons also relate to manufacturing performance more than electrical performance. Would you agree?

An interesting point that you have brought to mind, is if any designers ever consider blocking heat from travelling into a pcb due to electrically generated thermal output of components. It is conceivable that heat could be channelled away from the pcb by way of direct contact with the surface of a component with a heat sink for example. In that scenario, perhaps we can imagine the requirement to reduce thermal transfer through the pcb for heat dissipation.