PCB holes under transformers

[eng1]

What is the purpose of the holes that are drilled on the PCB's under transformers?
They are present in almost any AC/DC charger that I have on hand.

I don't think that the holes are drilled to maximize air flow through the transformer, because most converters are mounted inside plastic boxes with no air flow.

These holes clearly do not provide insulation (primary to secondary sides of the transformer), they are drilled on copper-free areas that already meet insulation requirements.

I thought that they are drilled for mechanical concerns during assembly or wave soldering, but I could not find any useful documentation on the web. Does anyone have experience with this tecnique?

Thank you.

 

[Nigel Goodwin]

I've always presumed they were for improved heat dissipation, allowing the transformer to breath more.

 

[Sceadwian]

I don't think that the holes are drilled to maximize air flow through the transformer, because most converters are mounted inside plastic boxes with no air flow.

That's a mistaken assumption, if the wallwart is at room temperature and then a load is applied to it the transformer will heat and there will be airflow within the housing through simple convection, heat is dissipated through the case which provides the temperature differential for continued air flow within the adapter. The holes are probably there for just the reason Nigel suggested, to allow the cooler air under the PCB to circulate up around the transformer creating a circular flow, it takes very little air flow to help dissipate heat, even tiny air currents can increase thermal transfer drastically, and I'd guess those current would be relatively high if the adapter become extremely warm.