esp-07s bottom exposed pad, how to use?

I have created an Arduino sketch for the ESP8266, which I have tested on a breakout for ESP-07.
Now I want to make a PCB for the ESP-07S (without built-in ceramic antenna) and when I studied the actual module received a few days ago I found that on the bottom side there is a square about 4x4 mm, which is not covered by the solder mask layer.
Instead it is gold covered so it seems like a connection pad.
Does anyone here know what to do with this "pad"?
I have downloaded the ESP-07S user's manual version 1.0 dated 2016-03-15 from Ai-Thinker and it says nothing about this exposed pad....

BosseB:
I have created an Arduino sketch for the ESP8266, which I have tested on a breakout for ESP-07.
Now I want to make a PCB for the ESP-07S (without built-in ceramic antenna) and when I studied the actual module received a few days ago I found that on the bottom side there is a square about 4x4 mm, which is not covered by the solder mask layer.
Instead it is gold covered so it seems like a connection pad.
Does anyone here know what to do with this "pad"?
I have downloaded the ESP-07S user's manual version 1.0 dated 2016-03-15 from Ai-Thinker and it says nothing about this exposed pad....

From the Google images, I think I see what you are looking at. It is a heat sink to dissipate the heat from the IC on the other side of the board. See the pattern of vias in the pad? They carry the heat from the pad/IC to the pad you are seeing. Be vary careful to not block air from moving around that part of the board.

The IC on the other side has a matching belly pad that is soldered to a copper pad under the chip. Removed with great difficulty.

Paul

Thanks for the explanation! Yes, there are 16 vias in a square pattern.

I wonder how people then utilize this module on their boards?
If you solder the pads on the sides to the suggested PCB pattern in the manual then it will be placed tightly against the PCB surface. So there is no air circulation possible.
Should I place a 5 mm diameter hole in the board at this location to allow air to ventilate through my own board and reach the pad?
I have a solid ground plane beneath the whole moduleā€¦

BosseB:
there are 16 vias in a square pattern.

You mean castellations.

Don't worry too much about heat.
An ESP module only consumes ~0.25watt. And it has two sides to dissipate that.
And the board you're putting it on also works as a heat conductor, assuming you use a ground flood-fill.
You could try a TO-220 silpad under the module.
I use tiny pieces of hot glue and a hot air rework station for that.
Leo..

OK, I cannot put anything between the module and the PCB surface because it will separate the pads and module so the solder would not be OK.
However, the module sits on a complete ground plane copper area so there will be heat transfer that way.
I guess we will have to see if any problems appear.

Normally you solder that exposed pad to a similarly exposed pad on the board you're soldering the module down to, which will be connected to the ground plane and act as a heatsink. If making board for hand assembly, you can put a plated through hole in the middle of the pad that it will be soldered to, and use that to solder it from the back.

However, for ESP8266, you don't really need that pad; lots of modules don't have it at all and work fine. It might be more important if it's crammed into a tiny space.

I put a 3 mm dia hole in the center of the 3.7x3.7 mm exposed square of the ESP-07S module just in case...