Thermal stabilization of TH LED (layout recommendations)
I need a datasheet to give specific recommendations. 

I have run many thermal experiments with LEDs in the past couple of years.

First off you should understand that heat flows from  warmer to cooler.  You must first provide a thermal path from the LED to the peltier plate. This is best done on the component side.  Thermal vias should be avoided if there is area on the component side. 

There must be a thermal pad on the PCB that attaches to the case or lead(s) of the LED. 

This PCB thermal pad must be as large as reasonably possible. 

The distance between the LED and peltier plate must be as short as possible.

PCB copper thickness minimum 2 oz.





This is a TO-18 case with a thermal base plate.  







This would be my recommended PCB footprint for the above case with a 40 mm² peltier plate 







UPDATE
Now have a datasheet 


  According to your idea, Peltier is mounted next to the LED body and
  heat exchanger (radiator, whatever thermal mass) goes on top of that.
  Thus it is likely, that the field-of-view is covered. 


Your half view angle is 12° and the top of the case height and width = 4.7 mm.  Using simple right angle trig, the height of a device placed 2.5 mm from the center (0.15 mm from side of LED) of the LED would need to have a height that extended 11.4 mm over the top of the LED to interfere with the beam.  11.4 mm + 4.7 mm LED height = 16.1 mm (0.63") total height.

You need a bar of aluminum or copper to mount and apply pressure to the peltier plate to the PCB.  You may as well use a heatsink as the mounting plate with a base less than 16mm - 3.5mm (height of peltier) = 12 mm and anything over 12 mm grind the ends of the fins at a 12° angle.  

Then use a low profile fan like the Sunon MagLev HA series with mounted on the heatsink far enough away not to interfere with the LED beam.

  



2" wide x 2" deep x 0.5" height
Heatsink USA 2" extrusion
  

Sunon MagLev HA Series





  Why not do
  thermal vias, similar to the way it's done with some ICs with exposed
  pads? Route the heat to the opposite side of the board through vias
  and remove it there? I'll make some drawings of these ideas and post
  them tomorrow


Conductive thermal resistance for thermal vias is much higher than a copper plate (PCB layer).   See Table 1 and read Sec 3.2 in TI App Note 2020: Thermal Design By Insight, Not Hindsight 

Then when the thermal flux reaches the other side of the PCB then you still need the copper plate to spread the heat.





This is how well I found thermal vias to work with high power LEDs:








Now that I have seen the datasheet and your 10 mW LED only generates 0.06 watts of heat.  1.4 Vf  x 0.05 A = 0.07 W - 0.01 Optical radiant watts = 0.06 thermal watts.

If you add copper area to dissipate heat, it is likely to generate more of a problem than be of help.  I doubt you will be able to determine if the LED is on of off by touching the TO-18 case. This means there is very little temperature difference &Delta; between the case and ambient. With a small temperature &Delta; there will be less thermal conductivity and convection and it will take longer to reach thermal equilibrium. So you would have to wait longer (hours) to use the LED.  


As the temperature changes Vf changes.
As Vf changes the amount of heat generated changes.
As the amount of heat generated changes the temperature changes.
Go to step 1.


This is why the datasheet has no thermal information for a 10 mW LED. 


  I actually expect the ambient (outside the device) temperature to be
  within -30'C to +40'C (outdoors usage in whole Europe from Italy to
  Norway/Finland all year round)


Oh!  


  ThorLabs offers up to 100mW electrical  


Good.  You will need a heater as well as a cooler.
It is much easier to heat on a cold day than cool on a warm day.  



A temperature sensor would be a very good thing to have also.
Two temperature sensors (ambient and LEDcase) would be good to have.

Then you can use a micro-controller to turn peltier plate off &amp; on and adjust the fan speed based on the case sensor.  

Use the ambient sensor to choose the methodology (e.g heat or cool). At a specific ambient temperature the peltier and fan will net be needed. A case temperature only sensor will not give enough information.

The question is can you lower the junction temperature on a hot summer day to the same temperature as a cold winter day?  If not, you have to add another heat source. You could attach the heat source to the opposite side of the PCB and use thermal vias.  If needed, I would recommend using an inefficient, low efficacy, CoB LED as a heat source with a dimable LED driver (e.g. TI LM3414).  You may even be able to use a thermal foldback circuit with the driver to turn the CoB on when it gets too cold.  The LM3414 has a thermal foldback but you'd need a different thermistor. 



IMPORTANT

I forgot to mention this previously.  Do NOT use HASL plating on the PCB, use ENIG.  HASL is much too uneven for an effective thermal interface.  ENIG has very good planar characteristics (therefore thermal interface) . 



You might consider a small board for just the LED.  Qty 5 PCB with 4 oz copper, ENIG plating, 0.4 mm thickness (thin PCB if thermal vias are needed 1), 100mm x 75mm costs $171 for qty 5 at PCB Way, $5 ea at qty = 100.

1Reducing PCB thickness by 50% (1.6mm => 0.8 mm increases thermal conductance of vias by 2X.  1.6 mm => 0.4 mm = 4X improvement.