50W iron here.

You may find this useful in case you connect pins you don't want:
http://www.ebay.com/itm/Soldering-Solder-Welding-Flux-Paste-ZJ-18-50g-US-Layout-/330910959743

But you need a broad tip to remove solder + best is a desoldering pump made from metal as well.

 

Here is the schematic diagram. Pdf.
I want to grab pin/legs 2,4,13 and 14.

Just to note pins 2 and 4 seem to be the power and ground pins, and these can be accesses *anywhere* they appear: then need not be poked at this chip but most anywhere. It's not unheard of to just power the entire board when programming (but your 78L05 needs about 7V to make 5V).

Also, pins 13 and 14 also travel to the temperature chip and may be easier to grab there.

If this is your board design then you may want to re spin it to provide some convenient access points: I oft use thru holes spaces 0.1" apart as a programming header: if you will be using pogo pins a smaller hole is desired to keep the pin from poking thru and making poor contact. You want it to center and hold the pin.

A 50W iron is way too powerful to do this board.

 

A 50W iron is way too powerful to do this board.

It doesnt rally matter how many watts it has, as long as it is temperature controlled.

 

The tqfp ic got heat up to 50 degree Celsius. Which is best to cool smd ic ? A heat sink or a thermoelectric cooler ?

 

The tqfp ic got heat up to 50 degree Celsius. Which is best to cool smd ic ? A heat sink or a thermoelectric cooler ?

Is that a 25 degree rise over a 25 degree ambient? That's nothing to worry about unless you intend to run your circuit in places where water will boil on its own.

With a device 85°C temperature range you are good up to a 60°C ambient.

 

ok but which is best to reduce heat ? Heatsink or TEC ?

 

Which is the best way to cut smd resistors and capacitors ?

 

Cut? Like cut in half? Or what do you actually mean?

 

I want to remove resistors but without hot solder machine.

 

That will most likely destroy the pads. You need to use soldering iron at least and some fresh solder.

 

I want to remove R3, R6, R7 , R8. I think i need to cut each R from centre.

 

Is there any way to stop Vdd going from atmega 8a to the motor driver circuit ?