How to tell if chips overheat while soldering

It's all about knowing how to solder.

You should use a temperature controlled soldering station. Set temperature to 300°C and you'll be able to heat each pin of DIP chip for 2-3 seconds without damaging the chip. Guaranteed.

See PDF by Adafruit.

First of all, it's unlikely that you will destroy your parts even if you hold the iron on the pins for a long time (over 5 seconds). Components are designed to withstand a good bit of heat and time (sometimes minutes) during mass production. However, an iron tip is usually hotter than the temperatures used in factory production, so there is risk of damaging a part if you hold the iron on it too long. Some spec sheets will give limits on soldering time, but these are usually aimed at mass production temperatures, not a hand soldering iron.

I, like many others here, have never fried a part from overheating it. But if you're working with a particularly sensitive part, there are a few techniques you can use to mitigate the risk of thermal damage (some CMOS or MOSFETs are known to be more easily damaged... CMOS technology is used in some digital logic ICs, for instance).

• Solder alternate pins, or give the chip time to cool off between pin
• Attach a thermal sink in between the chip and the solder joint to draw heat away before it damages the part. Note that this can make soldering more difficult as it will be harder to heat the actual joint.
• Use sockets (like you're doing already).
• Use a lower temperature (make sure to have a tip in good condition, and a little blob of solder already on the tip to help transfer heat--"tinning" the tip).

In general, though, if you spend no more than 2-3 seconds on the joint, you'll probably be OK. And for large wires, connectors, or ground planes, you may need to spend much more time on the joint to allow the solder to thoroughly wick and bond to all the surfaces. For joints with a lot of metal, try to keep the soldering time below 5-10s.

In terms of temperature, if you have an adjustable temperature iron, stay below 650°F for leaded solder, and 750°F for lead free. I will occasionally set the temperature to 800°F for large components or ground planes. You're better off finishing a joint in 5-10 seconds at a higher temperature than holding the heat on for much longer at a lower temperature. Long soldering times give the heat time to spread to the components where it can do damage.

How to tell if there's damage? If the component changes color, that's a bad sign. If the board browns or chars, that's also bad. The unfortunate reality is that you may do latent damage to a component by heating it too long and too hot. For example, a chip may initially work, but fail early, or some of its specs may be slightly off from their original design.

As an aside: Why is an iron tip hotter than temperatures used in mass production (and the temperatures noted in spec sheets)? During mass production, the entire board is usually heated, so the pcb, IC and joint are all at the same temperature. When you hand solder, the pcb and IC are much colder than the joint, and are constantly drawing heat away from the joint. Your iron must be much higher than the melting point of the solder to compete against these heat sinks.

I have been soldering for a couple of years now, and I have not damaged a chip yet. I am usually a bit careless about how hot the chips get (which bad practice). I guess my point is not to worry about it too much, as long as you are not applying excessive heat to a pin for too long a period.