Here's a true story regarding the service temperature of solder joints.
Yesterday, as I left work, I noticed that it felt really hot outside. I got in my car and cranked up the AC. I then noticed that my steering wheel was burning my hands. Next, I noticed the most alarming revelation; my new iPod (which was left in the car) was playing a different set of music than I had programmed it to play when I had got in the car that morning. Then, I saw a warning that the device was overheated and would not work properly until it had cooled down. I put the iPod in the path of the cool air blowing out of the vents and drove off.
Being in the solder industry, I spent the rest of my drive thinking about the solder joints inside of my device, and what effects they may have seen. Firstly, how hot was it in the car? The weather report said that it was about 85°F outside, and, comparing that to some studies that have been done to determine how hot the inside of a car gets in the sun, I estimated that the temperature inside the car was near but probably not exceeding 120°F.
Converting 120°F to Celsius, the more commonly used unit in our industry, I came up with approximately 50°C. Breathing a little sigh of relief, I was satisfied that this temperature was well below the melting temperatures of the alloys that were likely used on the circuit boards inside my iPod. Typical tin-bismuth alloys melt around 140°C, tin-lead alloys around 183°C, and tin-silver-copper (SAC) alloys around 217°C. I don't know what alloys are inside the iPod, but I'd bet that none melt lower than these common alloys, so I was confident that no solder joints had melted.
My next thought went to the solders themselves. One concern was the intermetallic layers at the interfaces between the solder joints and the pads. Heating, over time, can lead to increased intermetallic growth. One afternoon in the sun probably wasn't enough to greatly change the solder joints. As for the bulk solder joints, I've been told that one rule is that you need to keep the operating temperature below 90% of the alloy solidus temperature, in Kelvin units. When the solder is heated above this temperature for any period of time, the solder joint can begin to change in structure, becoming more brittle. In all of the alloy cases I've described above, I was still safe.
The last concern I had was with heat generated by the device itself. Luckily, my car shuts off power to the USB port when the car is not running, so the iPod had not been charging all afternoon. Excessive charging could have led to internal heat from the battery. Also, during the use of many electronics devices, components heat up. Many are designed so that the heat can be transferred away from the component, through the solder joint, and into the circuit board or surroundings. This prevents the component from overheating and burning out long before their time. I made a mistake here by continuing to listen to my music while the iPod cooled down. I should have turned it off so that no internal heat was continuing to be generated by the iPod itself as I was trying to cool it from the environmental heat it had absorbed.
So far my iPod seems to be working fine and, as I discuss above, I don't believe I'm in much danger after just one exposure to these temperature extremes. I am no longer going to keep it in my car when I'm not driving (one more thing to carry, but worth the hassle). For other good advice about keeping an iPod cool, please read this article.