Question: I have heard so many conflicting answers as to how many seconds it takes to make a solder joint. Can you tell me why it may take 1, 2, or 3 seconds to make a solder joint?
Answer: A solder joint is a metallurgical process; to make a sound solder joint there has to be a metallurgical reaction between the Tin in the solder and the basis material, which in our case is copper. This reaction is the creation of the intermetallic layer, which is identified as a compound of Cu3Sn, which transforms again to Cu6Sn5 through liquid state and solid state diffusion.
To make this metallurgical connection one must get the total area to be joined together with solder to at least 500F. Considering the time it takes the solder iron to heat the lead, the board and the solder it can take up to 1 to 3 seconds to do this, depending upon the total metallic mass of the solder joint to be created. For example a surface mount component would take less time to solder than a plated through hole, since a plated through hole has more metal to heat up.
The second part of this is whether or not the solder is a tin/lead or a lead-free material. If it is lead-free then the time to reflow will be longer still as the melting temperature of the solder is higher, so it will take more time to reach those temperature.
You may ask, how a solder pot is set at 500F and the solder iron is set at 650F to 700F? There is more mass in the solder pot which equates to more energy or more heat, therefore you can get away with lower temperatures as compared to a solder iron. A solder iron may be set at a higher temperature but it does not have the heat energy of the solder pot or a reflow oven. I call this Energy “Q”.
If you have a wave solder system, take a look at how long the board is in the wave during the soldering process. If the contact spot of the board going over the wave is 1 inch and the wave conveyor is set at 5 feet/minute, the board is in the solder for 1 second, which by the way is one of the ways which was used to determine how long it takes to make a solder joint.