Fred,
I checked my numbers; I actually have a 926 and a 936 Hakko!
I pay little attention to my soldering irons, just expect them to work on demand!
NOTE I HAVE CHANGED THE TEMPERATURE RATINGS FROM F TO C!!
The heat output of an iron on any given solder joint depends on just two factors. Temperature of the tip, and time exposure on the joint. In photography, you could say the temperature of the tip is the aperture, and the time at the joint is exposure. It is the product of the two - temperature and exposure - which determines the heat flow to the component above the joint. So, you could set your iron to 350C and spend three seconds at the joint soldering, or you could set your iron to 450C and spend maybe one second at the joint. The result would be similar. And there's the rub; it's personal choice.
At the low end of the scale you risk the solder not wetting the component wire and pcb pad probably; at the higher end you risk overheating the solder and the component and boiling out all the flux, leaving a potentially dry joint. Like everything, it's a compromise....
I personally go for very high temperatures, and I wipe clean the tip on a wet sponge before each and every use. This way I'm in and out of the job in a second, and can move quickly along the pcb. I always add that tiny bit of extra solder to the joint to ensure flux is visible at the edges of the joint when I'm finished; this means there is still flux left over after the job and it's unlikely to be a dry joint. Because this intense heat is applied so quickly, the number of calories injected into the joint is actually quite small, and consequently it quickly dissipates in the thermal mass of component lead and pcb pad. Therefore there is little danger of damaging a sensitive component.
Twenty years ago active devices like transistors were not as robust as today. Most semis are specced to withstand 300C for 10 seconds; I solder around 400C with 175C solder, and am normally done after no more than two seconds. I never use additional flux; it's in the solder anyway and all my component and pad surfaces are normally clean and highly solderable. This does not endanger anything, and in any event I set reasonably long leads when I fit transistors, further assisting with dissipation.
There are courses on High Reliability soldering conducted by industry and the military. As a young Officer I once served in the Australian Army School of Signals where they ran a HRS course. Like any technology, there are a few tricks but most of it is common sense, and a moderate approach. I'm very impressed with both my Hakkos, nice irons, and would never use anything else now.
Most industry soldering, except on the very highly priced gear from the big boys, is wave soldered on a machine, often using surface mount components, which are completely immersed for a brief second or so in solder during the operation. This operation is fascinating to watch if you ever get the chance; a standing wave of molten solder is set up by a solder pump in a small container. The position of the wave is adjusted so it is stationary, and a prescribed height, although the solder is moving through it at considerable speed. The solder bath is constantly monitored for flux content to minimize dry joints. The pcb is then inverted and passed across the crest of the wave, leaving a marvellous job. Very elegant!). Now this approach is quite traumatic, so components have had to get tougher to withstand it. The problem with wave soldering is that human inspection is really required to ensure complete reliability, partially offsetting the benefit of using a machine anyway. The other problem with SMD technology is heat generation, which can be so severe - as these components are tiny - that the solder can come unstuck! In the design phase a heat profile of the board is calculated, and the boundary temperatures of every joint are computed. As long as they do not exceed about 100C all is well.
Hope this gives you some pointers. Nothing can really help you divine a good solder joint except experience, but, OTOH, a bit of experience does the job every time!
Cheers,
Hugh
