I'm using a dead Sonus amp as a practice piece so I can learn some reflow techniques. I have a hot plate made to preheat boards and a reflow hot air gun. I tried 140C preheat and 400C hot air and nothing comes close to liquifying the solder. Even my manual iron had difficulty melting the solder on most of the components. Do they use some really high-temp solder or is it just the run of the mill non-leaded stuff and I just need to persist? Suggestions please!
400C will toast FR4 - are you sure its actually 400? Perhaps its a 6 or 8 layer board which conduct heat away much better. Expect heating to take several minutes - if it melts faster than that you are risking crisping the FR4 permanently.
Also try placing the PCB on an insulating substrate (ceramic) and only using hot-air. A hot plate at 140C is also a heatsink if you think about it!
Also try placing the PCB on an insulating substrate (ceramic) and only using hot-air. A hot plate at 140C is also a heatsink if you think about it!
I can't be sure of the temp, I'm just going by the readout on the reflow unit, which may be optimistic. I didn't use any flux; I'll try that the second go-round.
You are trying to desolder a populated board? Flux isn't needed for that as everthing is already wetted in solder, but otherwise flux is essential. Try removing from the hotplate just before applying hot air.
Because a hotplate at 140C is simply a heatsink from the point of view of solder that melts at 230C.