reflowing a board

[flippineck]

I have a board which was built around 2000. It's a control board for a boiler.

It has some kind of microprocessor (size and format of a Z80), and a bunch of assorted discrete passive components and semiconductors - some surface mount, some with thru hole wire leads.

I'm suspecting the fault I have with it might well be due to dry solder joints - it works fine at high room temperatures especially if you give it a good hard slap, but when it's cold it misbehaves. Also a google search indicates many others have experienced faulty soldering on these particular boards.

Having looked good long and close at the track side, I fancy I can see a few dark hairline rings around a few of the component leads where they sit in their solder bobbles.

So anyway.. I had success fixing an Xbox a while ago by reflowing the board using a kind of non-corrosive liquid flux in a pen, and a glorified high temperature hairdryer. ISTR the webpage i was following instructed me to use the pen to get the flux right under the main chip on the xbox's pcb, then heat the whole board up with the blow gun until the solder remelted.

I was extremely sceptical that it'd work (all through my 70's & 80's childhood all I ever seemed to hear was 'heat kills semiconductors' 'always use a clip on heatsink when soldering' 'only use the briefest touch with the iron') so I thought the wholesale 'cooking' of the board would just kill it. But it was a dead board anyway so what was to lose. I was amazed, it worked..

So I was going to try doing the same with this boiler board but.. what would be the best flux to use, where to get it, does it need washing off, what with if so.... memory not so good these days lol.

 

[Arouse1973]

I personally would do the following. Get a small soldering iron about 18W will be fine some leaded 60/40 solder with built in flux. Get some solder wick and do the following. Re-solder the joint and then apply a small amount of solder to the iron tip and place the wick onto the same solder joint. Apply the iron to the wick and watch as the solder is absorbed into the wick. Don't leave the iron on for too long as this may damage the solder pads. Then re-solder the joint as normal, don't spend too long again doing this. Do not use a heat-sink as this can cause dull dry joint. Just be quick... Clean the joints with a flux cleaner designed for PCBs
Adam

 

[BGB]

AFAIK:
a lot of early semiconductors were heat-sensitive, so getting too hot could destroy them.

most modern semiconductors are a lot more resistant to heat, and are typically soldered by them putting the entire PCB in a specialized oven (heats to a certain temperature for a certain amount of time). so, ICs can get pretty hot, just not *too* hot, and not during operation or with voltages applied (temperature effects conduction, so overheating disrupts electrical properties, and can lead to self-destruction such as by excessive conduction).

though, this is not universal, and some (typically through hole) components seem to still be temperature sensitive (for example, getting too hot can apparently melt the plastic housing on LEDs and other things).


as for fixing the PCB, I would have thought it sufficient to touch the iron to the solder joints for long enough to melt/reflow the existing solder?...

then again, I am kind of new to electronics, only recently discovered (after making a mess and potentially wasting a piece of perfboard) that one can't solder onto nichrome (doesn't wet/stick, 1). also, the solder apparently keeps eating the soldering iron tips for some reason...

1: was trying to use it for connectors as it had fairly good mechanical properties (gauge and stiffness/springyness).