Alternative hot air soldering method

[spiyda]

Just sharing a method for soldering button LEDs onto aluminium boards  that seems to work pretty well.

I'd always found it hard to solder button LEDs to Ally board as even with low melt solder paste, some of the LEDs crap out at not much more than soldering temperatures.
( It depends on the LEDs themselves, I nneeded to use a particular wavelength only available from one supplier which seems to be particularly heat sensitive)

The problem was getting an even controlled heat so that the solder flowed, but the LEDs stay as cool as possible.

After several failed attempts and a lot of head scratching, I came up with this method which has 0% led Failures.

An MDF jig made in the shape of a box. The top cut just a little smaller than the pcb
One end of the box has an inlet to fit the hot air nozzle of the soldering station
the other end a couple of exhaust holes..

the temperature of the air needs to be set by experimentation, but  roughly double the soldering temperature.
(the top of the board is losing heat by convection as the bottom is absorbing heat by convection.)

In this case with the air set to 300C the boards solder perfectly, at 310, the LEDs start to fail!
my guess is that the board itself is at 310 - 20 / 2  C  or about 145C   (where 20 is ambient)

It takes a little while for the board to heat up, but because the excess hot air is being directed away (through the vents)
you can work on the board at its soldering temperature, correcting any soldiers etc.

pic below

[trev]

Do you think vapour phase would be a good solution for this problem? Seems a lot of metal to heat up but you get assured 230C temperature with no chance of overshoot?

Trev

[Gopher]

That would be the idea certainly.
With other processes you would expect a longer profile with a higher temperature in the early zones to get energy into the board , then you need to carefully set the peak temperature so that it doesn't overshoot or stay above liquidous too long. At the end you might expect to need some additional cooling or make one of your final zones a cooling zone rather than a heating one. That's obviously tricky to replicate in an oven with no zones/conveyor and little control, boards with lots of metal or ceramic PCB's will prove tricky in many small ovens not least because the parameters will be be very different from some boring FR4.

[spiyda]

Do you think vapour phase would be a good solution for this problem? Seems a lot of metal to heat up but you get assured 230C temperature with no chance of overshoot?

Trev

It would probably work, but my guess is it would be more expensive, less environmentally friendly, and would be harder to correct by hand during liquidus. It still has the disadvantage of holding the components at a higher temperature than the board while the thermal mass of the aluminium board absorbs the heat.

That is the disadvantage of applying heat to both sides of the board by convection ( by convection oven or vapour phase)  it will always raise the temperature of the LEDs before the board.
Applying it to the back only, means the components are the last to heat, always at a lower temperature than the board and therefore held at high temperature for less time.

The other one sided alternative, a hotplate doesn't work, my early experiments with an aluminium hotplate were a failure, the thermal mass of the hotplate makes it too difficult to control the temperature accurately enough. The rework station method can hold the board to within a degree or two.

[trev]

I thought with Vapour phase the only problem would be how long the LEDs were kept at 230C? They might reach temperature before the metal PCB? I did not think they would go beyond it?

[spiyda]

I thought with Vapour phase the only problem would be how long the LEDs were kept at 230C? They might reach temperature before the metal PCB? I did not think they would go beyond it?

Sorry, that's what I meant Trev,

Chris