Solder paste not wetting at all

Question

I have an issue with solder paste, I would like to know its origin so I can fix the problem and solder by components properly.

I use a lead free Sn42/Bi57.6/Ag0.4 solder paste, manufactured by ChipQuick.

Here is the datasheet.

The syringe I'm using was opened three weeks ago and stored at ambient temperature until now. (I close it with the protective cap between each use, of course.)

I ran some tests before actually using it to solder components. I simply deposited several bits of it on a copper board, which I previously wiped with alcohol.

I have at my disposal a soldering oven (not some salvaged toaster, a real oven designed for this application).) However, it works like regular timer-ovens: set a time with one button, and set a temperature with another.

This is the process I used so far:

1. I put the board in the oven, at ambient temperature
2. I start the oven at 90°C and I wait one minute
3. I set it to 140°C and wait for two minutes
4. I set it for 180°C and wait for the solder paste to "melt" and get transformed to actual solder
5. Finally, just after the activation, I turn off the oven and open the door to allow a quick return to ambient temperature.

Problem is, I always end up with a nice sphere instead of observing spread solder across the copper face. Exactly like this :

I want to know if I am doing something wrong during the process, or if this is linked to the storage conditions of the solder. Note that the manufacturer indicates a good "shelf life" but I don't know if it implies that the container should not be opened.

Answer 1

My guess would be that the copper board is not being given enough time to heat up. Due to its thermal mass the copper heats up much more slowly than the solder, and the solder melts before the board reaches the correct temperature. If you choose a smaller piece of copper, or an etched PCB with less copper on it, or leave the copper board in the reflow oven longer, the solder will eventually flow as expected. It's probably just that the large thermal mass can't heat up enough before the solder melts.

Answer 2

I've heard bad things about desktop ovens like this. They don't necessarily have the oomph to get the job done correctly. Saying "I turned knob X to temperature Y and waited Z minutes" does not mean that you have any idea what's happening to your board. The only reliable way of knowing would be to measure, maybe with a thermocouple in contact with the board (not perfect, but likely close enough).

You're obviously reaching an adequate temperature, because the solder is melting. It's certainly possible that your oven doesn't provide enough oomph to actually heat the board, and the solder is melting on top of cold componets. You may also be having problems with the flux. Either the flux in the paste is past it's prime, or the heating profile you're actually getting isn't giving the flux enough time to do its job, or the flux is activating too long before you bring your solder past the melting point, and a new oxidation layer is forming.

My advice is actually to forego no-lead solder unless there is some regulatory reason why you need to work with it. It's just harder to use- requires higher temperatures, which makes it tougher to come up with the right temp profile short of using real equipment. You may still have problems with lead, but probably less so.

Just as an aside, regardless of the nature of your oven, if it doesn't have heat-ramp-soak control with feedback, it isn't "meant for this purpose".

Update -- given the low temp nature of the Chipquik, the comments on no-lead solder don't apply. I think it might highlight the issue of premature and prolonged activation of flux though, if the oven is a very powerful one. No real way of telling whether its that or a cold board, though, without measuring. Temp crayons might shine some light on this.

Lead solder might actually help. Flux activation temps are better documented, so the soak profiles can be tweaked to slow things down before activation to avoid oxidation.

Answer 3

I suspect a problem with the copper board. What about the surface, is it bare copper only, or is it covered with tin? I would try a conventional soldering iron and lead rosin-core solder to do some test joints. If the solder does not flow well, there is something wrong with the board. The surface may be oxidized or the copper areas are to large to get heated. Wipping the board with pure alcohol does not remove copper oxide from the surface, very fine abrasive paper does.

Answer 4

There's a problem with your solder paste. May be the paste you are using is expired. If you mix it with flux it will look like mud and if put it in oven it will never melt. You can verify that paste with heat gun or solder iron. You will get the same results as in the oven. In the past I suffered the same problem and that was resolved by changing the solder paste. Hope this will help you 🙂