lead free soldering - Why are some joints shiny?

[LL0rd]

Hi,

some days ago, I tried to solder lead free the first time in my life. So I bought the lead free solder Amasan BF32-3. It's a Sn95Ag4Cu solder. Some people said about this solder, that it contains 4% of Silver (Ag), so the joints are shiny and indistinguishable to normal Sn60Pb40 joints.

Well, I tried the solder and got some verry different results. Some joints were shiny but the most of them are dull. I attached a photo of a perfboard, where I tried to solder. The row at the bottom is verry dull. The row in the middle is kind of shiny, there I used just a little bit of solder and the row on the top, I applied some flux to the pads, took the solder to my iron and just draw over the pads, like I would solder SMD ICs.

So I don't understand, why some joints are shiny and some are not. Well, nearly all of my joints are dull, but when I take some boars with through-hole components, their solder joints are shiny. So I took another look to some boards I have. And well. Some of their joints where shiny and some were dull.

Can someone tell me the reason, why some joints are dull?

[lapm]

Make sure you use correct temperature on soldering, would be my first guess on such variation in soldering..

[LL0rd]

I tried different temperatures on my station. The most consistant results I got with 330°C. But even with the same temperature, I get different results. And I have no clue why. The worst results I got with 370°C. The solder didn't flow. When I took the iron up, it sometimes drag some solder up.

[Niklas]

There might be a small difference on solderings of actual components depending on their surface treatment. The complete soldering that you look at is a mixture of the solder, the component's surface treatment and the solder pad's surface treatment. Many of the low price chinese manufacturers, like Seeed, Itead etc, use leaded solder as default alternative for surface treatment.

You should wash the solderings throughly to remove all the flux residues before doing any comparison. The older Rosin based flux with brown residues were easy to spot, but the transparent no-clean flux can cause a lot of misinterpretations as they can act as lenses, resulting in a distorted view. The transparent no-clean flux is also shiny in its appearance, so various thicknesses of it could also be an explanation. Heat a bit longer and more of the flux is burnt off, resulting in a less shiny result and so on.

[romantronixlab]

Im not an expert but have found to have similar result to yours. And I made an experiment.
I first soldered with the PCB at room temperature and found the same inconsistencies worst with A/C on at 21°C. But by preheating the board even with A/C the solder 85% of the time gave the same  results which were not dull.
So I concluded that the time/speed of cooling the solder can affect quality(looks) of the solder point.

If someone with more experience can provide more details, welcome they are.

[Psi]

If you stop feeding the joint solder then wait to long before removing the heat you can get issues. Try playing around with your timing, when you do things and for how long.

Sometimes i find a double solder method works better.
ie, Feed the joint 1/2 the solder, stop&wait 1 sec, then feed it the rest and remove the heat almost instantly.

[Niklas]

There could be two more things to consider, both somewhat described above:
- The ramp down temperature after the solder peak temperature. For reflow soldering this is specified to be less than 6'C per second if I remember it correct. A "slow" ramp down could cause the crystalline structure to change and become more brittle. I read that in a datasheet for SnPbAg (60/37/3) solder paste a couple of weeks ago, but this might also be valid here?

- The peak temperature could be above what is safe for the flux in the solder. If the flux is burnt off, then the risk of getting oxides in the soldering joint is increased. There is also a risk that the flux is boiled down to a thick residue that prevents the solder wetting. This could explain the bad flow at 370'C and also the 2 step procedure described by Psi. I have also experienced bad wetting of the tip of my soldering iron using lead free solder at high temperatures. The tip seems to have an invisible plastic film that prevents the solder wetting. The solder melts but it will not stick to the tip.

[nctnico]

In my experience you need to use the right (oversized) tip for a soldering job with the temperature set at 330 deg C. I get shiny lead free joints all the time. It's even hard to see the difference between leaded solder joints.

[Araho]

There might also be a difference in thermal layout of the board itself, ie. some of the pads may lead more/less heat away, leading to different results with same temperature on the iron.

[LL0rd]

Sometimes i find a double solder method works better.
ie, Feed the joint 1/2 the solder, stop&wait 1 sec, then feed it the rest and remove the heat almost instantly.

Thx, I'll try that out.

There could be two more things to consider, both somewhat described above:
- The ramp down temperature after the solder peak temperature. For reflow soldering this is specified to be less than 6'C per second if I remember it correct. A "slow" ramp down could cause the crystalline structure to change and become more brittle. I read that in a datasheet for SnPbAg (60/37/3) solder paste a couple of weeks ago, but this might also be valid here?

Yeah, well, the ramp down temperature can be a problem for me. Well, may be it's a piece of the puzzle.
When I solder, I normally use the Amtech nc-559 flux. The original, not the crap from china. It is no problem for me to breath the fumes of the flux. The nc-559 smell kind like the burned resin in the meditation class

When I solder lead free, I smell the burning flux. And it was like hell, so I had to cough. And richt after some joints, I put on the fan on my bench to get the fumes away to the ventilation system of my lab.

May be, I try the next time to solder without the fan cooling the joint.

[marshallh]

Flood the worksite with any other inert gas besides oxygen and you will see nice shiny joints.

[Macbeth]

Chuck the lead free in the bin. Return to 60/40 and don't look back. (If only!)

[LL0rd]

Chuck the lead free in the bin. Return to 60/40 and don't look back. (If only!)

I wish I could. But I'm from germany. And if I want to sell some of my soldered boards, I have to solder lead free. And at the moment, it looks just like shit.

I already tried to solder without any air flow of the ventilation or with a running preheater and fan to 100°C, but I didn't get the shiny joints, I want. The next step for me is to try the inert gas. In another lab I have an argon tank, so I try to solder with it.