Use Indium-Tin solder (In52/Sn48) with regular solder tip?

[jusaca]

We want to try using Indium-Tin solder (In52/Sn48) for a sensistive device because of the low melting temperature of 118°C.

But can we just use our regular soldering tips? Or will we destroy them somehow? I know that you dissolve the iron coating of older tips pretty fast when used for lead-free soldering with high tin content, but how is the In52/Sn48 behaving? I could not find any information about what kind of tip I need to reliably solder this kind of solder.

[Kleinstein]

Unless the tips are really expensive I would not worry so much. The temperature setting would be relatively low (e.g. 150 C) and thus diffusion slowed down a lot. It would likely also be more like infrequent use, not a hot iron the whole day.

One may need an iron that also regulates at low temperatures (not all do). It may also need a different flux, that is active already at a lower temperature.

I have used pure indium with no problem with a normal tip - though only for a few joints.

[jusaca]

That sounds good, but would you use the tip for normal solder joints again after using it with indium? No idea what kind of alloy might be formed, when a little bit of indium left at the tip diffuses in the "normal" solder joints.

The solder we bought was paired with SMD291 flux, it activates at 140°C. So that sould be all right.

[Ian.M]

You can assume that every time you clean and re-tin your tip, the concentration of undesirable metals on it will drop by at least a factor of ten due to dilution (provided the solder you use to re-tin has a negligible concentration of the undesirable metals).  A two orders of magnitude drop is very possible if you clean thoroughly and re-tin generously.  A further factor of ten will be gained by dilution in the first normal alloy solder joint  you make afterwards.

Therefore if you thoroughly wipe and generously re-tin your tip three times, the residual indium concentration in the first joint you make afterwards will certainly be under 0.05% and probably entirely negligible, perhaps even undetectable by chemical analysis.

The same applies to going from Sn/Pb to Pb-free ROHS solder, so unless you need guaranteed traceable freedom from cross contamination, which demands separate work areas and separate color coded (or otherwise distinctly marked where color coding is impossible) tools and solder reels, unless you need to switch solder alloy compositions frequently during the course of your workday, one set of soldering tips and other tools will suffice.

[jusaca]

That is great, so no worries at all. Thanks!

[jpanhalt]

About 15 years ago, I was looking for a low-temperature alloy and had a discussion with tech service at Indium American.  He warned me that gold will migrate into the indium and can cause joint failure.   Nothing in writing.   I ended up using ordinary tin-lead solder.

Here is something related that may be of interest: https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1431927606066049

Edit:  Indium and its salts are also toxic.  Current PEL (8 hour exposure) is 100 ug/m^3. For comparison, lead is 50 ug/m^3 currently.  Much less is known about indium toxicity.  It might be a situation where a known danger is less dangerous than an unknown one.  Laws, of course, must be obeyed and may not really make sense.  My advice is to not treat indium as innocuous.

[T3sl4co1l]

Heh, interestingly enough, gold platings (at least as in ENIG; you should rarely if ever need to solder hard gold (>30u")?) dissolve almost instantaneously at normal soldering temperatures; I suppose they may well survive for a moment at lower temperatures, making that a possible failure mode.

The traditional solution is to tin and wash the joint -- just as you do with the tip as described above.  Same as rinsing in any other chemical process: fresh solder is your solvent and sponge and wick are your towels.

BTW, main downside to leaving indium or regular solder in the joint, is that it ends up frosty -- the alloy is far from eutectic so some part crystallizes out first, leaving a bumpy surface.  The solder also takes longer to solidify (eutectic hardens as soon as it crosses solidus temperature).  Uh, unless there's some creepage or corrosion problems with small amounts of indium in conventional solder, Idunno, but again as mentioned, it should be well enough washed away not to be a problem.

One possible gotcha is, standard rosin isn't very active at those low temperatures.  Low melt solder I think usually comes with, or suggests, a more reactive flux for this reason.  So just use that and you're fine.

Tim

[jusaca]

Thanks for the warning about toxicity, we will have to take that into account!

Regarding the dissolving of gold: As far as I know, that is actually the intended task of gold on ENIG pads: It forms an oxidation barrier and is then dissolving into the tin when soldering to allow the tin to connect with the nickel below the gold layer. The joint is only weakend, when the gold content is higher than 3%, so this is only a concern when soldering to massive gold (or thick gold layers), while the thin gold layer of ENIG pads is not a problem.

I'm not sure if it is the same with indium solder, but I could not find any information that this might be a problem.

[helius]

Some attention should be paid to the residual stress on the solder joints when using In52. "Indium-Tin alloys have low tensile strength." This says to me that creep may be an issue.

Also note that when using conductive epoxy to attach components, there does not need to be any heating at all.

Edit:  Indium and its salts are also toxic.  Current PEL (8 hour exposure) is 100 ug/m^3. For comparison, lead is 50 ug/m^3 currently.  Much less is known about indium toxicity.  It might be a situation where a known danger is less dangerous than an unknown one.  Laws, of course, must be obeyed and may not really make sense.  My advice is to not treat indium as innocuous.

Neither lead nor indium is normally present as a vapor during soldering operations, so the PEL is not directly relevant. What is known is that indium is poorly absorbed by the GI tract, so handling the material does not require greater precautions than handling lead.

Regarding the dissolving of gold: As far as I know, that is actually the intended task of gold on ENIG pads: It forms an oxidation barrier and is then dissolving into the tin when soldering to allow the tin to connect with the nickel below the gold layer. The joint is only weakend, when the gold content is higher than 3%, so this is only a concern when soldering to massive gold (or thick gold layers), while the thin gold layer of ENIG pads is not a problem.

That is true, ENIG is simply one method of preserving solderability when boards are stored for some time after production, like OSP or HASL. Issues arise with specific components such as PME ceramic capacitors, which can experience joint failure if the wrong alloy is used. This is the reason for silver content in some solders.

I'm not sure if it is the same with indium solder, but I could not find any information that this might be a problem.

Again it may be an issue for PME capacitors. Check with the datasheet.