Soldering sponges: the "thermal shock" myth

[MaximRecoil]

I believe you mentioned, before, that you did thru hole soldering at your work. Extra flux is not needed. You need to feed so much solder wire into the joint, the flux will be there, anyway. With SMD soldering, fluxing the joint and then adding solder is commonplace in industry. Google "wave soldering."

At work I mainly did through-hole soldering (terminal blocks) on PCBs that were already populated with SMDs when I got them (the huge Panasonic SMT PnP machine did that). However, I also did rework on the SMDs that failed in the Hewlett-Packard HP3070 bed-of-nails tester or that were crooked. To solder an SMD in place, I put a small amount of solder on one of the pads, placed the SMD with tweezers and tacked one of its legs to the pad with my iron, soldered the other legs one at a time (feeding solder wire to each one, usually 0.015" diameter), then reflowed the tacked joint.

One thing I can see separate flux coming in handy for is soldering a wire to a surface-mount pad. A wire doesn't like to stay in place on its own, and unlike an SMD there's no way to tack one end of it in place to keep the end you're soldering from moving, unless it's a particularly stiff wire. I might buy some flux just for doing stuff like that.

By the way, most through-hole soldering at the place I worked was done in the wave; I have no idea why they had us solder in all those terminal blocks manually (I did thousands of them every night). No one I asked there knew either.

And sometimes you simply need flux. Try installing a BGA without flux.

As far as I know, that's not a hand-soldering procedure. That's an oven procedure.

[MaximRecoil]

And sometimes you simply need flux. Try installing a BGA without flux.

As far as I know, that's not a hand-soldering procedure. That's an oven procedure.

Hot air station. I consider anything soldered with hands, not fully automated, hand soldering, no matter direct or convection heat transfer.

By "hand-soldering" I meant with a soldering iron. Doing it with a heat gun could be considered hand-soldering I suppose, though it's effectively the same thing as doing it in a reflow oven.

[KL27x]

To solder an SMD in place, I put a small amount of solder on one of the pads, placed the SMD with tweezers and tacked one of its legs to the pad with my iron, soldered the other legs one at a time (feeding solder wire to each one, usually 0.015" diameter), then reflowed the tacked joint.

There are a lot of things in industry that are done, simply because they work, so don't F with it. Your terminal block example, for instance. Perhaps there was a good reason (thermal mass of the legs too high?) Or maybe they just need something to keep you guys busy in downtime for when they really need you?

In practice, if you used the same flux that is in the wire. And you preloaded the tip with solder. And you let the flux IN that solder drop off  and/or burn off onto the edge of tip where chrome plating starts. Then you touch the crusty solder to the pads which are fluxed with the appropriate amount of flux and drag solder it.... the end result is same as point to point soldering with solder-wire, alone.

Perhaps due to being high end test equipment, they are concerned with excess flux residue, if too much liquid flux were to be applied. Because in frequency approaching GHz, rosin/resin residue can cause unintended effects. In your home projects, this is probably not a concern. I've watched NASA soldering video where not only tip of the iron must be cleaned before each and every joint, but also the solder wire must be cleaned with a GD Kimwipe before making each joint. Am I going to do that for all my soldering?

[MaximRecoil]

Or maybe they just need something to keep you guys busy in downtime for when they really need you?

Maybe. There were only two to four people there at any one time doing what I did, usually two, and the job was called "inspecting". We got the boards after they came out of the Panasonic PnP machine and had to visually inspect each solder joint. If we saw any problems, such as crooked SMDs, or SMDs with a contact that didn't get soldered, we fixed it. Then we had to place and solder in all of the terminal blocks (like these). That part always seemed like an afterthought, given that the name of the job was "inspecting". Then we trimmed the terminal blocks' legs with flush cutters and sent the board to the HP3070. Depending on the workload, there might be someone assigned to the HP3070 for the night, or I might walk over and do the tests myself. The HP3070 would give a printout of any failures and their locations, and then we'd fix them manually, by replacing the SMD(s) that failed, and run the test again. It had to pass that test 100% before it could leave our area.

In practice, if you used the same flux that is in the wire. And you preloaded the tip with solder. And you let the flux IN that solder drop off  and/or burn off onto the edge of tip where chrome plating starts. Then you touch the crusty solder to the pads which are fluxed with the appropriate amount of flux and drag solder it.... the end result is same as point to point soldering with solder-wire, alone.

Yes, I know that the results of drag soldering can be perfectly fine; they didn't allow it though. They would always point out that we were building "life-saving equipment" to justify their fussiness about how things were done.

I've watched NASA soldering video where not only tip of the iron must be cleaned before each and every joint, but also the solder wire must be cleaned with a GD Kimwipe before making each joint. Am I going to do that for all my soldering?

They didn't take their fussiness that far. They were very concerned about ESD though. Our area, in a small corner of the factory near the tail-end of that Leviathan of a PnP machine, was the only area that worked on those particular "life-saving" boards. The area was enclosed in glass walls and temperature- and humidity-controlled. We wore ESD blue smocks, ESD wrist straps (along with a hand-cream type substance which was conductive, applied to the wrist beneath the conductive part of the strap), and ESD heel straps. When carrying PCBs out of that area, they had to be placed in ESD bags. Our Soldapullts were the ESD-safe variants of the original blue DS017, i.e., the black DS017LS (which is what I bought for use at home) and the chrome (or whatever that plating is) AS196.

They also had us attend ESD training classes every now and then.

[RobertHolcombe]

I prefer a wet sponge with a hole in it, as it provides a convenient location to wipe contaminants off the tip before wiping clean on the top surface of the sponge. In my experience metal wool has always trapped solder, components, component leads, etc, thus requiring more effort to maintain a clean tip - but 99% of the work I do is rework.

Question then for people who prefer wool; do you work on production of new boards/circuits, rework, or a mix of both, and how do you keep the wool from trapping debris?

[tautech]

Question then for people who prefer wool; do you work on production of new boards/circuits, rework, or a mix of both, and how do you keep the wool from trapping debris?

Mix of both but like you but mainly rework.
I never bothered with brass wool and use stainless wool in a short stout ceramic tumbler, easy to clean.
If you've ever tried to solder to stainless you'll know why I use it. 

[KL27x]

Bulk of my soldering is assembly, by volume. Of course if you do any assembly, it often works out that way. I also routinely do fine rework, usually when building prototype from scratch, or "optimizing" less than perfect pcb prototypes. I also simply build one-off stuff for my own use.

Debris goes into the wool, and it usually stays there. Every year, empty out the crud in the bottom of the holder, shake out the wool, and then put it back in. If removing a lot of parts, I'll wipe them off on a paper towel (or tap them off into a dish, if I want to save them), but the occasional 0603 ends up jabbed into the wool and occasionally comes back out on the tip. No big deal. Not building life-saving equipment in my lab. That said, wool does not really clean the tip very well, IMO, for how I solder. It is usually just a stop gap measure or to remove the odd part that gets stuck and is easier to dump in the wool than to re-tweezer. When I need to clean... brass tube and paper towel (dry). This is usually about once per session, often before I start. Works cold or hot. (The part that needs the deep cleaning is the chrome plated part that is not covered with solder.)

I have nothing against a wet sponge. It requires water, and that requires work and/or bench space for the bottle. Brass wool is always on.

[timb]

I've used the wet sponge for a long time and never had any issues with tips becoming bad early because of this. A few years ago I've moved to brass wool, and I like the convenience of not having to wet the sponge each time. The solder which is removed from the tip while cleaning simply falls down into the wool's container. And no corroded sponge holders anymore.

I keep a bottle of distilled water nearby for my sponge. I find that tap water causes the neck of the soldering iron and the base of the top to form white streaks after awhile. This is caused by the minerals in the water I imagine. (I'm on well water here and it's high in fluoride and other minerals. Great for preventing cavities, not so great for ultrasonic humidifiers and soldering iron sponges!)

I also use brass wool (or brass tribbles, as I like to call them). I use the sponge to wipe flux off the tip after a couple of joints; I use the brass a few times per session to keep the tip nicely tinned. If you apply a little solder to the tip then poke it in the brass and rotate it a few times, it comes out perfectly tinned.

The Hakko FX's base has both a sponge holder and a brass wool slot, so it's not a big deal.

[Someone]

I'm not a fan of brass wool because it seems to abbrasive to me.

Brass: 3 mohs
Steel: 4-4.5 mohs
Harddened steel: 7-8 mohs

http://www.jewelrynotes.com/the-mohs-scale-of-hardness-for-metals-why-it-is-important/

Wow, thats a radical oversimplification of abrasion. Just comparing hardness is almost worthless in this application, stainless steel works:

Question then for people who prefer wool; do you work on production of new boards/circuits, rework, or a mix of both, and how do you keep the wool from trapping debris?

Mix of both but like you but mainly rework.
I never bothered with brass wool and use stainless wool in a short stout ceramic tumbler, easy to clean.
If you've ever tried to solder to stainless you'll know why I use it. 

[nanofrog]

I'm not a fan of brass wool because it seems to abbrasive to me. I only use it if a tip is so crusty it cannot be cleaned with a sponge. IMHO the biggest disadvantage of a sponge is that it needs to be wet (I have a squeeze bottle with water for that purpose) and many people use it upside down so it falls apart quickly.

Do keep in mind, that although brass wool is harder than a damp sponge, it's not as hard, and therefore abrasive, as the steel plating on your soldering iron tip.    So it's not like you're putting your tips to a grinder. FWIW, tips will wear out faster with lead-free alloys IME. Not only due to the higher temps cracking the iron plating when using a sponge vs. brass wool (greater temp differential), but lead-free alloys also have a greater affinity for iron than lead based alloys.

Steel/stainless steel are approximately equal in hardness to the iron plating on your tips (depending on the specific alloy), but will also remove more stubborn deposits than brass wool or a damp sponge.

[nanofrog]

Question then for people who prefer wool; do you work on production of new boards/circuits, rework, or a mix of both, and how do you keep the wool from trapping debris?

Mostly rework (repairs), but some new (my own projects), as my gear is used purely in a hobbyist setting.

I find that the brass wool used makes a difference. For example, I find Hakko's wool to be a lot better than what I get from Weller (my soldering station is Weller), as it's coated with flux. I don't have definitive proof, but I don't think the Weller wool has flux on it. Or if it does, not nearly as much as Hakko's.

At any rate, when it works properly, I find solder balls & other debris falls to the bottom of the well/housing that contains it.  If not, it's an annoyance at best.

[stj]

FWIW, tips will wear out faster with lead-free alloys IME. Not only due to the higher temps cracking the iron plating when using a sponge vs. brass wool (greater temp differential), but lead-free alloys also have a greater affinity for iron than lead based alloys.

but lead free solder usually has less agressive flux in it than older solder formulations.
i have not used a lead-free solder that leaves burned flux residue on my iron yet, but most 60/40 formulations always did that.
IMO the biggest threat to the iron tip is large amounts of rosin in the flux.
before lead-free i used to go through tips every few months (production level use),
now tips last years with lead-free solder with no-clean fluxes.

[tautech]

FWIW, tips will wear out faster with lead-free alloys IME. Not only due to the higher temps cracking the iron plating when using a sponge vs. brass wool (greater temp differential), but lead-free alloys also have a greater affinity for iron than lead based alloys.

but lead free solder usually has less agressive flux in it than older solder formulations.
i have not used a lead-free solder that leaves burned flux residue on my iron yet, but most 60/40 formulations always did that.
IMO the biggest threat to the iron tip is large amounts of rosin in the flux.
before lead-free i used to go through tips every few months (production level use),
now tips last years with lead-free solder with no-clean fluxes.

There's the difference^^^ production where comparatively little flux is required as most everything is clean. Rework is an entirely different proposition where aggressive fluxes are often required.

Horses for courses. 

[MaximRecoil]

There's the difference^^^ production where comparatively little flux is required as most everything is clean. Rework is an entirely different proposition where aggressive fluxes are often required.

Horses for courses. 

I use the same 1.1% (P1) no-clean flux-core solder wire at home for rework (mostly on arcade PCBs from the 1980s, some of which are pretty nasty, especially CRT monitor chassis) that we used at work on brand new boards. I've never run into a situation where I needed a lot of and/or more aggressive flux. To replace a part or resolder a joint, I first desolder it (I don't use any flux to do that) and then the pads are inherently clean and lightly tinned after that, because they've been protected by solder fillets since they were manufactured; they accept new solder just as easily as brand new pads do.

I am curious though, about what sort of situations you encounter that require a lot of and/or aggressive flux.

[tautech]

There's the difference^^^ production where comparatively little flux is required as most everything is clean. Rework is an entirely different proposition where aggressive fluxes are often required.

Horses for courses. 

I use the same 1.1% (P1) no-clean flux-core solder wire at home for rework (mostly on arcade PCBs from the 1980s, some of which are pretty nasty, especially CRT monitor chassis) that we used at work on brand new boards. I've never run into a situation where I needed a lot of and/or more aggressive flux. To replace a part or resolder a joint, I first desolder it (I don't use any flux to do that) and then the pads are inherently clean and lightly tinned after that, because they've been protected by solder fillets since they were manufactured; they accept new solder just as easily as brand new pads do.

I am curious though, about what sort of situations you encounter that require a lot of and/or aggressive flux.

Yeah, I accept that the fillets DO protect pads but often a lead has degraded with age and if one leg of a component has needed to be pulled sometimes a neat fillet can be hard to make when resoldering.

When I've mentioned rework it's probably a little misleading. Electronics and things electrical have been a hobby for decades. From distant Scots heritage I will have a go at most things that many wouldn't give a second look and some are in poor state. Also part of being frugal by nature I have all manner of componentry and cabling tucked away for the day it might come in handy, some 2nd hand, some new, despite some being new it all degrades over time and gets harder to solder.
So there's joints that require solder with active fluxes in order to be properly soldered and they're just part of the arsenal of goodies I keep. The last new 1lb roll of solder I bought was ~20 yrs ago, some fancy RS stuff with 2% silver.......reserved for special occasions. Day to day solder is anything I can get my hands on for bugger all, formulation....well I don't care as long as it's not that lead free muck. 
Of my collection some have more aggressive fluxes than others .........how do I know ? Use one on shitty work......doesn't work.....use another til one cuts through the gunk and wets the joint properly.
Magnification while soldering is my friend. 

There's been times when I've wanted addition flux and to date I have only added fluxed solder to get flux to a joint but I grabbed a flux pen recently from a supplier so we'll see how that goes when I need some flux next. Might be the dawn of a new world. 

[nanofrog]

FWIW, tips will wear out faster with lead-free alloys IME. Not only due to the higher temps cracking the iron plating when using a sponge vs. brass wool (greater temp differential), but lead-free alloys also have a greater affinity for iron than lead based alloys.

but lead free solder usually has less aggressive flux in it than older solder formulations.
i have not used a lead-free solder that leaves burned flux residue on my iron yet, but most 60/40 formulations always did that.
IMO the biggest threat to the iron tip is large amounts of rosin in the flux.
before lead-free i used to go through tips every few months (production level use),
now tips last years with lead-free solder with no-clean fluxes.

Huh?  What does this have to do with a sponge v. brass or even a steel/stainless-steel wool?

As per the burnt/crispered flux, that tends to have to do with the operating temperature IME. FWIW, I've dealt with that as well as a clean tip, depending on operating temp (iron's recovery v. set temp has had a HUGE impact on this IME).

Regarding flux activity, what are you talking about?  I ask, as there's variations in the activity level of no-clean, organic (water-soluble), and of course, rosin-based fluxes. Please understand, I'm not trying to be an ass, but there' multiple variations in all formulations' activity, and they do matter IME. That said, it can be harder to determine what's-what regarding no-clean & water-soluble formulations in particular vs. traditional rosin based formulations.

Specifically, the newer fluxes designed for lead-free alloys have a shorter time to activate before the solder is molten & is looking for clean surfaces to join (higher temp to activate, and therefore a shorter time between cleaning off the oxides and joining the metallic surfaces = more aggressive vs. rosin formulations, even RA).

Granted, the actual cleanliness (oxidation) is both a function of both temp & time, but I've found that I prefer the results of rosin-based, including modified rosin based no-clean formulations, as a general rule.*

*Not only do I like the performance in regard to cleaning the oxides off of both new & older boards, but whether declared as rosin-based or modified-rosin no-clean,  I find they're much easier to clean than their synthetic no-clean counterparts. Water-soluble/organic fluxes are easy to clean of course, but they must be cleaned off, and within a particular amount of time before it causes problems (oxidizes traces & component leads). For a hobbyist, the latter is more trouble than it's worth IMHO.

[KL27x]

As per the burnt/crispered flux, that tends to have to do with the operating temperature IME. FWIW, I've dealt with that as well as a clean tip, depending on operating temp (iron's recovery v. set temp has had a HUGE impact on this IME).

Yup. Pretty much 2 main settings on my iron. Low: I'm either making something up as I go, where iron is on for hours but making joints only once per 20 minutes. High: I'm assembling a batch of PCB's, and the iron is either singing along making a joint every couple seconds or it's frying in the stand. This is perfectly fine. I'm done for the day by the time tip needs cleaning.

Steel wool, brass wool... I am pretty sure steel wool will scratch elemental iron. Who really knows what kind of iron is actually in your soldering iron tip? Cast irons* are harder than many steels and are commonly used in industry for extreme resistance to wear and oxidation in high temp environments. And contrary to "iron plating" which is the common way to refer to it, the iron skin on say Hakko tip is far from a plating. It's quite thick and I doubt it is plated onto the copper core. My guess would have been that pure copper is swaged into the iron jacket, just by looking at the remains of a Dremeled-in-half iron tip. Anyway, if it works, who cares?   

*For some reason, if iron has less than 0.3% carbon alloyed into it, it is called iron, or wrought iron. If it has between 0.3 and 2%, it is steel. And if it has >2% carbon, it is called cast iron. My numbers might be slightly off, but that's the gist.

** Found via google, random forum:

Yes you can soft solder cast iron. The best advise I can give you is keep everyting clean. A fresh ground surface works better than one that has sat for several days. Use a high tin alloy and the best flux you can get. Even heat with a propane torch kept moving works better than anything I have found. Tin the parts well and use a clean, steel bristle brush to rub the solder into the area.

So, regular iron is highly susceptible to oxidation. It is a very reactive metal. And it's not particularly hard. Copper tip plus relatively soft plating = soft tip. Soldering iron tips are not particularly malleable. Unless they're by Radio Shack.
Cast iron is hard and resists oxidation and scratching. But unlike stainless steel, it has a high thermal conductivity.... and it will hold solder. Unless you leave it bare for a few days.... Hmmm..? Maybe at some point pure iron was commonly plated onto a copper tip...

[Someone]

OKi makes no mention of thermal shock in their lengthy document discussing tip life:
http://www.newark.com/pdfs/techarticles/oki-metcal/extendingTipLife.pdf
And suggest the only problem with a sponge is the cleanliness of it!

[tszaboo]

So, I have to replace the tips (if I use it often) every 4-5 months, without sponge, they are like 50% more durable, than I can save enough money in a few years, to eat an ice cream.

[madires]

Question then for people who prefer wool; do you work on production of new boards/circuits, rework, or a mix of both, and how do you keep the wool from trapping debris?

Hobbyist usage, i.e. repairs and projects. Most remains simply fall through the wool down to the bottom of the container. So I empty the container and slap the wool a few times against the inside of my bin.

[TheBay]

I believe that the tips cracking is more prevalent on lead free systems due to the composition of the tip.

lol - BS
the tips are the same, they never changed.
some people will blame anything on lead-free!!

I blame the EU 

[MaximRecoil]

So, regular iron is highly susceptible to oxidation. It is a very reactive metal. And it's not particularly hard. Copper tip plus relatively soft plating = soft tip. Soldering iron tips are not particularly malleable. Unless they're by Radio Shack.
Cast iron is hard and resists oxidation and scratching. But unlike stainless steel, it has a high thermal conductivity.... and it will hold solder. Unless you leave it bare for a few days.... Hmmm..? Maybe at some point pure iron was commonly plated onto a copper tip...

Well, pure iron doesn't exist, for all intents and purposes. There are laboratory examples that are ~99.99% pure, and it is softer than lead; it can be cut with a knife. It doesn't take much carbon to drastically increase its hardness though.

I just skimmed through the Metcal article that Someone linked to above, and it is bizarre. For starters:

A tip typically consists of a solid copper core, a plated layer of iron, a plated layer of nickel behind the
working surface, and a plated chrome layer. Copper is used for the core primarily to ensure good heat
transfer. The nickel layer is a non-wetting layer designed to keep the solder from wicking away from the
tip's working surface. Without this layer, the solder would travel preferentially up the tip toward the heat
source, making it impossible to apply solder to the solder joint. The chrome layer is applied as an
additional protective layer.

We know that can't be technically accurate, because they can't simply be using iron, because it effectively doesn't exist; it has to be an iron alloy (steel) of some sort. Also, the Metcal tips use the Curie point for temperature regulation, which is determined by the type of alloy they use, but it's the heater that's made of that alloy, and I have no idea what that alloy is (I assume it is copper-based though). I wonder if the heater is integral with the tip:



It would make sense for the heater and "copper" core of the tip to be one piece, as that would give the best thermal response and thermal regulation, and Metcals excel in both areas.

I also find it strange that nickel is a non-wetting material while chrome is a wetting material. I would expect both of them to be non-wetting under typical circumstances. The chromium content of stainless steel is the reason it is pretty much a non-wetting material. The chrome very quickly forms a passivation layer of chromium oxide (i.e., oxidation, the enemy of soldering), which is what protects stainless steel from further corrosion (rust in this case). So how does solder wet to a chrome-plated tip, given chrome's rapidly-forming, tough, and tenacious passivation layer?

But the most bizarre part of the article is how they talk about iron's wetting characteristics. For example, in the section called "Why Iron Plating?", it says:

Must Be Wettable
The working surface of the tip must wet to transfer molten solder to the joint and to aid heat transfer. Iron
wets. Molybdenum doesn't.

And in another section:

Dewetting is the most common form of plating failure and is preventable, for the most part, with good
daily tip care. Thermal dewetting is caused by oxidation of the iron plating to iron oxide. Iron oxide is
non-wetting.

But how is that relevant to a tip that's chrome-plated? The solder will never contact the iron plating beneath it, so what do the wetting characteristics of iron have to do with anything? The iron plating can't even oxidize in the first place as long as the chrome plating is intact, because the chrome plating prevents oxygen from contacting the iron plating.

I wonder if the chrome plating is thin "decorative chrome" (thickness measured in the millionths of an inch) or thick "industrial hard chrome" (thickness measured in the thousandths of an inch). Industrial hard chrome is tough as a bag of badgers; it is what they plate e.g., hydraulic cylinders on big earth-moving machinery with, for example. It is also what they commonly plate the barrel bores of military rifles with, such as the M16 and AK-47. In my experience, the Metcal tips never fail, nor do they even need to be tinned when you're done with them. When a Metcal tip cartridge fails, it is always a heater failure, so it wouldn't surprise me if the plating is industrial hard chrome, and it wouldn't surprise me if the iron "plating" is a jacket rather than actual plating, like the copper jacket on a lead bullet.

It's funny that Metcal wrote such a lengthy article called "Extending Soldering Iron Tip Life", when their tip plating never fails to begin with, even if you don't keep it tinned (still speaking in the context of my own experience).

[KL27x]

^If you look at the cross section, the chrome plating is only over the shaft part of the tip. It stops where the solder bead begins. The chrome plating is there to prevent solder from adhering. If you look closely at a tip, you will see where the tip doesn't wet, it's thicker. It's a chrome plating OVER the wettable "iron" plating. This is how is appears on my Hakko tips, anyway. On a cheaper station I used before, the base of the tips were covered with some thicker/crustier stuff that almost looked like ceramic.

We know that can't be technically accurate, because they can't simply be using iron, because it effectively doesn't exist;

I wonder if early irons were plated with pure iron. I am not sure you can electroplate an alloy? So unless case hardened, after, perhaps some tips used to be made with pure iron plating. Radio Shack copper tips actually come with a thin plating that lasts about 3 days... perhaps this might be example of electroplated "pure" iron?

Some gears are coming loose in the attic of my brain. IIRC, what qualifies "steel" is ability to undergo martensic hardening. Probably misspelled it. Whereas cast iron is hard even without a heat treat. Which seems like a good idea for something which operates at over 700F for extended periods of time, which is above tempering level of common carbon steel. So carbon steel sucks for an iron due to corrosion and need for heat treat. Stainless steel sucks because it can't be wetted and lower thermal conductivity. Cast iron would seem to be pretty much ideal. It seems to me, modern iron tips probably have nothing to fear from unhardened steel wool.

[MaximRecoil]

^If you look at the cross section, the chrome plating is only over the shaft part of the tip. It stops where the solder bead begins. The chrome plating is there to prevent solder from adhering. If you look closely at a tip, you will see where the tip doesn't wet, it's thicker. It's a chrome plating OVER the wettable "iron" plating. This is how is appears on my Hakko tips, anyway. On a cheaper station I used before, the base of the tips were covered with some thicker/crustier stuff that almost looked like ceramic.

The way you're interpreting that diagram doesn't match what the text says, and if your interpretation were correct, then the nickel plating would be useless, because it would be completely covered by the chrome plating. The text says that the nickel plating is there specifically because it is non-wetting, thus it prevents the solder from wicking up the tip away from the joint. It couldn't serve that function if it were completely covered by chrome. Also, the implication is that the chrome plating is wettable, else they could just use chrome in place of nickel for the non-wetting barrier.

I wonder if early irons were plated with pure iron. I am not sure you can electroplate an alloy?

Here is a paper called "Electrodeposition of Iron and Iron Alloys":

http://www2.bren.ucsb.edu/~dturney/port/papers/Modern%20Electroplating/11.pdf

So unless case hardened, after, perhaps some tips used to be made with pure iron plating. Radio Shack copper tips actually come with a thin plating that lasts about 3 days... perhaps this might be example of electroplated "pure" iron?

If it were "pure" iron, you couldn't case harden it. Only steel can be case hardened, and even then you need a certain amount of carbon to do so. Some low-carbon steels (and pure iron could be called no-carbon steel) have no hardenability at all.

I wouldn't be surprised if cheap tips such as on a $7 Radio Shack wall iron were just solid mild steel, i.e., the same thing as a nail from the hardware store. In fact, I've heard of some cheap irons that actually did use an ordinary nail for the tip.

Edit: Scratch that. I just checked my 64-2067C Radio Shack iron (with severely deformed tip, despite not having been used much), and a strong magnet has no perceptible attraction to the tip.

[nanofrog]

The chrome plating is there to prevent solder from adhering. If you look closely at a tip, you will see where the tip doesn't wet, it's thicker. It's a chrome plating OVER the wettable "iron" plating.

Chrome won't adhere directly to iron, but it will adhere to nickel, which will adhere to iron. So plate iron with nickel, and the nickel with chromium, and the tip we're all familiar with is formed.