tldr: The scope of this question is potentially larger than it appears. Disadvantages apply to personal, hobby, research-level soldering such as prototype design by engineers. Disadvantage also apply to industrial production, all industries involving electronics.

Disadvantages of lead-free solders:

* The flux core in the wire and lead-free fluxes contain harsh reducing agents that are highly irritating to wet membranes like the sinuses and the eyes * The flux used in lead-free soldering has a shelf life because exposure to oxygen reduces the effectiveness of the core in the solder. For this reason, many solder companies have introduced expiration dates for their flux-core solders instead of just printing the DOM (date of manufacture). * lead-free solder is mostly, tin, silver, copper :: Sn, Ag, Cu, or other metals. The temperature required to get a lead-free solder alloy into its molten, “eutectic”* state which allows for proper wetting is greater than that required for soldering with Sn60Pb40 or Sn63Pb37 Lead-tin solder; significantly hotter in centigrade or Fahrenheit (C or F). Table source link [ https://fctsolder.com/lead-free-solder-paste/#contact ]

* Good old Sn60Pb40 (or Sn63Pb37) undergoes a phase change from solid to liquid (molten) at a [specific] eutectic point. 183 degrees C (highlighted in yellow) * SAC (SnAgCu305) is likely the most common lead-free solder. SAC305 is a lead-free alloy that contains 96.5% tin, 3% silver, and 0.5% copper. The phase change from solid to liquid occurs over a range (217–220) * * * The phase change from solid to liquid is not at a single [eutectic] point or temperature value for this lead-free alloy, SAC 305, and so is technically a non-eutectic solder.

* Wetting is a property of liquids. Wetting is crucial to the formation of acceptable or superior solder joints. Wetting occurs as a phase change from solid to liquid (flux is crucial for this wetting as well), then heating it to the eutectic temperature. * Flux allows for heated solid solder to wet and be wet in open air. This is similar to how surfactants such as soaps help water (liquid) become “wetter.” * Flux helps solder wet properly, as the solder moves to the two surfaces being heating temporarily with the soldering iron tip. * * The less-than-desirable “eutectic range” for the alloy itself can account for * less than ideal wetting * a different appearance from other metallic alloys

* Lead-free solders also differ in their greater surface tension, specifically, and which can be seen exaggerated in specialty solder alloys such as Sn96Ag04, tin solder or Silvergleem. This is a solder alloy used for jewelry, for example. The surface tension of lead-free solders results in playfulness on the tip, flux is essential (but caustic or corrosive) * * technique must be modified, if accustomed to normal electronics

* lead-free solder will require specialized, (upgraded) tips, especially if the tip temperature is determined by the model number of the tip (e.g. Metcal brand), and not a variable temperature dial, tips which could be expensive to replace lead-solder tips * * there is a good amount of reason to consider that the higher temperatures required for making good solder joints with lead-free solder could be responsible for the premature failure of active semiconductor components as a trend, some of which have low temperature exposure tolerances.

* When soldering is done with leaded-solder, there is somewhat of a correlation between the qualitative appearance of the joint (shiny, smooth) and the electrical conductivity and mechanical strength, the two most important qualities provided by Mr. solder joint. * When lead-free solder is used, appearance and qualities like “shininess” are not really an indication of a good solder joint. * If you are totally new to soldering and you have a lot of it to do (for example, to complete a large engineering project that will determine your final grade) * * you may not learn how to solder sufficiently well if you use lead-free when learning to solder. (i.e., don’t waste your time!!) * This situation would be made worse if one is also attempting to learn using lead-free solder that is old, expired, oxidized or using lead-free flux that is expired. * It would be worse or least ideal to the also use old, oxidized soldering tip(s) and/or a soldering iron lacking power in Wattage available, or temperature prevision.

* Learning what good soldering looks like, through first-hand experience is easiest achieved by using the old leaded solder, it is the starting point for soldering (even with mandated lead-free ROHS in industry) and how to adjust your technique to the idiosyncratic demands of lead-free alloys * In addition, the inability to make good solder joints leads to other issues: * * hidden resistances in solder joint(s) and which become worse over time * solder joints so poor that they begin to act as thermal intermittents * cold, dry joints that are not conductive as they should be and lack mechanical strength * trying to troubleshoot the source of an issue when the problem was that one or two solder joints on an IC chip simply do not have a good electrical connection (in terms of conductivity) to the circuit because of a lack of experience in spotting defective solder joints when using lead-free. This could lead to hours pr days of frustration and running in circles when circuit analysis won’t help you figure it out because it could be one crappy solder joint.

The alternative:

* Use leaded solder for your projects * * Wash your hands every time you touch solder, just as you do after say, doing woodwork or working on a car. * Do not eat, drink, smoke, apply cosmetics, till you wash your hands thoroughly and you are done soldering, just like doing any DIY project. * Keep your area clean and out-of-reach to children or pets. Wipe up your work area and static-dissipative mat to clean up solder balls and bits.

* Use a solder (lead-waste) container, or substitute for one, e.g., a medium sized coffee can where contaminated items can be dumped (when they are cool). * * Contain leaded solder waste such as solder balls and bits in that container such as solder wick, brass wool for conditioning your solder tips, bad components, etc. * Dispose of this waste at electronic-recycling collection events or contact your waste management company.

Unless you are a business that requires so much soldering that you would be classified as a small, medium, or large waste-generator:

* you don’t have to use lead-free. Doing so doesn’t mean you are more conscientious or a “better person.” Simply manage the handling and disposal of the waste, just like you do when you change the oil in your car.

Alloys containing lead versus lead salts

Solder as an alloy is composed of tin and lead and which has a super-strong outer surface layer of oxidation.

Solid solder, as in the case of solder wire, has an oxidative coating on the surface. This coating results from exposure to ambient air which contains oxygen that bonds to surface atoms. The oxidative coating on metals may be held strongly. For example, the oxidative coating on aluminum is an invisible barrier that makes it corrosion proof.

Years ago, certain products had lead incorporated into them, notably in indoor house paint. The paint contained lead, but not as metal but as lead salts, or molecular compounds containing lead. If ingested, as paint chips that eroded off walls and eaten by infants, the lead would be easily digested and easily absorbed by the body as toxic, heavy metals.

Is it possible to handle pure lead (Pb) metal slabs and get some on the hands? Then not wash one’s then eat a bag of popcorn? Pure lead is “soft” and can rub off on hands but solder is mostly tin, not pure lead metal.

Leaded solder is best to learn. In fact, if you paid money to learn from a training organization that offers certifications, you would be trained using leaded solder.

%3E They don’t have the time and patience for you to not show some good results from training.

Leaded solder is how people have learned soldering and continue to learn to solder correctly

edit, in case this question was meant for industry, I touch upon here, a case where the rubber meets the road regarding lead-free versus leaded solder allows used in automated production:

An ideal solder joint with have two (2) key characteristics:

* good electrical conductivity that results from the proper reflow and wetting within a non-oxidized and contaminant free solder joint * mechanical strength of the solder joint that connects two different surfaces together In electronics, through-hole components and through-hole board designs were dominant. The processes for soldering through-hole components into plated through-hole barrels (PTH) that transect the board, were highly reliable yet they were based on lead-containing alloys.

Wave machines solder pots, utilizing molten solder pools, to solder all the components to a board well, reliably, and verifiable quickly by visual inspection, were the standard.

That said, currently it is not feasible or even possible to maintain pools of molten lead-free solder, using the lead-free alloys currently available.

* to be clear: It is possible to create a molten pool that is restricted in area during the process of selective soldering, for brief periods, while under or inside a non-oxidative environment. The process is done in a nitrogen gas saturated system and not done in ambient air. I could be done under argon Ar(l) a completely unreactive or noble, gas. * here is a wonderful video the demonstrates selective soldering: link [ https://www.youtube.com/watch?v=wAOLOSky9P4 ] I know the video is 10 yrs old but selective soldering for lead-free processes still looks similar to that today in 2020. * it’s optional, premium technology Most electronics today are fabricated using surface mount technology or SMT technology. The components are attached only to the surface of a board, and not through it.

SMT technology utilizes solder paste, distributed in precise volumes onto the board surface using stencils and a squeegee. Components (SMD or surface mount devices) are tacked onto a board using a pick and place machine (thousands of them and teeny-tiny). The solder paste provides enough tackiness (stickiness) to hold the components in place and during convection reflow. This process for surface mount components works using lead-free solder pastes works generally well.

The exception is for components that are so important that they must be soldered into a board designed for through-hole technologies using plated through-holes and this regards many I/O connectors that provide, for example, many functions or many signal channels and many small pins within many holes.

Because molten solder pools are not great with lead-free alloys, such as with wave-soldering machines, many manufacturers must now rely on a process called “intrusive soldering.”

Intrusive soldering is the use of surface mount convection reflow ovens to solder through-hole components, while still using SMT-purposed solder paste. Intrusive soldering is also called Pin-in-Paste (PIP) or Pin-in-hole-paste (PIHP) soldering. This is done when waves [of molten solder] were used previously for those. Intrusive soldering is a new approach to soldering based on, and also resulted from, RoHS (lead-free) prescribed protocol.

Source, images: http://www.ami.ac.uk/courses/topics/0226_pip/index.html

Notice the concave shapes (versus convex) or bumpy appearance to the solder. Lead-free solders tend to have greater surface tension. Often will also see darker solder at the surface, usually as partially reflowed solder paste when everything out of a wave machine looks as shiny metal that clearly reached a state of liquidus, displaying properties of wetting and of solder that reached its eutectic temperature—the point or narrow temperature range for phase change; solid becomes liquid.

Intrusive soldering however, is mostly experimental since each process and oven temperature profile must be tweaked for every particular assembly. It is an ad hoc process, mostly trial-and-error—tweaking or adjusting oven temperature profiles to produce desirable outcomes. Often the first production run of a single circuit board with these connectors will be defective and scrapped (something now expected), and the oven temperatures tweaked on the next run.

Due to the experimental nature of intrusive soldering, agreement by convention will be difficult to standardize or attain.

Not everyone agrees that Pin-in-Paste (PIP) or Pin-in-hole-paste (PIHP), aka intrusive soldering, can allow for standardized improvements or even produce acceptable solder joints (results similar to those allowed for previously with leaded solders) with the currently available alloys.

For this reason, through-hole connectors must be soldered by hand for lead-free alloys and cannot depend on convection reflow methods whenever a connector is used in a very important circuit board, especially in the case of mission-critical class-3 circuit boards, such as those placed in satellites which are very difficult to service once they’re in orbit. For this reason, intrusive soldering has not yet been accepted by IPC as a reliable method that can be standardized. Instead, IPC urges direct oversight by an astute soldering technician/process engineer and the onus is on them to get the process right by tweaking it for every individual assembly, but for class-2 boards (consumer level) at best.

There are some perceived advantages to cost, to using this unconventional means of installing through-hole components with convection reflow of solder pasted just as a reduction in the number of process steps, taken with a grain of salt because the solder joints created with intrusive soldering simply cannot reach target criteria, but only acceptable conditions at best.

* A solder joint can be Target (nearly perfect), or Acceptable (not too shabby), or non-conforming or Defective. * Only hand-soldering of through hole components with lead-free solder wire and not paste, can allow for Target solder joints. * Intrusive soldering of through-hole components using solder paste in a convection reflow process: * * results in “Acceptable” solder joints, at best * more easily allows for non-conforming characteristics (solder joints with insufficient solder, solder joints with voids or pockets of air, cold (non-reflowed) solder joints * comes with a known trade-off of having solder joints that do not have the same conductivity and mechanical strength of through-hole components installed with leaded solders (but may suffice for say, cheap consumer electronics).

Soldering through-hole components with lead free solder paste and using SMT convection reflow is often problematic compared to the older process which was highly consistent, highly reliable in producing target solder joints.

There was a huge advantage to established processes involving lead-containing solders and technologies, but these are no longer in use whenever lead-free alloys are required, and also because of the different chemistries involved with leaded-and lead free solders.

The closest thing today for lead-free solders to the past wave technology with leaded solder, is called selective soldering. Selective soldering requires a specialized machine with cameras and PC, but also cooling with liquid nitrogen. So it add cost to the process where arguably the electronics don’t have to be 100% perfect in aesthetics as long as they’re 100% functional.

The disadvantage was that lead-waste in electronics is not and was not being processed as waste, properly, only an estimated 15% of old electronics is recycled appropriately.

This is an example of a challenge or disadvantage presented by the transition from leaded to lead-free solders.

edit: One other big disadvantage to lead-free solder alloys that should be mentioned involves the formation of microscopic metallurgical “whiskers [ https://en.wikipedia.org/wiki/Whisker_(metallurgy) ],” more specifically “tin whiskers” and also “zinc whiskers” that have been increasingly associated with lead-free solder alloys.

Image [ https://engineering.stackexchange.com/questions/535/how-to-design-electronics-to-mitigate-tin-whiskers ]:

Image, same source: [ https://engineering.stackexchange.com/questions/535/how-to-design-electronics-to-mitigate-tin-whiskers ]

The formation of these whiskers is as yet not well-understood. What is clear from just the images above is that these develop over time and essential create tiny short-circuit or bridging conditions, instead of discrete, isolated components.

For this other reason, highly and critically important soldering electronics does not use lead-free solders. The best examples of highly critical soldering involves Class 3 electronics [mentioned above] (mission-critical electronics with absolutely the lowest possible fail rates, such as in jet planes, defense applications, rockets and satellites, other examples follow), which still utilize lead-based alloys still, because of the tin-whisker problem correlated with lead-free solder alloys in electronics.

[Effects of [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#Effects ]]:

%3E Whiskers [ https://en.wikipedia.org/wiki/Whisker_(metallurgy) ][Wikipedia] can cause short circuits [ https://en.wikipedia.org/wiki/Short_circuit ] and arcing [ https://en.wikipedia.org/wiki/Electric_arc ] in electrical equipment. The phenomenon was discovered by telephone companies in the late 1940s and it was later found that the addition of lead [ https://en.wikipedia.org/wiki/Lead ] to tin solder [ https://en.wikipedia.org/wiki/Solder ] provided mitigation.[4] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-4 ]

The European Restriction of Hazardous Substances Directive [ https://en.wikipedia.org/wiki/RoHS ] (RoHS), that took effect on July 1, 2006, restricted the use of lead in various types of electronic and electrical equipment. This has driven the use of lead-free alloys with a focus on preventing whisker formation, see § Mitigation and elimination [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#Mitigation_and_elimination ]. Others have focused on the development of oxygen-barrier coatings to prevent whisker formation.[5] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-5 ]

Airborne zinc whiskers have been responsible for increased system failure rates in computer [ https://en.wikipedia.org/wiki/Computer ] server rooms [ https://en.wikipedia.org/wiki/Server_room ].[6] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-6 ]

Zinc whiskers grow from galvanized [ https://en.wikipedia.org/wiki/Galvanization ] (electroplated) metal surfaces at a rate of up to a millimeter per year with a diameter of a few micrometres. Whiskers can form on the underside of zinc electroplated [ https://en.wikipedia.org/wiki/Electroplating ] floor tiles [ https://en.wikipedia.org/wiki/Tile ] on raised floors due to stresses applied when walking over them; and these whiskers can then become airborne within the floor plenum [ https://en.wikipedia.org/wiki/Plenum_space ] when the tiles are disturbed, usually during maintenance. Whiskers can be small enough to pass through air filters and can settle inside equipment, resulting in short circuits [ https://en.wikipedia.org/wiki/Short_circuit ] and system failure.

Tin [ https://en.wikipedia.org/wiki/Tin ] whiskers don't have to be airborne to damage equipment, as they are typically already growing in an environment where they can produce short circuits. At frequencies above 6 GHz or in fast digital circuits, tin whiskers can act like miniature antennas [ https://en.wikipedia.org/wiki/Antenna_(radio) ], affecting the circuit impedance [ https://en.wikipedia.org/wiki/Characteristic_impedance ] and causing reflections. In computer disk drives they can break off and cause head crashes or bearing failures. Tin whiskers often cause failures in relays [ https://en.wikipedia.org/wiki/Relay ], and have been found upon examination of failed relays in nuclear power [ https://en.wikipedia.org/wiki/Nuclear_power ] facilities.[7] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-7 ]Pacemakers have been recalled due to tin whiskers.[8] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-8 ]

Research has also identified a particular failure mode for tin whiskers in vacuum (such as in space), where in high-power components a short-circuiting tin whisker is ionized into a plasma that is capable of conducting hundreds of amperes of current, massively increasing the damaging effect of the short circuit.[9] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-9 ]

The possible increase in the use of pure tin in electronics due to the RoHS [ https://en.wikipedia.org/wiki/RoHS ] directive drove JEDEC [ https://en.wikipedia.org/wiki/JEDEC ] and IPC [ https://en.wikipedia.org/wiki/IPC_(electronics) ] to release a tin whisker acceptance testing standard and mitigation practices guideline intended to help manufacturers reduce the risk of tin whiskers in lead-free products.[10] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-10 ]

Silver [ https://en.wikipedia.org/wiki/Silver ] whiskers often appear in conjunction with a layer of silver sulfide [ https://en.wikipedia.org/wiki/Silver_sulfide ] which forms on the surface of silver [ https://en.wikipedia.org/wiki/Silver ] electrical contacts [ https://en.wikipedia.org/wiki/Electrical_contact ] operating in an atmosphere rich in hydrogen sulfide [ https://en.wikipedia.org/wiki/Hydrogen_sulfide ] and high humidity [ https://en.wikipedia.org/wiki/Humidity ]. Such atmospheres can exist in sewage treatment [ https://en.wikipedia.org/wiki/Sewage_treatment ] and paper mills [ https://en.wikipedia.org/wiki/Paper_mill ]. (Another such atmosphere that is high is hydrogen sulfide gases and involve humidity is at the deep sea vents at the bottom of the ocean. So very small deep-sea submarines or so submersibles such as the ALVIN may also be susceptible.)

%3E Whiskers over 20 µm in length were observed on gold-plated [ https://en.wikipedia.org/wiki/Gold_plating ] surfaces and noted in a 2003 NASA internal memorandum.[11] [ https://en.wikipedia.org/wiki/Whisker_(metallurgy)#cite_note-11 ]

The effects of metal whiskering were chronicled on History Channel [ https://en.wikipedia.org/wiki/History_(U.S._TV_channel) ]'s program Engineering Disasters 19. image from the link below:

%3E Doctoral student unravels 'tin whisker' mystery [ https://phys.org/news/2012-12-doctoral-student-unravels-tin-whisker.html ]

Yong Sun, a mechanical engineering doctoral student at the University of South Carolina's College of Engineering and Computing, has solved part of the puzzle. [ http://%3E*https://phys.org/news/2012-12-doctoral-student-unravels-tin-whisker.html#jCp* (https://phys.org/news/2012-12-doctoral-student-unravels-tin-whisker.html#jCp) ]

…

Sun's findings were published in the Scripta Materialia, a materials science [ https://phys.org/tags/materials+science/ ] journal. This fall he won the prestigious Acta Student Award, one of only six to receive the honor. A team of editors from Acta Materials, Scripta Materialia and Acta Biomaterials evaluated the applications and Sun beat out students from the world's top universities, including MIT.

"This shows that our research in material science [ https://phys.org/tags/material+science/ ] is reaching an international audience," Sun said. "It is nice to be recognized for our work."

The importance of that work goes well beyond extending the operating life of consumer electronics. NASA has verified multiple commercial satellite failures it attributes to tin whiskers. Missile systems, nuclear power stations and heart pacemakers also have fallen victim to tin whiskers over the past several decades and they are also considered a suspect in reported brake failures in Toyota vehicles.

While manufactures had been able to control some whiskers by mixing small amounts of lead into tin solder, the 2006 European Union ban on lead in most electronic equipment had ignited a debate among scientists about whether whiskers would remain a perpetual problem. Some observers even predict that it's only a matter of time before miniature devices built after the ban start failing en masse.

Xiaodong Li, a professor in USC's Department of Mechanical Engineering who served as an adviser on the research, said Yong's work likely will prompt manufacturer to design lead-free products that diffuse stress.

"This (research) is a very big deal. As we move toward nano-scale devices, this is a problem that needs to be solved," Li said.

Read more at: https://phys.org/news/2012-12-doctoral-student-unravels-tin-whisker.html#jCp Finally, it should be noted that IPC, the industry standard setting organization for electronic solder joints, has received and documented examples of 63Sn/37Pb solder with tin whiskers, provided by documented cases from NASA and Goddard Space Flight.

So even the usual solder used for aerospace application has been demonstrated to present a potential for tin whiskers if the unit was ultimately retrievable.

My first experiences with soldering did not include any mention of safety or the implications of heavy metals, but played with my elder brother’s soldering equipment when he took a course on electronics. I played with solder casually, didn’t even wash my hands, so I look back and wondered if I had been exposed to more-than-healthful levels of lead.

There are many threats that are toxic to biological life forms, many of which are never publicized or addressed. The article I recently added mentions that there have been few or singular solutions to mitigate the problem of tin whiskers, besides adding small amounts of lead to mitigate tin whiskers and device failure in the field.

Tin whiskers have also been documented by NASA for aerospace electronics on Sn63Pb37 alloy, the standard military/aerospace solder alloy, in very-long-service satellites.

It’s “two steps forward, one step back,” (adding lead minimizes the problem better than any alternative)while the good intentions may be misplaced in the first place and possibly a hindrance to engineering pursuits. In the case of cardiac pacemakers, I would think that lead has a sufficiently good reason to be used, perhaps with special insulation, polymeric, or conformal coatings, the RoHS directive in medical devices may be overly rigid when exceptions should be made, such as with pacemakers.

Thank you for reading

The major disadvantages in no particular order are:

A "good" lead-free solder joint is grainy and rough. Please note that both of these solder balls are cold, yet look how shiny the leaded solder is next to the rough lead-free solder:

I have found that lead-free solder often requires tight temperature regulation on your soldering iron, depending on the exact mix of metals. For many years, I and others have used unregulated irons or those with interchangeable tips that are at 600F, 700F, or 800F with 63/37 and 60/40 solder without problems. When I switched to lead-free, I had to get a digital adjustable iron because many lead-free solders have only a 10 or 15F working range.

The flux in lead-free solder is much more active than the rosin (literally pine sap) flux in leaded solder. In addition, you -must- use a lot of added flux when soldering lead-free solder. You should not be breathing in smoke from any kind of soldering, but the lead-free flux is much more irritating to your eyes and respiratory tract. And unless you use no-clean flux, the flux must be cleaned from the board or it will corrode the metal.

Tin whiskers! Lead was originally added to solder because of tin whiskers. No one is really sure why they grow, but they can grow at prodigious rates and cause failures. There is no clear, repeatable correlation, but elevated temperature, humidity, and voltage seem to have some correlation. Tin whiskers have been known to grow right through conformal coatings. This is especially a risk in modern high density, low current circuit boards. Page on nasa.gov [ http://nepp.nasa.gov/Whisker/background/index.htm ]

The major concern from solder smoke is the flux, not lead. Lead does not tend go into vapor at soldering temperatures. For that, lead mining and smelting are the main pathways, and leaded gasoline for countries that still allow it. Leaded paint being an ingestion risk for children and amateur home renovators. Keeping in mind, of course, that lead absorption is cumulative as it is slow to leave the body.

As a side effect of lead-free solder, much more aggressive fluxes must be used, and in much greater quantities. In my many years of soldering, I've rarely added flux when using leaded solder, but -always- end up adding flux for lead-free joints. In either case, the smoke is very bad for you and should be aggressively removed. I've found the common tabletop smoke fan is a huge waste of money. I have a 200mm fan that is in a flex arm lamp in place of the magnifying lens. It is like a mini fume hood.

As for skin absorption, I've commonly used empty solder wick rolls to hold the solder I'm using. And wash my hands before doing anything else.

Lead (Pb) Toxicity: How Are People Exposed to Lead? [ http://www.atsdr.cdc.gov/csem/csem.asp?csem=7&po=6 ]

The Problem with Solder Smoke [ http://www.garypalamara.com/Articles_Solder_Smoke.htm ]

Coal burning releases a lot more lead into the environment and has been cited as the major source of lead found in children's blood. Lead in Children’s Blood Is Mainly Caused by Coal-Fired Ash after Phasing out of Leaded Gasoline in Shanghai [ http://pubs.acs.org/doi/abs/10.1021/es9039665 ]

Disadvantages of Lead Free Solder:

1. Lead free solder need higher temperature to melt, 2. It is environmental friendly but not circuit board friendly, 3. Hard to solder and more harder to de-solder, 4. Not shiny, 5. Life span is less Disadvantaged of Leaded Solder:

1. Not Environmental friendly.

For most applications, you will use lead free solder. The one place where lead solder works GREAT (and lead free does not) is repairing brass radiators in trucks and old cars. I had to track down a roll of lead solder to fix the outlet of an old heavy truck radiator that was leaking where it met the tank. Lead solder is still sold for leaded glass. DO NOT under any circumstances use lead solder for plumbing. I don’t care what your great grand dad did. The lead leeches out, and into the water, particularly hot water.

Lead-free solder is an abomination. It has a gap between the Liquidus [ http://en.wikipedia.org/wiki/Liquidus ] and Solidus [ http://en.wikipedia.org/wiki/Solidus_%28chemistry%29 ]temperatures between which it goes pasty and semi-solid and won't flow, and when it is liquified it has low surface tension and won't wick into joints. It's particularly nasty when you have to remove it to rework something, because all the traditional methods of solder removal fail - you have to swamp it with lead solder before you can either suck it out or wick it up. When it's solid, it's brittle, and in damp conditions it can grow tin whiskers that short out adjacent connections. The military won't use it - military equipment is exempt from the RoHS directive. That alone should tell you all you need to know.

By comparison, 63/37 tin/lead is a eutectic [ http://eutectic ]with excellent melt and flow properties and strong surface tension.

There's a reason lead has been used for soldering and brazing for something over 2000 years - it does the job better than anything else that's ever been tried.

Lead-free solder is the culprit behind people’s Playstation 3s dying, and laptops, and anything with a circuit board and very hot silicon chips. Some of the bigger chips in compact computer systems (e.g. laptops and game consoles) generate a lot of heat, and the built-in cooling is just adequate enough to keep the chips and solder from getting too hot. As people use their laptops and consoles, dust builds up over time, creating “dust bunnies” in the heatsink fins, hindering the cooling process, and ultimately overheating solder joints until a micro-fracture occurs in as few as one solder joint. The same can happen when the intake or exhaust ports on a laptop are blocked, say, by placing it on bed, blanket, or sofa. Leaded solder softens when exposed to excessive heat generated by hot silicon chips, but it doesn’t crack.

As for lead-free solder being cleaner for the environment, I thought anything with a circuit board is supposed to be properly disposed of, like batteries, and motor oil. So it’s not the lead that’s a problem, it’s the senseless people who don’t give a damn or know better.

%3E I don't recommend using lead free solder for any electronic work: design, manufacturing or repair / restoration / modification. Unleaded solder requires a higher soldering iron temperature, precise temperature control and the use of special types of flux to rework solder joints. Lead is an essential ingredient for reliable and quality, shiny solder joints. There was a time when I had only lead free solder and pure lead solder to do a job. I had to mix the two together to avoid dull, dry solder joints that can easily crack and crumble. There is a great potential for a phenomenon similar to dendrite growth known as tin whiskers that can cause circuit failure with lead free solder. I would not trust any electronic equipment manufactured or serviced using lead free solder. I believe that the RoHS banning lead in solder and cadmium in batteries in EU is totally misguided and irresponsible. Recycling and safe handling of such toxic metals should be the main focus.

“What are the disadvantages of lead-free solder vs. lead solder?”

That depends on the particular solder alloys. Initial leadfree solders were very problematic and inconvenient to work with, but metallurgy did not stop improving.

For hand soldering, there is little difference these days. You simply cannot tell, whether it’s lead solder or leadfree. Yes, the eutectic Pb-Sn alloy would melt at 181°C, and most leadfree alloys melt above 230°C. In practice, most lead alloys have a slightly higher melting point, and we’re anyway using around 350°C iron temperature, because both alloys have poor wetting at lower temperatures and the flux is not activated properly either. This temperature is needed for lead solder, and it’s sufficient for leadfree, too.

Reflow soldering of SMD parts is a bit different. Peak temperature is lower, because the parts cannot withstand 350°C. It’s more like 250..270°C. The problem is that the temperature range from sufficient wetting to part damage is very narrow. Process parameters must be strictly controlled to minimize the waste. Lead solder was much more forgiving. You could reliably solder big BGA packages in a grill oven. Not going to happen with leadfree. However, it’s worth mentioning that there are leadfree solders with very low melting point available. They melt at much lower temperature than lead solder. Might be a solution, I have no experience with them.

I can list some further disadvantages, but most are not relevant anymore with modern alloys and technologies:

* Initial leadfree alloys destroyed solder tips very quickly, like ten times faster than lead alloys. * Leadfree alloys had the potential to grow tin Whisker (metallurgy) - Wikipedia [ http://en.wikipedia.org/wiki/Whisker_(metallurgy) ]. Whiskers can cause short circuits and destroy the equipment. At least one satellite was destroyed this way. Modern alloys won’t grow whiskers. * Lead solder was eutectic. It melted and solidified at one temperature, and formed very fine crystal structure. Joints looked shiny, inter-crystal corrosion was minimal. Most leadfree solder are not eutectic. When solidifying, first one metal crystallizes, and the crystals are swimming in a closer to eutectic liquid phase. The alloy feels like mud, before solidifying completely. The resulting crystal structure is coarse, looking matte. Inter-crystal and electrochemical corrosion are problems.

I can’t stand lead-free solder. The solder joints are brittle, and more inclined to structural failure than with lead/tin solder. After a few years, the lead-free solder often develops a spate of loose or dying connections.

I had some ultra-ultra-ultra-ultra-Leftist kid come into my store, intoxicated on populist rather than scientific environmentalism, who told me that Monsanto was the maker of lead, and the EPA should go after them and make them stop.

I asked him if he’d heard of the PERIODIC TABLE OF ELEMENTS. He didn’t know about the periodic table, or that lead was an element. But he had a mouth the size of a volcano.

Lead free solders biggest disadvantage has been that it's killer of electronic components. You will see your laptop, GPU, gaming consoles fail a lot more now a days than before. The reason is lead free solder. Lead free solder is one of the worst thing that has happened to electronic industry.

You might heard of reballing of GPUs, laptop or gaming console. Reballing is process of removing lead free solder with leaded ones. This process has a 99% success rate and the hardware normally performs better after reballing and doesn't fails much.

Lead free solder is said to be safer hence used. But researches have found out that lead free solder isn't any more safer than lead based solder.