Electronics manufacturers have stipulated that they must use lead-free solders for the manufacture of their assemblies. According to this law, “lead-free” means solders that do not contain more than 0.1% lead and are therefore below the specified limit values. This EU directive was replaced by 2011/65 / EU in 2013. Nothing has changed in the permitted limit values for solders for soft soldering; a few flame retardants that were used in the field of circuit board production have also been added.
In June 2018, lead was added to the list of substances of very high concern in the European Chemicals Act (SVHC – “Substances of Very High Concern”). This leads to a changed labeling requirement for all lead-containing solder products (e.g. solder wires) with more than 0.3% lead in solid form, since lead is rightly classified as toxic to reproduction from this date. This is not a chicane, but reflects the necessity that the handling of leaded solders must be adapted to the current knowledge about their dangerousness.
This classification as toxic to reproduction meant that solder containing lead could only be sold to specially trained people. Various legal requirements came together, so that today many private electronics hobbyists also have to deal with the topic of alloy conversion. Of course, there are exceptions, the RoHS refers to the “placing on the market” of electronic assemblies. Does this a private user who works out his hobby projects himself, soldering and then using them – and who does not “put them on the market”? Probably not, but that doesn’t change what we know about the dangers of handling lead.
The switch to lead-free solders
The switch from leaded to lead-free solders has worked well on a large scale in the industry. Of course, adjustments to the system technology were necessary, and of course production parameters had to be adjusted. One or the other component as well as circuit boards had to be optimized in terms of temperature resistance. But in the field of hand soldering with a soldering station, the necessary adjustments are kept within narrow limits. Some of the first lead-free assemblies have been in operation for up to 15 years – and there are no causes of failure that can only be traced back to the use of lead-free alloys.
What are the major differences between the alloys used in the alloy sector? Contains lead Sn63Pb37 as a eutectic alloy with a melting point of 183 ° C or variations with more lead or a little copper / silver as an additive with melting ranges around 179 – 190 ° C. These formed the standard for many years. These alloys could be processed well with a 30-50 W soldering station with a set soldering tip temperature of 300-320 ° C. The solder became liquid, the flux removed the oxides from the surfaces to be soldered, the tin from the solder dissolved the copper (or other metallizations) on the component or circuit board and an intermetallic phase was created. This made the solder joint well formed, resilient and durable.
The lead-free alloys usually have a much higher proportion of tin. Instead of 63% it is more like 95 – 99% Sn. As a result, the melting point of the alloy rises to 217 – 227 ° C, but the tin has always been the component in the solder that forms the intermetallic phase and can “loosen” all solderable metal surfaces. Lead was always only the “inert” (inactive) component of the alloy, with the advantage of making it cheaper and reducing the melting point of tin from 232 to 183 ° C. However, more tin in the solder in combination with a higher soldering temperature now means that you have to pay more attention to your tools and component metallization. The solder not only dissolves copper surfaces more quickly, it also dissolves more quickly. Before you know it, the soldering eye of the circuit board has dissolved.
Standard lead-free alloys
But let’s take a closer look at some lead-free standard alloys: If you want to achieve the above-mentioned 217 ° C as one of the lowest possible temperatures in the SnAgCu range, the composition with Sn95.5%, Ag3.8% Cu0.7% be used. The advantage is the relatively low melting point, the disadvantage is the almost 4% silver in the solder, which can almost double the price of the solder wire. In principle, this silver-containing alloy can be made a little cheaper by reducing the silver content to 3%. Then the alloy has a melting range of 217 – 223 ° C, but this is not really noticeable either when processing or when considering and longevity of a soldered joint. As the cheapest alloy, the tin-copper alloy (Sn99.3% Cu0.7%) can be used as a eutectic alloy with a defined melting point of 227 ° C without silver. With this alloy, too, the temperature does not necessarily have to be increased by 10 ° C at the soldering tip compared to an alloy containing silver.
For all lead-free solders, the same rule of thumb applies to determining the required soldering tip temperature as for leaded solders:
Liquidus (liquefaction point) of the alloy + 120 ° C = working temperature at the soldering tip
So arithmetically, a lead-containing solder with a melting range of 183 – 190 ° C results in a soldering tip temperature of 310 ° C as a starting value, with Sn99.3 Cu0.7 with 227 ° C, 350 ° C is a good starting point for soldering begin. If 10 – 20 ° C more is required to enter a certain amount of heat in a short time, this can definitely be an option. Temperatures above 380 ° C usually damage the circuit boards and components more than they help. The flux in the wire also burns much faster, so that it can only do its job for a certain time at a certain temperature. Every 10 ° C increase in temperature halves the active duration of the flux. The time it takes to remove the oxides becomes shorter – at some point it is too short. Ultimately, it’s not about absolute temperatures that are necessary. Soft soldering is always about the input of the necessary amount of energy and the achievement of certain minimum temperatures. The solder must be liquid, it must be a certain temperature above the liquidus in order to allow the metallization to dissolve and thus to form the intermetallic phases and thus a resilient soldering point.
The lead-free alloys briefly outlined above all have sufficient long-term resistance: one can roughly define the silver-containing solders as more suitable for applications with higher temperature changes, often associated with permanent mechanical stress (vibration). Use in cars can be cited as an example. The low-silver or silver-free solders are often, but not exclusively, used in consumer electronics. No excessive temperature swings, lower mechanical load. These are the areas where you can do without silver. In addition, factors such as the amount of solder, the layout of the soldering geometry and the metallization used on the component and circuit board also have a significant influence on the assessment of the long-term reliability of a soldered joint. So it is never just a question of which solder is used to break down and answer it.
Entwicklungen der Lotlegierungen
In addition, there have been many developments in the field of solder alloys in recent years that can optimize the long-term reliability and other properties of standard solders. These micro-alloyed solders are based on the above-mentioned tin-copper or tin-silver-copper base solders, but around 500 ppm of controlled micro-components are added. These are often nickel, cobalt or other metals and semi-metals. These reduce the dissolution properties and create a refinement of the microstructure in the solder joint. What does a refinement of the microstructure mean? Finer grain boundaries in the solder allow the solder joint to absorb significantly more mechanical energy before it is mechanically destroyed in thermal shock tests – long-term reliability is better. The soldering tips also live longer again because the wettable iron layer on the tip dissolves much more slowly. Copper will also dissolve in the solder much more slowly, the solder eye on the circuit board will remain longer, and the repair process can be more reliable. A well-known lot series are the lots of the FLOWTIN series. Longer service life of the soldering tips compared to standard solder of 30 – 50% can be achieved with careful handling of soldering parameters and tools.
Since a soldering wire does not only consist of the alloy, but also the flux it contains is an essential component, here is a brief excursion into the difference in the fluxes that are used in lead-free / lead-containing solder wires. The task of the flux is to remove the oxides on the components involved: component, printed circuit board and of course also liquid solder. It should do this as long as possible in order to have a large process window when soldering. Depending on the type and amount of oxide on the item to be soldered, the activity must be adapted. There are halogen-free fluxes, as well as the somewhat stronger halogen-containing fluxes. Both groups remove the oxides by means of an acid-metal oxide reaction. However, the flux for the lead-free solders must carry out this reaction mechanism at a higher temperature and thus be active longer even at a higher soldering temperature. The flux must be able to flow in sufficient quantity before the solder, remove the oxides, transport the resulting salts away before the solder and leave the liquid solder with a nicely clean, pure metal surface. Then the diffusion process can take place and the solder joint is formed. With the increased soldering temperatures, the flux must also be adjusted so that the spraying of the flux and the wetting are optimized. This is where the two differently activated fluxes Kristall 600 and 611 come into play. These were developed in combination with the lead-free and micro-alloyed solders and can thus play out their full potential on differently oxidized surfaces.When selecting the flux, you should always use the weaker one. Anything that does not remain on the assembly as activators and their reaction products in the residue cannot cause any problems with long-term reliability. Always as strong and as much as you need to achieve a good wetting reaction.
Another advantage of these solder wires is that they are only made with tin from the Stannol Fairtin range. Not only the quality of the raw materials plays a role here, but also other factors such as working conditions when mining the tin, environmental standards used and much more.
Working with lead-free solder
Let us now consider the process of working with the lead-free solder. A lead-free solder joint requires more energy than a lead-containing joint under the same conditions. This applies to all soldering processes, regardless of whether you use solder wire or solder paste.Since the amount of energy required is higher, the heat transfer to the soldering point must also be regarded as the most important aspect when soldering. Here it is important to create an optimal contact surface for the heat transfer. In this context, optimal means as large as possible! This is quite easy to do: just use the soldering tip with the largest contact area and don’t leave the thin needle tip attached all day. Every soldering task therefore has an optimal soldering tip. Due to a larger transition area, this provides a higher amount of energy at the same time, so that the higher energy requirement for melting the lead-free solder does not have to be compensated for by increasing the working temperature. That would again lead to faster wear of the soldering tip.
Investigations have shown that a temperature increase from 360 ° C to 410 ° C increases the soldering tip wear almost exponentially when using lead-free alloys. The service life of the soldering tip is not only halved, it is shortened considerably more. You should therefore generally consider a slightly longer soldering or contact time for the soldering point in order not to have to unnecessarily increase the working temperature.
Another important factor is choosing the right tool. The heat transfer technology plays an essential role. Fast reaction times of the soldering iron to increased heat demand is a fundamental factor in keeping the working temperature as low as possible.
Active soldering tip technologies, in which the soldering tip forms a “unit” of heating element, sensor and wettable area, have a very fast heating-up time (approx. 3 seconds) and can be readjusted accordingly quickly. This advantage of the directly heated tips goes hand in hand with a much higher price. But thanks to the fast heating-up time, these soldering tips can automatically go into standby temperature more quickly, which reduces wear and tear and power consumption.
Passive soldering tip technologies separate the control electronics in the soldering iron (heating element / sensor) from the soldering tip, which can then be replaced as a wear part – and is cheaper. In order to be able to optimally utilize the efficiency of the passive technology, a good contact surface between the soldering tip and the soldering iron is important and a powerful soldering tool with at least 80 W or more is essential (e.g. Weller WSP 80, WTP 90, WXP 120).
If you have then selected the soldering wire with the optimal flux, which can remove the oxides present, you will be able to achieve a good wetting reaction.
The appearance of lead-free solder joints is a little different from lead-containing solder joints. While the “3G rule” (even, smooth and shiny) still applied to lead-containing solder joints, these criteria only apply to a limited extent for lead-free solder joints. The most important criterion for a lead-free solder joint is the cleanly formed “meniscus”. This visible wetting angle can be seen on the surface of the solder joint. Since the composition of the lead-free, silver-containing alloys means that the surfaces are rougher, they cannot shine as nicely and they cannot really meet the “3G” rule. But here, too, there are silver-free solders with micro-alloy components that can create a shiny solder joint with a SnCu base solder. Flowtin TC or SN100c are mentioned here as examples.
Conclusion
Lead-free soldering isn’t difficult – it’s just different from soldering with leaded solders.
The quality criteria change and the soldering tools to be used may have to be adapted. But the electrical safety of a lead-free solder joint is in no way inferior to a lead-containing solder joint! If you have familiarized yourself with the changed spreading and wetting behavior of a lead-free alloy and accept a slightly longer soldering time in order not to unnecessarily increase the temperature, you will quickly find that the soldering is actually unchanged.
Test different lead-free alloys as inexpensive test wrapping cards without great financial outlay to find the right solder for your application:
Solder HS10 FAIR lead-free with copper content, Ø 1.0 mm, 5 g
Stannol solder wire HS 10 FAIR FLOWTIN TC + Sn99 Cu1 + ML in 1.00 mm Ø on wrapping card (5g)
This contribution comes from the company STANNOL GMBH & Co. KG
