Hand soldering with lead-free solders (2002/95/EC)

Hand soldering - According to EU Directive 2002/95/EC (Waste Electrical and Electronic Equipment Act), the majority of industrial electronics manufacturers are no longer permitted to use solders containing lead. This law was introduced in 2006 and lead-free solders must now be used. Lead-free is defined as containing no more than 0.1% lead. In 2013, this directive was replaced by 2011/65/EU, but nothing has changed significantly. In 2018, lead was added to the list of substances of very high concern in the European Chemicals Act. This resulted in a change to the labeling requirement for all lead-containing solder products with more than 0.3% lead.

It is fair to say that the changeover from leaded to lead-free solders has worked well in the industry. Even if some adjustments had to be made to the processes and materials.

What is the difference?

Alloys containing lead, such as Sn63Pb37 (63% tin and 37% lead) have a melting point of 183°C. Alloys with added copper/silver have a melting point of 179-190°C. This used to be the standard for many years. These solders could be processed well with a soldering tip temperature of 300-320°C. In most cases, lead-free alloys have a higher proportion of tin. No longer 63% but between 95-99%. Consequently, the melting point of the alloy rises to 217-227°C. However, more tin in the solder in conjunction with a higher soldering temperature means that you have to take a little more care with your tools and component metallizations. The solder not only dissolves copper surfaces faster, it also dissolves them faster. Before you know it, the soldering eye of the circuit board is dissolved. If you are aiming for the lowest soldering temperature of 217°C, you should use the alloy Sn95.5%, Ag3.8% Cu0.7% (95.5% tin, 3.8% silver and 0.7% copper). 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 somewhat cheaper by reducing the silver content to 3%. This results in a melting range of 217-223°C. If you want to use the most favorable composition, you can use a tin/copper alloy. Here the melting point is defined at 227°C. With this alloy, as with the other alloys, the temperature at the soldering tip does not necessarily have to be increased to the same extent, but the rule of thumb applies:

Liquidus (liquefaction point) of the alloy + 120°C = working temperature at the soldering tip

This results in a soldering tip temperature of 350°C for a lead-free alloy (Sn99.3 Cu0.7) and a soldering tip temperature of 310°C for a composition containing lead. However, temperatures above 380°C are generally more harmful to the circuit boards and components than helpful. The flux in the wire also burns much faster, so that it can only fulfill its task 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 is not about absolute temperatures that are necessary. Soft soldering is always about the input of a necessary amount of energy and reaching certain minimum temperatures. The solder must be liquid, it must have a certain temperature above the liquidus in order to allow the metallization to dissolve and thus form the intermetallic phases and thus a resilient solder joint.

What influence does the composition of a wire have on the durability of the sounding point?

When it comes to hand soldering applications with high temperature fluctuations and constant mechanical stress (vibration), the silver-containing solders can roughly be classified as more suitable. An example of this is the use in cars. Low-silver or silver-free solders are often, but not only, used in consumer electronics. No excessive temperature increases, lower continuous mechanical stress. These are the areas where silver can definitely be dispensed with. 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 long-term reliability of a solder joint.

Another development is micro-alloyed solders. These are based on the tin-copper or tin-silver-copper base solders mentioned above, but around 500 ppm of controlled micro-components are added. These are often nickel, cobalt or other metals and semi-metals. These reduce the deposition properties and produce 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 alternation tests - long-term reliability is improved. The soldering tips also have a longer service life, as the wettable iron layer on the tip is dissolved much more slowly. Copper is also dissolved much more slowly in the solder, the soldering eye on the circuit board is retained for longer and the repair process can be carried out more reliably. The FLOWTIN solder series is a well-known solder series. Longer service life of the soldering tips compared to standard solders of 30 - 50% can be achieved with careful handling of soldering parameters and tools.

The flux is another component of a solder wire for the hand soldering process. The task of the flux is to remove the oxides on the components involved: component, circuit board and, of course, liquid solder. It should do this for as long as possible in order to have a large process window for hand soldering. Depending on the type and quantity of oxide on the soldering material, the activity must be adjusted. 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 fluxes for the lead-free solders must carry out this reaction mechanism at a higher temperature and therefore be active for longer at a higher soldering temperature. The flux must be able to flow in sufficient quantity in front of the solder, remove the oxides, carry away the salts formed in front of the solder and leave the liquid solder with a nice clean, pure metal surface. The diffusion process can then take place and the solder joint is formed. At higher soldering temperatures, the flux must also be adjusted to optimize the spraying of the flux and wetting. This is where the two differently activated fluxes Kristall 611 and 600 come into play. These were developed in combination with the lead-free and micro-alloyed solders and can therefore exploit their full potential on surfaces with varying degrees of oxidation. When choosing a flux, the basic rule is to always use the weaker one. Activators and their reaction products that do not remain on the assembly in the residue cannot cause any problems with long-term reliability. Always use only as much and as strong a flux as is required to achieve a good wetting reaction.

Another advantage of these solder wires is that they are only manufactured using tin fromStannol's Fairtin range. Not only the quality of the raw materials plays a role here, but also other factors such as the working conditions when mining the tin, the environmental standards applied and much more.

The working process with lead-free solder

A lead-free solder joint requires more energy than a leaded joint under the same conditions. This applies to all soldering processes, regardless of whether solder wire or solder paste is used.

As the amount of energy required is higher, the heat transfer to the solder joint must also be considered as the most important aspect of manual soldering. Here it is important to create an optimum contact surface for the heat transfer. Optimal in this context means as large as possible. This is quite easy to achieve: Simply use the soldering tip with the largest contact surface and do not leave the thin needle tip mounted all day. Every soldering task therefore has an optimum soldering tip. This provides a higher amount of energy in the same amount of time due to a larger contact surface, so that the higher energy requirement for melting the lead-free solder does not have to be compensated for by increasing the working temperature. This would again lead to faster wear of the soldering tip. Studies have shown that an increase in temperature from 360°C to 410°C increases 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. A slightly longer soldering or contact time should therefore generally be taken into account for the soldering joint in order to avoid having to increase the working temperature unnecessarily.

Another important factor is the selection of the right tool. The heat transfer technology plays a significant 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" consisting of heating element, sensor and wettable area, have a very fast heat-up time (approx. 3 seconds) and can readjust accordingly quickly. However, this advantage of directly heated tips is accompanied by a significantly higher price. However, the fast heat-up time means that these soldering tips can automatically switch to standby temperature more quickly, which reduces wear 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 wearing part - and is cheaper. In order to make optimum use of the efficiency of 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.

The appearance of lead-free solder joints differs slightly from leaded solder joints. While the "3G rule" (uniform, smooth and shiny) still applies to leaded solder joints, these criteria only apply to a limited extent to 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. As the composition of the lead-free, silver-containing alloys means that the surfaces are rougher, they cannot shine as beautifully and do not really fulfill the "3G" rule. But here too, there are silver-free solders with micro-alloy components that can produce a shiny solder joint with a SnCu base solder. Flowtin TC or SN100c are mentioned here as examples.

Conclusion on hand soldering

Lead-free hand soldering is not difficult - it is just different from hand soldering with leaded solders.

The quality criteria change and the soldering tools to be used may have to be adapted. However, the electrical safety of a lead-free solder joint is in no way inferior to a solder joint containing lead! Once you have familiarized yourself with the changed spreading and wetting behaviour of a lead-free alloy and accept a slightly longer soldering time so as not to increase the temperature unnecessarily, you will quickly discover that hand soldering is actually unchanged.

Source: STANNOL GMBH & Co. KG

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