Why is lead-free solder wire used

soldering

Soldering is the joining of metal parts with a metal alloy (solder) under the influence of heat / heat. The melting temperature of the solder is below that of the other metals to be joined. When the temperature is right, the molten solder flows between the two metal parts. Under favorable conditions, there is a firm, tight, corrosion-resistant, current and heat-conducting connection between the solder and the metals.

All further explanations refer to soldering in electronics and electrical engineering (soft soldering). The so-called hard soldering should only be mentioned here for the sake of completeness.

The solder (soft solder)

Soft solders are alloys made from tin (Sn) and lead (Pb). In addition, small amounts of antimony (Sb), silver (Ag) or copper (Cu) are added. The composition of the solder depends on the area of ​​application. Soft solders are standardized according to DIN 1707 and identified by abbreviations.

AbbreviationComposition in%Working temperature in ° Capplication
PbSn4060 Pb, 40 Sn235Sheet metal
Sn60Pb60 Sn, 40 Pb183Tin
Sn50PbCu50 Sn, 49 Pb, 1 Cu190Electrical appliances
Sn60PbAg60 Sn, 36 Pb, 4 Ag180electronics

The solder, which is common in electrical engineering, is known colloquially as solder and is a tin-lead alloy with a melting point between 180 ° C and 260 ° C. Its working condition is a kind of wire that has a tube in its center that contains rosin. The rosin serves as a flux because the solder sticks rather than flows during the melting process.
Soft solder is used when the connection between two metals should be tight and conductive and no high demands are placed on mechanical strength. The following metals can be soft soldered:

  • tin
  • zinc
  • lead
  • copper
  • Copper alloys
  • soft steel
  • Gunmetal

Lead-free solder / tin solder

Since the introduction of the RoHS directive, the specter has been that soldering containing lead is forbidden. However, lead-free solder is only required in commercial devices according to the RoHS directive. This means that in the private sector you can still work with leaded solder.
A lead-free solder is, for example, the solder alloy S-Sn99Cu1 with tin and copper. You may find it easier to use a higher proportion of flux (tin / silver / copper), which is of course more expensive.

Soldering process

A general distinction is made between the soldering processes soft and hard soldering. Soft solders have a melting point below 450 ° C. Hard solders have a melting point between 450 ° C and 1100 ° C. The purpose of the application decides which soldering process is used.

Soldering devices

There are basically two types of electrical soldering devices: soldering iron and soldering gun.
The soldering iron is the classic soldering device that is used for soldering printed circuits. It is a handle with a cable on one side and a metal soldering tip on the other. The cable is either plugged into the socket or in an adjustable soldering station, where the temperature of the soldering tip can be regulated and serves as a shelf for the soldering iron. There are also soldering irons with battery or gas operation.
The soldering gun has a typical pistol shape with a soldering tip at the front and a connection cable in the handle. The advantage of the soldering gun is that the tip gets hot more quickly. It can also be used to solder thermoplastic materials and large-area metal connections.
If you solder more often and, above all, for a long time, you are on the safe side with a temperature-controlled soldering station (50 to 80 watts). It can also be used for lead-free soldering.

The soldering tip is very hot. Objects that cannot withstand these temperatures will be damaged or destroyed by touching the soldering tip. Be careful, there is also a risk of fire. The skin of the person doing the soldering is also at risk. Depending on how long the contact with the soldering tip lasts, the skin at the point of contact can have permanent or lifelong burn marks or scars. The ambient temperature of the solder joint must not be underestimated.
Zinc ointment can prevent blistering of minor burns and also makes the whole thing less painful. It is important that the zinc ointment is ready to hand. If you want to do without zinc ointment, you should go to the tap in the event of a burn and let lukewarm (!) Water flow over the site of the fire.
Under no circumstances should cold water be used to cool the combustion. If you take very cold water, the blood vessels constrict and this means that in the deeper tissue layers of the skin, the dissipation of heat by the circulating blood is no longer guaranteed.

The soldering / the soldering process

Before soldering, the following things must be ensured:

  • The soldering device has the correct soldering power (temperature / watt).
  • The size of the soldering tip is suitable for the size of the solder joint.
  • The solder is suitable for the soldering performance.
  • The soldering point is metallically pure, i. H. free of contamination and oxidation layers (clean with alcohol if necessary).
  • The strength of the solder joint is higher than that of the solder.
  • The soldering tip must be tinned and free from contamination.
  • A suitable shelf for the soldering device must be provided (e.g. metal stand).
  • A small wet sponge is suitable for cleaning the soldering tip.
  • Once soldered, areas can no longer be removed from the solder without leaving any residue. Therefore, solder should be used sparingly.
  • Too much solder forms balls.

If you pay attention to a few things when assembling a circuit board, you will save yourself a lot of problems afterwards. In the case of finished kits with printed circuit boards, most errors are caused by poor soldering points and components damaged by soldering (heat death).

Soldering - step by step

  1. As far as possible, there should be a mechanical connection between the two metals to be connected. Wires should be twisted together. The pins of electronic components should be bent over at the soldering point on the circuit board so that they do not slip.
  2. Metals that cannot be mechanically joined together before soldering should be tinned.
  3. Soldering electronic components without a mechanical connection takes longer and can therefore lead to heat damage to this component. The component should therefore be fixed with metal needle-nose pliers or similar during the soldering process.
  4. To solder two metals, they are brought to the correct working temperature at the soldering point. The soldering device is used for this. If the soldering tip of the soldering device is at the right temperature, it takes about 2 to 3 seconds.
  5. Then the solder is briefly added (approx. 1 to 3 seconds). In the best case scenario, the solder starts to flow between the metals. A small thread of smoke rises and something crackles at the soldering point. Depending on the size of the solder joint, this takes between 2 and 5 seconds. Clean flowing solder has a silvery sheen. Once the tin has spread, the soldering device is removed.
  6. Then the solder joint cools down. If the metals have a mechanical connection and are not exposed to any vibrations (shaking, trembling), the solder joint has a silvery sheen.
  7. If the soldering point has no way of solidifying without vibration, a so-called cold soldering point is created in which the metals could not be alloyed. This soldering point leads to a very high contact resistance which, under unfavorable circumstances, cannot lead to an electrical connection. This soldering point must be reworked again (from step 4).
  8. Any excess solder or flux on the soldering tip should be removed immediately with a wet sponge.

Tin before soldering?

A very common process in practice is tinning the contact surfaces before the actual soldering process. This means that the contact surfaces are lightly coated with tin with the soldering tip. The advantage here is that the contact surfaces can be brought together better even when they are slightly heated. But if you do this in a clumsy way, the soldering surfaces will stick together nonconductively afterwards. The solder joint then breaks apart even with slight mechanical vibrations. If the soldering point still holds, the already heated solder will oxidize and disrupt the connection. In the worst case, you have caught a cold solder joint. Later, there may be an interruption, possibly even sporadically.
Soldering is really clean when both soldering surfaces are heated together and the solder is placed on them. This means that you have to somehow fix the contact surfaces together before soldering. This is relatively easy with wires. Here you simply twist the wires together. Components with wire connections are clamped in the holes on the circuit board. And SMD components are glued to the circuit board before soldering.

Cold solder joint

A "cold solder joint" is a solder joint with very poor contact properties and leads to a poor electrical connection. In the best case, the contact resistance is just high. In the worst case, there is no electrical connection at all, only a mechanical connection. This means that everything looks fine at first glance. But there is still a disconnection. Therefore, a "cold solder joint" is the worst that can happen to a hobby electronics technician while soldering.
A "cold solder joint" occurs when ...

  • the power of the soldering iron is too weak.
  • the tip is too cold.
  • the solder is applied too early and the contact points are not yet hot enough.
  • the contact points are moved when cooling.

A "cold soldering point" can be recognized by its lack of gloss and often also by the fact that it has the shape of a pearl. With a good solder joint, the solder forms a volcanic shape that shines. Only lead-free solder will result in matt or dull surfaces.

Lead-free soldering / soldering with lead-free solder

Every now and then there are difficulties soldering with lead-free solder. In particular, simple soldering irons are only suitable to a limited extent. Your tip gets very hot after a while and after repeated use it cools down too quickly.
To make matters worse, the quality of the solder joint can hardly be determined with the naked eye. A cold soldered solder joint with lead-free solder always looks somehow like a cold solder joint. Beginners in particular find it difficult to determine whether or not they soldered properly. The experienced hobby electronics technician knows from experience whether the soldering process that has just been completed was successful or not.
If you observe the following rules for lead-free soldering, something could still come of it:

  • Work with a clean and tinned soldering tip.
  • Solder with a temperature of approx. 320 to 340 degrees.
  • You should stay on the solder joint for about 3 seconds longer than usual (so that the solder can run properly).
  • The cooled solder joints have a gray, matt and rough surface. They look like a "cold solder joint".

Desoldering

Solder points are mechanically permanent connections. Soldering points can also be removed again. Usually you do this when replacing components. Sometimes it is enough to heat the solder joint again and then pull out the component. As a rule, however, soldering points can only be removed with great effort and expense.

If solder has to be removed, two different aids can be used. One of the tools is the desoldering pump. It is tensioned by pressure, placed on the heated and liquid solder joint and released. It creates a brief vacuum cleaner effect in which it pulls the hot and liquid solder upwards into itself. The soldering point is then exposed with a slight residue of tin. The contact points can usually be separated from one another.
But be careful, when operating the desoldering pump a recoil occurs, which can damage the circuit board. Therefore, handle the desoldering pump with care.

The second tool is the desoldering braid. It is a fine wire mesh made of copper that soaks up liquid solder when heated. To do this, the desoldering braid must be between the soldering point and the soldering tip. The soldering iron needs a lot of heat so that the solder becomes liquid through the desoldering braid. Desoldering with the desoldering braid removes the tin solder with almost no residue. Only a small layer of solder remains on the conductor track.
Desoldering is very professional with a soldering station with an electric pump. The solder is sucked off through a hole in the desoldering tip. This is the cleanest, but also more expensive solution.

When desoldering, you have to take into account that there is a risk that the desoldered component will no longer work afterwards. During the desoldering process, the soldering process takes considerably longer than during the soldering process. This means that the component is exposed to the heat for longer. Some components break if they are exposed to excessive heat for too long.
When it is certain that the component is no longer needed, it is best to use the desoldering braid. Then the solder joint is also clean. If you still need the component, then the desoldering pump is more suitable.

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