Differences between lead-free and leaded solders

For a long time, lead-containing alloys were the basis of the electronics industry. Their use was due to the following advantages:

• Excellent wettability, providing better spreading of molten solder over the entire contact pad and flowing into small holes on the board.

• Formation of a strong soldered connection between contact pads and leads of components made of different metals (for example, copper and nickel);

• Low melting point (within 179-190ᵒС) — therefore, less heat load on heat-sensitive components;

• A wide technological window when soldering by reflow of printed circuit boards.

But lead solders also have disadvantages, the main one of which is the toxicity of lead itself. Its vapors, called hydrogen cyanide and carbon monoxide, formed when the solder melts, penetrate the human respiratory tract, accumulate in the body and cause severe poisoning. Lead also accumulates in the soil and groundwater when it enters the environment (for example, when old electronics are disposed of).

That is why the RoHS Directive was adopted in the EU in 2006, which limits (but does not completely eliminate) the use of lead solders in electronics. Instead, tin-based alloys with the addition of copper, silver or zinc (less often gold, nickel and other metals) are used today. The absence of highly toxic lead has had a positive effect on the safety and environmental friendliness of soldering, but at the same time lead-free alloys have their drawbacks:

• Low wettability, reducing the quality of soldered joints;

• High melting point (217-227ᵒС), and as a result, higher peak temperatures during soldering (235–250 °С), long exposure to which can damage heat-sensitive components.

• Shorter service life of electronic products, due to increased sensitivity to humidity.

These problems of lead-free solders are partially solved by introducing additional metals into their composition, as well as by using fluxes that improve the wettability of molten solder by contact surface melts. However, to date they have not yet been able to completely replace lead solders.

The Impact of Lead-Free Soldering on SMT Technology

Surface mounting is much more dependent on solder quality than through-hole mounting, as SMDs do not have mounting holes to improve the quality and reliability of connections. Although the introduction of lead-free compounds has not changed the overall surface mounting process, it has forced technologists to adjust its individual parameters:

• Temperature profile. The main difference between a leaded and a lead-free profile is higher temperatures in the solder paste stabilization and reflow stages, as well as an increase in the preheating and cooling time. The profile construction depends on the selected solder paste and the alloy composition included in it; different alloys have their own melting point. Before constructing a thermal profile, it is necessary to familiarize yourself with the recommendations of the solder paste manufacturer.

• Reflow oven design. Lead-free soldering requires reflow oven models with a large number of heating and cooling zones to ensure the temperature profile parameters.

• Flux composition. They reduce the surface tension of the lead-free melt and improve its spreadability over the contact pads. At the same time, the fluxes themselves must be more heat-resistant than those used in lead soldering. The increased soldering temperature with lead-free solders leads to the fact that flux residues are more difficult to remove from the surface of the printed circuit board. At the stage of checking the board as a whole, unwashed flux residues can distort the operation of measuring devices, showing the presence of false breaks in the circuit. More aggressive cleaning agents are used to remove flux residues at the stage of cleaning the PCB. For electronic devices where cleaning is not a mandatory process, no-clean fluxes developed specifically for lead-free soldering are actively used.

In general, lead-free soldering has had a negative impact on the quality of surface mounting due to more frequent solder joint defects and a shorter service life of electronic products. It is also possible to note worse maintainability associated with the technology of printed circuit board production and the use of higher temperatures. At the same time, it has significantly improved the safety of personnel involved in the production of electronic products, as well as the environmental friendliness of the industry as a whole.

Our company HONG KONG CROSS OCEAN TECHNOLOGY offers customers a choice of both lead-free and lead soldering, mixed installation, technologists who work in our company will analyze your project and advise which type of soldering is most suitable for a specific project, you can send us a request by email [email protected]