What is Lead-Free (RoHS) Soldering?

The soldering process which uses a lead-free solder material is called RoHS or Lead-free Soldering. Lead (Pb) based elements are Mercury, Cadmium, Hexavalent Chromium, Polybrominated Biphenyls, and Polybrominated Diphenyl Ether. Over time these materials can form toxic substances which can threaten human life and the environment.

To avoid the use of Lead (Pb) based soldering materials, the European Union created a directive called RoHS (Restriction of Hazardous Substances) which restricts the use of electronics which use lead-based solder materials.

We are using more and more electronic devices every day. All these devices have components which are connected to a PCB using solder. Lead (Pb) is a dangerous material and it takes very long to decompose. So it is best to try and avoid the use of this material in our daily lives. As a result, most components are soldered on to PCBs using Lead-Free ROHS solder.

However, lead-free solder does have some disadvantages compared to non-RoHS Soldering:

• Thermal Stress: Lead-free solder has a higher melting point. This means it melts at a higher temperature than lead-based solder which can cause more thermal stress to the components being soldered.
• Worse Wettability: The wettability is a measurement of adhesion between the solid and liquid phases. When a high temperature is applied to the lead-free solder, it melts and joins the component to the pad by forming a bond, but the strength of this bond is not as good as that of the Lead-based solder. The bond strength is weak due to the bad quality of the adhesive bond between the contact pad and the component terminal.
• Defects: Due to worse wettability in the lead-free solder joints, the chances of defects like Vacancy, and Displacement are quite high.
• Residue Formation: Lead-free solder generates surface oxide and flux contaminant which hampers the strength of the solder joint. Prolonged application of high temperature at the time of soldering also degrades the soldering due to oxidization.
• Grainy joints: This happens due to high temperature and dissolution of the metal to be joined while soldering.
• Cold solder joints: This happens due to insufficient flux in the solder wire.
• De-wetting: Due to the prolonged tip contact and dissolution of the plated metals to be joined.
• Flagging: Low flux volume and high temperature are responsible for flagging.
• Difficulty with the cleaning of residues

However, in spite of these issues, RoHS solder has now become a standard as non-RoHS solder is very harmful.