How Solder Paste Works

During the PCB assembly Services process, solder paste is applied to the PCB pads using a stencil or dispensing equipment. The component leads are then placed on top of the solder paste. When the PCB is heated in a reflow oven or wave soldering machine, several key processes occur:

• Melting: The solder paste’s solder alloy particles melt at a specific temperature, typically above the eutectic point for the chosen alloy. This temperature is lower than the melting points of the components being soldered, ensuring that the components are not damaged during soldering.
• Wetting and Bonding: As the solder alloy melts, it wets the surfaces of the component leads and the PCB pads. The flux in the solder paste facilitates this wetting process, ensuring that the solder forms strong and reliable solder joints.
• Solidification: Once the solder alloy cools and solidifies, it creates solid solder joints that mechanically and electrically connect the components to the PCB.

Solder paste comes in various types, each designed to meet specific soldering requirements and manufacturing processes. The choice of solder paste type depends on factors such as the components being soldered, the assembly method, and environmental considerations. Here are some common types of solder paste:

Lead-Based Solder Paste:

• Lead-based solder paste contains a significant percentage of lead in the solder alloy. Common alloys include Sn63/Pb37 (63% tin, 37% lead).
• Lead-based solder pastes were widely used in the past but have seen a decline due to environmental and health concerns associated with lead. However, they are still used in some applications where RoHS (Restriction of Hazardous Substances) compliance is not required.

Lead-Free Solder Paste:

• Lead-free solder paste is formulated without lead to comply with environmental regulations like RoHS.
• Common lead-free solder alloys include Sn96.5/Ag3.0/Cu0.5 (SAC305) and Sn95.5/Ag3.8/Cu0.7 (SAC387). These alloys may contain silver (Ag), copper (Cu), and other elements.
• Lead-free solder pastes are now the standard in many electronic manufacturing processes.

No-Clean Solder Paste:

• No-clean solder paste contains a type of flux that leaves minimal to no residue after soldering. This reduces the need for post-soldering cleaning.
• No-clean pastes are commonly used when residues might be difficult to remove or when cleaning processes are not desirable.

Water-Soluble Solder Paste:

• Water-soluble solder paste contains flux that can be easily cleaned with water after soldering.
• It is often used in applications where thorough residue removal is essential, such as in medical devices or military electronics.

Rosin-Based Solder Paste:

• Rosin-based solder paste uses flux derived from natural rosin. It is known for its excellent wetting properties and reduced spattering during soldering.
• It can leave some residue, but it is generally considered easier to clean than traditional fluxes.

Halide-Free Solder Paste:

• Halide-free solder paste is formulated without halogen-containing compounds (e.g., chloride and bromide).
• It is used in applications where the presence of halides could lead to long-term reliability issues.

Low-Temperature Solder Paste:

• Low-temperature solder paste contains solder alloys with lower melting points, designed to reduce the risk of heat damage to sensitive components.
• It is used in applications where temperature-sensitive components must be soldered.

High-Temperature Solder Paste:

• High-temperature solder paste is designed for applications where the assembly will be subjected to elevated temperatures during its lifetime.
• It contains solder alloys with higher melting points to ensure the solder joints remain stable.

Specialized Solder Pastes:

• Some applications require specialized solder pastes, such as those with enhanced thermal conductivity for power electronics or those designed for high-reliability aerospace and automotive applications.