Understanding solder Paste for PCB Assembly

older paste plays a crucial role in the assembly of printed circuit boards (PCBs).lt is used to create solder joints between the electronic compo-nents and the printed circuit boards. Solder paste is typically applied to the PCBs using a SMT stencil. After the PCBs are populated typically bya pick and place machine, they are placed in a soldering oven also known as reflow oven. During the reflow process,the PCBs are heated to aspedific temperature, causing the solder paste to melt and form a liquid state. The liquid solder then solidifies as it cools, creating a permanentelectrical and mechanical connection between the component leads and the PCB pads.


In this artice, we will dive into the world of solder paste;exploring the different types available and the significance of solder reflow profiles;also known as thermal profiles in achieving successful PCB assembly.

Solder Paste

Solder paste is a mixture of metalaly such as tin-lead (Sn-Pb) or lead-ree like tin-iver-copper(Sn-Ag-Cu) or tin-copper (Sn-Cu), flux, and abinding agent. The metal alloy provides the necessary conductivity and strength for the solder joints.

The biding agent in older paste givesit the necessary consistency and adhesion properties.lt holds the powdered solder particles and fluxtogether and helps the solder paste adhere to the PCB pads and component leads during PCB assembly.
gure 1. Solder Paste printed on a printed circuit board

The binding agent in solder paste gvesit the necessary consistency and adhesion properties.It holds the powdered solder particles and fluxtogether and helps
the solder paste adhere to the PCB pads and component leads during PCB assembly.
Figure 1.Solder Paste printed on a printed circuit board


Solder Paste Types

A common metal lloy til used in solder paste is a combination of tin (Sn) and lead (Pb),commonly known as Sn-Pb solder or leaded solder.However, due to environmental concerns lead-ree alternatives have gained popularity, with tin-siver-copper (Sn-Ag-Cu) and tin-siver (Sn-Ag)alloys being widely used.Hence, current solder paste types can be categorized by their composition as leaded or lead-free:


'Sn-Pbl Leaded Solder Paste:This type of solder paste, consisting of tin and lead, has been the industry standard for many years. lt offersexcellent weting properties and relatvely low meltng temperatures, which make it suitable for a wide range of applications. However, due toenvironmental regulations restricting the use of lead, Sn-Pb solder paste is gradually being replaced by lead-free alternatives

·Lead-Free Solder Paste: Lead-free solder pastes have become the preferred choice in modern PCB assembly processes. They comply withRoHS(Restriction of Hazardous Substances) directives and offer comparable or even better performance than Sn-Pb solder.The mostcommon lead-reeally compositions are Sn-Ag-Cu(also known as SAC) and Sn-Ag.These alloys provide reliable solder joints with good mechanical strength and thermal stability.

* Most electronic contract manufacturers in the USA now use lead-free solder paste.

Flux Types


Flux is a chemical compound that cleans and prepares the metal surfaces to be soldered. t removes oxides and contaminants, promotingproper wetting and adhesion of solder to the components and printed circuit board.Flux also acts as a protective layer, preventing furtheroxicdation during soldering and contoling the solder fow to prevent solder bridges. t facilitates heat transfer, ensuring uniform melting and