PCB Solder Paste 101 | Everything You Need To Know

The use of PCB solder paste is integral in SMT PCB assembly and production. PCB solder paste is generally used in large scale PCB assembly with infrared reflow machines during the PCB assembly process. It provides significant advantages and its form enables the process to become a simple and easy process. In today’s article, we will go over PCB solder paste in detail, its types, and need in PCB assembly & production.

PCB Solder Paste

PCB Solder paste or solder cream is a mixture of minute tinning solder spheres adhering to a special form of solder flux. It is one of the most important materials in the manufacturing of SMT PCB boards; As it connects SMD components on SMT PCBs. Solder paste also makes it possible to solder through hole pin in paste components by printing solder paste in the holes. It’s a suspension of powder metal solder in a thick fluid medium, which is called flux.

Types Of PCB Solder Paste

Classification on the Basis of Solder Particle Size

Solder Paste Type	Solder Paste Particle Size (Micron)	Gullwing Lead Pitch (mm)/(in)	Square/Circle Aperture (mm)/(in)	Dispense Dot Dia. (mm)/(in)
II	45µm – 75µm	(0.65 / 0.025)	0.65 / 0.025	0.80 / 0.030
III	25µm – 45µm	GLP —> (0.50 / 0.020)	0.50 / 0.020	0.50 / 0.020
IV	20µm – 38µm	GLP —> (0.30 / 0.012)	0.30 / 0.012	0.30 / 0.012
V	15µm – 25µm	(0.20 / 0.008)	0.15 / 0.006	0.25 / 0.010
VI	5µm – 15µm	(0.10 / 0.004)	0.05 / 0.002	0.10 / 0.004

Classification on the Basis of Flux Used:

Rosin based solder pastes

Solder pastes based on rosin are easily cleanable after the soldering process using any off the shelf solvent containing chlorofluorocarbons. These solder pastes contain rosin, a natural extract from pine trees.

No Clean Solder Paste

A no clean solder paste flux contains resins and various levels of solid residues. No clean PCB solder pastes not only cuts down on cleaning costs; but, also saves capital expenditures and floor clearance. However, these pastes need a very clean assembly environment and may need an inert reflow environment.

Water Soluble Flux Solder Pastes

Water soluble solder paste contains fluxes made from various different organic materials and glycol bases. Therefore, they can easily be cleaned using DI water.

Applications & Uses

Reflow Soldering Process

PCB solder pastes have an immense role in SMT PCB reflow soldering process. They are also are used in soldering of SMD PCB assemblies. Moreover, solder paste plays a vital role in creating connection between the leads or termination of surface mount components.

Solder Joints

Solder paste is a homogenous mixture of solder alloy, flux and vehicles. The paste can create a strong metallurgical bond between SMT parts, given the right soldering conditions. This allows PCB designers to create reliable solder joints.

PCB Lead Components

Solder pastes are used to create surface mount technologies (SMT) assemblies with leaded components, which is good for simplifying the design, lowering the fabrication cost, improving the component density, as well as increasing the reliability of the project.

Use in PCB Stencil Printing

PCB solder paste is also used in stencil printing process. The solder paste is layered on the polyester mask or stainless steel to create the necessary pattern on the PCB. It can be done with pin transferring or jet printing through the nozzles.