What Are the Different Types of Solder Used in SMT Assembly?

Different Types of Solder Used in SMT Assembly

In smt assembly, a stencil is used to print a proper amount of solder paste on the pads where components are to be fixed. This is a critical step, since incorrect placement will result in failure of the assembled circuit board. After the solder paste is printed, the component positions are inspected to ensure that they have been properly placed. Then the PCB is inserted into a reflow soldering oven, which will melt the solder paste and permanently fix the components to the pad.

Before the era of SMT, through-hole technology was the standard method for mounting components onto a PCB. Through-hole technology required the creation of holes in the PCB that were sized to fit each component. This process was complex and labor intensive, but SMT allows the manufacturer to eliminate this step and dramatically reduce production costs.

To perform SMT, the PCB is first cleaned with an alcohol-based IPA solution. This will remove any oxidized material from the surface and prepare it for the reflow soldering process. A suitable flux is also applied to the surface, which will remove any oxide and promote wetting of the solder.

Depending on the application, different types of solder are available. Lead-alloy solders were the original choice for use in electronics, and they still have good wetting properties and resistance to oxidation. However, due to environmental concerns, these have been largely replaced by lead-free alternatives.

What Are the Different Types of Solder Used in SMT Assembly?

The main reason for this shift was the lower risk of toxicity for workers. Typically, lead-free solders are composed of tin, silver and copper with small amounts of other metals such as bismuth, zinc, antimony and nickel. The most popular alloys are SAC305 (96.5% tin with 3% silver and 0.5% copper) and SAC387 (96.5% tin with 3.8% silver and 0.7% copper).

Once the solder has been applied, the assembly is then heated in a reflow soldering oven. The temperature of the oven needs to be carefully controlled in order to ensure that the solder is melted and consolidated in a reasonable time. The temperature must also be stable to avoid overheating, which can cause the solder to deform and fail to form a good joint.

The reflow soldering process is then completed by the addition of a finishing agent. This can be either a water-soluble flux, which is made from organic materials and glycol bases or an acid core soldering flux. These are both designed to remove the oxide from the surface of the melted solder and promote wetting, while also promoting good conductivity between the parts being joined. The final product is then inspected for quality and function.

Depending on the specific application, this may include visual inspection (AOI), ICT and function test. If the final product meets all of the requirements, it can then be shipped to customers. The entire SMT assembly process can be a very delicate and complex task, which is why it requires experienced professionals to complete it successfully.