Solder paste is a crucial material in the domain of modern electronic assembly technology. It is used for soldering electronic components onto printed circuit boards (PCBs), this enabling the creation of ever-evolving technological products.

The invention of solder paste played a pivotal role in the miniaturization of electronic product assembly technology. It transformed bulky devices, such as early large sized mobile phones, into pocket-sized, feature-rich smartphones.

The term “paste” in solder paste is used because its pre-melted form is similar in consistency to the toothpaste we use daily. Before soldering, the paste-like solder can be applied to hold electronic components in place on the surface of the PCB. This ensures that these components remain in position, even under slight vibrations. Its primary function, however, is to solder electronic components onto the PCB, facilitating the connectivity of electronic signals.

But did you know what other materials are concealed within the solder paste, besides “tin,” which is its main component?

Main Components of Solder Paste: Flux and Solder Powder

Solder paste consists primarily of two components, flux, and solder powder, which are thoroughly mixed. Below, I’ll briefly explain the content and considerations of these two components:

Solder Powder:

Solder powder refers to particles of metal alloy used to manifest solderability and bonding strength. Its main constituents include the following metals:

• Sn (Tin)
  

• Ag (Silver)
  

• Cu (Copper)
  

• Bi (Bismuth)

The composition of solder powder varies depending on the solder paste type and brand. Even with the same type number, different brands may have slight variations, sometimes to circumvent patents or due to unique proprietary formulations.

For instance, the widely used SAC305 comprises tin (Sn, 96.5%), silver (Ag, 3%), and copper (Cu, 0.5%). Another example, SAC0307, utilizes tin (Sn, 99%), silver (Ag, 0.3%), and copper (Cu, 0.7%).

Additionally, low-temperature solder pastes employ SnBi or SnBiAg alloys. The SnBi alloy has tin (Sn, 42%) and bismuth (Bi, 58%) with a melting point of 138°C, while the SnBiAg alloy consists of tin (Sn, 64%), bismuth (Bi, 35%), and silver (Ag, 1%) with a melting point of 178°C.

Furthermore, for copper-based PCBs (OSP, bare copper boards), some manufacturers add a small amount of nickel (Ni) to the solder paste. This addition suppresses and slows down the transformation of benign Cu₅Sn₆ intermetallic compound (IMC) into the detrimental Cu₃Sn over time. To delve deeper into the concept of IMC, please refer to the article on What purpose of alloy metal of Cu, Ag, Zn, Sb, Bi added to solder paste?

The composition and particle size of solder powder vary. The size is indicated by a designation, but this designation is not entirely standardized across suppliers. Generally, smaller numbers denote larger particles.

The designation and diameter sizes are approximately as follows. Please note that the table is based on the IPC J-STD-006A specification, which has been replaced with IPC J-STD-005 after version B. Therefore, the table may not be the most accurate and is for reference only.

Besides industrial standard designations, solder paste suppliers may use their own designations, such as Type 4.5 or Type 7, Type 8, to accommodate specific customer requirements. These specifications outside the standard may vary slightly among different solder paste manufacturers, so detailed specifications should be obtained from individual suppliers.

Solder powder size plays a crucial role in the efficiency of electronics manufacturing. Smaller particles generally result in better solder deposition. This is because smaller particles can more easily pass through the opened apertures  in the stencil, facilitating precise transfer onto the PCB. Additionally, smaller particles are less likely to leave residue on the vertical surface of the stencil apertures, contributing to improved accuracy when printing solder paste for fine-pitch components. Smaller solder particles also enhance the paste’s resistance to collapse and exhibit better wetting characteristics.

However, smaller solder powder is more prone to oxidation. To mitigate this, nitrogen gas (N₂) may be used to reduce the oxidation rate, especially when soldering large pads (e.g., shielding frames) with a designation of type 5 of solder powder or above. This is because smaller particles have a larger surface area in contact with air, making them more susceptible to oxidation. Therefore, the selection of solder paste should consider not only the size of solder particles but also the uniformity of particle size, depending on the product requirements.

A user asked why smaller solder powder has a larger surface area in contact with air. We can visualize this by considering a cube composed of solder particles with a diameter of r. This cube can accommodate 16 particles. On the other hand, a cube with solder particles of diameter 2r can only fit 2 particles. Calculating the surface area of each:

• Surface area of diameter r solder powder: 16 x 4πr² = 64πr²
  
• Surface area of diameter 2r solder powder: 2 x 4π(2r)² = 32πr²

Therefore, within the same volume (same stencil opening), smaller solder particles have a larger total surface area, making them more prone to oxidation.

In SMT soldering, type 3 solder powder is commonly used, while type 4 is used for fine-pitch or small pad soldering. Types 5 and 6 are typically employed for bumping in processes like Flip Chip or CSP. The cost of solder powder tends to increase with decreasing particle size due to the challenges in manufacturing smaller particles.

Despite efforts to prevent oxidation, a certain level of oxidation may still occur. This residual oxidation, especially on the surface of metal particles, can be beneficial. It helps prevent solder paste from self-fusing before being officially printed on the circuit board. This is because pure metals placed together tend to fuse, illustrating the concept of “like attracts like.”

Oxidation primarily depends on three factors: temperature, air, and water.

Solder powder comes in two main shapes—spherical and elliptical. Spherical solder powder is preferred for its broad applicability, smaller surface area, lower oxidation, and bright solder joints. In contrast, elliptical solder powder has comparatively inferior characteristics.

Flux:

Flux is the powerhouse that transforms solder paste into a paste-like consistency. It contains solvents that blend all the materials together to create a paste.

For further reading: Why is washing necessary after PCB soldering? Differences between washing and no-clean processes; Types of flux.

The primary purpose of flux is to remove metal surface oxides and impurities. During high-temperature operations, it forms a thin coating on the metal surface, isolating it from the air and preventing solder paste oxidation. Flux composition mainly includes four components:

• Rosin or Resin: 40-50%
  Rosin can be natural (Rosin) or artificially synthesized (Resin). Typically, leaded solder paste uses Rosin, while lead-free solder paste employs Resin. Rosin forms a protective layer on the soldered metal surface, isolating it from the air, preventing oxidation. It is adhesive and has the ability to clean the metal surface slightly.

• Activator: 2-5%
  Composed of organic acids and halides, it has a powerful ability to clean the metal surface. Often used as a cleaner during reflow soldering, it dissolves metal surface oxides, improving soldering efficiency. Halides are highly toxic. To meet modern environmental standards, halogen-free solder paste is becoming a trend. However, due to the strong deoxidizing ability and cost-effectiveness of halides, they are still frequently used in some solder pastes.

• Solvent: 30%
  Includes components like ethanol. These solvents evaporate during the preheating process of solder paste, so they do not affect the soldering properties of the paste. They help dissolve and mix different chemical substances in the flux more uniformly, enhancing the effectiveness of the flux. Solvents also make the flux easier to handle, allowing control of the viscosity and flowability of the solder paste. If the preheating temperature during reflow increases too quickly, it may cause immediate boiling of these solvents, resulting in solder paste splattering issues.

• Rheology Modifier: 5%
  Provides thixotropy or shear-thinning to control the viscosity of solder paste, achieving the paste-like consistency. It enhances the printability and anti-collapse properties of the solder paste, ensuring that the paste retains its original shape after printing on the circuit board, preventing solder shorts.

Additionally, there’s one more thing to remind you of. When calculating the ratio of solder paste components by weight, the ratio of solder powder to flux is approximately 90% to 10% because solder powder is heavier. However, if you calculate the ratio by volume, the ratio is approximately 50% solder powder to 50% flux. This can impact the calculation of the solder paste amount after reflow soldering. In other words, one unit of solder paste, after reflow soldering, will result in approximately 0.5 units of solder.

For further reading: A visual explanation of how to calculate the printed solder amount in the SMT through-hole reflow soldering (PIH) process.

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• How to print solder paste on PC Board and screen printing notices?