Solder Paste

Solder paste is used in the manufacturing of printed circuit boards (PCBs), serving as the “adhesive” that connects components to the board. This specialized material consists of a mixture of metal solder particles suspended in a flux medium, which, when heated, allows the printed circuit board’s components to be securely attached. The accurate dispensing and application of solder paste onto PCBs is critical, directly affecting the board’s performance and stability.

There are various methods for applying solder paste to a PCB, with stencil printing and jet printing being two of the most common techniques.

  • Stencil printing, as its name implies, involves using a stencil to control the deposit of solder paste onto specific areas of the board.
  • Jet printing, in contrast, employs a digital process that allows for precise control and even distribution of the solder paste, without the need for a physical stencil.

Factors such as the solder paste’s viscosity, the dispensing method, and the specific application technique contribute to the overall transfer efficiency. By optimizing these factors and employing suitable printing methods, PCB manufacturers can create high-quality boards with consistent reliability.

Composition of Solder Paste

Solder paste is most often used in PCB fabrication involving surface mount technology (SMT). It is a suspension of metal alloy particles in a specialized flux medium. There are three primary types of solder paste alloys that are commonly used, these are: SAC alloys, Lead-Free Alloys, and Tin-Lead Alloys.

SAC Alloys

SAC (Sn-Ag-Cu) Alloys are a widely used lead-free option. They contain tin (Sn), silver (Ag), and copper (Cu) in various proportions. A popular composition is SAC305, which consists of 96.5% Sn, 3.0% Ag, and 0.5% Cu. This alloy has a melting point of around 217-220°C and offers:

  • Good strength and fatigue resistance
  • Higher thermal and electrical conductivity
  • Reduced silver content, which lowers costs

Another common SAC alloy is SAC387 (95.5% Sn, 3.8% Ag, 0.7% Cu). It has a melting point of 217-222°C and provides enhanced melting properties.

Lead-Free Alloys

There are several lead-free alloys available, which have gained importance due to environmental and health concerns associated with lead exposure. Some examples include:

  • Tin-Silver-Copper (SAC): As mentioned earlier, SAC alloys are a popular lead-free choice. They provide excellent mechanical and electrical properties, making them suitable for a wide range of applications.
  • Tin-Copper (SnCu): SnCu alloys contain tin and a small amount of copper. These alloys, such as SnCu0.7 (99.3% Sn, 0.7% Cu), are more affordable than SAC alloys but may provide lower performance in certain aspects.
  • Tin-Silver-Bismuth (SnAgBi): SnAgBi alloys, like Sn42Bi57.6Ag0.4, offer low melting temperatures (138°C) and good wetting properties. However, they are less common due to their higher costs and potential compatibility issues with existing equipment.

Tin-Lead Alloys

Traditional tin-lead (Sn/Pb) alloys are known for their excellent wetting and reliable joint properties. The eutectic Sn63Pb37 is widely used, consisting of 63% tin and 37% lead. This alloy features:

  • A melting point of 183°C, which is lower than the lead-free variants
  • Greater wettability and ease of use
  • Lower costs compared to lead-free alloys

However, due to the health and environmental risks associated with lead, regulations have pushed the industry towards adopting lead-free alternatives.

The Role of Flux in Solder Paste

Flux plays a crucial role in solder paste, serving as a cleaning and deoxidizing agent that ensures a strong bond between metal surfaces during the soldering process. It removes any contaminants and oxides from the metal surfaces, allowing for improved electrical contact and good adhesion of the solder.

There are several types of flux, which can mostly be divided into three categories:

  • Water-soluble fluxes are easy to clean off after soldering since they can be removed with water.
  • No-clean flux, on the other hand, leaves almost no residue after soldering, reducing the post-soldering cleaning process.
  • Halogen-free also represents an important category of flux. This type of flux is designed to have a minimal environmental impact, as it does not contain harmful halogenated compounds.

Different types of solder paste can be made according to the specific type of flux incorporated in the mixture. No-clean solder paste, for instance, uses no-clean flux to achieve residue-free soldering results.

Flux paste is a blend of flux and solder powder, specifically designed for intricate soldering tasks that require additional flux beyond what is included in the solder paste.

Properties of Solder Paste

Flow and Viscosity

One of the most significant properties of solder paste is the flow and viscosity of the paste. The viscosity refers to the thickness and resistance to flow, which determines how easily it can be applied through a syringe or stencil.

Lead-free solder pastes may be more environmentally friendly, but they tend to have higher viscosity compared to traditional lead-based pastes. To achieve optimal flow, it is essential to maintain the right balance between pressure and viscosity during application. This ensures proper wetting and spreading of the paste on the intended surface and prevents issues such as bridging and voiding.

Low-Temperature Solder Pastes

Low-temperature solder pastes are designed to accommodate the requirements of heat-sensitive components and substrates. These pastes melt and solidify at lower temperatures, typically below 200°C, reducing the risk of damage during reflow.

No-Clean Solder Pastes

No-clean solder pastes are formulated with minimal residue fluxes, mentioned earlier, that do not require cleaning after reflow. They create a benign residue which doesn’t negatively impact the performance or reliability of the electronic assembly. No-clean pastes can save manufacturers time and resources by eliminating additional cleaning processes and reducing environmental waste generated by cleaning solutions.

However, there are scenarios where a no-clean solder paste may not be suitable, such as:

  • High-frequency circuits: In these applications, residue-induced parasitic capacitance or impedance issues may arise.
  • High-reliability systems: Products with stringent performance requirements, like medical or aerospace devices, may benefit from additional cleaning processes to ensure longevity and performance.

Assess specific application requirements and weigh the benefits of using no-clean solder pastes against potential drawbacks before deciding on an appropriate solution.

Solder Paste Application

There are several methods of applying solder paste to circuit boards, one of which is the stencil printing process. The stencil, designed according to the board’s layout, assists in depositing an accurate volume of paste. A squeegee glides over the stencil, filling the apertures with solder paste and leaving a consistent layer. Accomplishing a precise application helps to avoid various defects such as insufficient wetting or bridging between solder joints.

Jetting is another reliable technique that proves to be effective for high-density boards with smaller component pitches. In jetting, a specialized dispenser ejects precise, tiny droplets of solder paste directly onto the PCB pads, providing excellent accuracy and repeatability.

In choosing the type of solder paste, factors like the reflow process and desired level of reliability should be considered. During the reflow process, the applied solder paste goes through a series of stages, for instance, preheating, reflow, and cooling. Proper temperature control is required to allow the solder paste to wet metal surfaces and form strong bonds. The reflow profile must be adhered to as deviations can lead to solder joint defects or reliability issues in the final product.

As soldering paste plays such a crucial role in the production of electronic assemblies, it is common to inspect the solder joints for defects after the soldering process. Inspection can be done visually or using automated optical inspection (AOI) systems, which check for potential issues like voiding, insufficient wetting, or tombstoning of components.

Frequently Asked Questions

What are the different types of solder paste?

There are several distinct types of solder paste commonly used in electronics and other applications. They are classified based on the alloy composition, viscosity, and melting temperature. Some common types include lead-based solder paste, lead-free solder paste, low-temperature solder paste, and high-temperature solder paste. Each type has its specific uses and applications in the industry. Additionally, there are other factors to consider that apply to all these solder paste types, such as the size of the balls suspended in the flux (they go by “T” numbers, most commonly T4 and T5)

How is solder paste used in PCB assembly?

Solder paste is primarily used in the surface-mount technology (SMT) process during printed circuit board (PCB) assembly. The paste is applied to the pads on the PCB where components will be attached. The components are then placed on the paste, and the entire assembly is heated in a reflow oven, causing the solder paste to melt and create a secure electrical and mechanical connection. This process ensures that the components are firmly and accurately attached to the PCB.

What are the key specifications to consider when choosing solder paste?

When selecting a solder paste for a specific project, one should consider the following specifications:

  1. Alloy composition: The choice of metal alloys affects the melting temperature, mechanical strength, and conductivity of the paste. Common compositions include tin-lead (Sn-Pb) and lead-free alternatives such as tin-silver-copper (SAC).
  2. Viscosity: Good solder paste should have an optimal viscosity to allow easy printing and component placement, without causing issues like slumping or bridging.
  3. Melting temperature: The paste should have a melting temperature suitable for the components and materials involved in the assembly.
  4. Shelf life: Depending on the usage frequency, one must consider the shelf life of the solder paste to ensure proper performance and storage conditions.

Solder paste vs solder wire: what are the pros and cons?

Solder paste and solder wire each have specific advantages and disadvantages.

Solder paste:

  • Pros: Ideal for surface-mount applications, excellent for high-volume production, and well-suited for precise and automated processes.
  • Cons: Requires a reflow oven, may have a limited shelf life, and can be difficult to apply in smaller, manual operations.

Solder wire:

  • Pros: Versatile for various soldering applications, used for both through-hole and surface-mount components, and suitable for manual soldering.
  • Cons: Slower and less precise compared to solder paste, more challenging for miniaturized components, and generates more fumes during soldering.

How to properly apply solder paste for jewelry?

To apply solder paste effectively for jewelry-making, one should follow these guidelines:

  1. Use an appropriate paste with a low melting point to prevent damaging delicate pieces.
  2. Clean the surfaces of the metal pieces to remove any dirt or impurities.
  3. Apply a small amount of solder paste to the joint using a fine applicator or toothpick.
  4. Position the jewelry components so that the joint is secure and in proper alignment.
  5. Heat the joint with a soldering torch until the solder paste flows and bonds the components. Allow the pieces to cool naturally before handling or cleaning.

What is the role of flux in solder paste?

Flux serves several essential functions in solder paste:

  1. It removes oxide layers from the metal surfaces being soldered, ensuring proper wetting and adhesion.
  2. It reduces solder surface tension, allowing it to flow more readily and evenly.
  3. It prevents the formation of new oxides during the soldering process, which could impede the formation of reliable joints.
  4. It aids in heat transfer, enabling better and more consistent soldering results.

Flux is typically present within the solder paste itself, but additional flux may be applied in certain applications for improved performance.