RMA solder paste is a type of solder paste that utilizes RMA (Rosin Mildly Activated) flux as its fluxing agent.
Solder paste is a material used in the manufacture of printed circuit boards (PCBs) to connect electronic components to soldering pads. It is a mixture of solder powder and flux. The flux's role is critical; it cleans the metal surfaces to be soldered, preventing oxidation during the soldering process and ensuring a strong, reliable connection.
Understanding RMA Flux
The core component defining RMA solder paste is the RMA flux it contains. According to the reference, RMA (Rosin Mildly Activated) Liquid Solder Flux is:
- A flux type containing rosin and activators. Rosin, derived from pine trees, provides the basic fluxing action and encapsulates residues. Mild activators enhance the cleaning power of the rosin.
- Widely used in electronics soldering. This indicates its common application in assembling electronic devices.
- Characterized by its ability to provide a balance between soldering performance and residue cleanliness. This is a key feature of RMA flux, offering effective soldering while leaving residues that are often considered less corrosive and sometimes left on the board, depending on the specific formulation and application requirements.
Composition of RMA Solder Paste
RMA solder paste is primarily composed of:
- Solder Powder: Tiny spheres of solder alloy (e.g., Tin-Lead, Lead-Free alloys like Tin-Silver-Copper). The specific alloy determines the melting temperature and joint properties.
- RMA Flux: A viscous mixture containing rosin, mild activators, solvent, and rheology modifiers. The flux cleans the surfaces, carries the solder powder, and holds the paste in place until reflow.
Characteristics and Usage
RMA solder paste is favored in many electronics manufacturing processes due to several characteristics:
- Effective Cleaning: The activators, while mild, are sufficient to clean copper and component leads.
- Manageable Residues: Compared to more active flux types (like RA - Rosin Activated), RMA flux leaves residues that are less corrosive. While cleaning is often performed, some RMA residues can be left on the board in certain less critical applications, depending on the flux formulation and reliability requirements.
- Good Printability: Formulated for deposition onto PCBs using stencils or dispensing equipment.
- Stability: Offers a reasonable shelf life and remains stable during the printing and reflow process.
| Feature | Description |
|---|---|
| Flux Type | Rosin Mildly Activated |
| Key Components | Solder powder, Rosin, Mild Activators, Solvent, Additives |
| Primary Use | Electronics assembly (SMT - Surface Mount Technology) |
| Residue Corrosivity | Low (compared to RA flux) |
| Cleaning Requirement | Often optional or application-dependent, but recommended for high reliability |
Comparison with Other Flux Types
RMA stands in contrast to other flux types like:
- RA (Rosin Activated): Contains more aggressive activators for stronger cleaning, but leaves more corrosive residues that typically require cleaning.
- No-Clean: Designed to leave minimal, non-corrosive residues that do not require cleaning after soldering. These fluxes often contain synthetic resins and weaker activators than RMA.
- Water-Soluble: Use organic acids as activators, which are highly active and require thorough cleaning with water after soldering.
RMA offers a middle ground, providing reliable soldering performance with residues that are less problematic than RA, although often still requiring cleaning for optimal long-term reliability.
Practical Considerations
- Residue Management: While sometimes called "no-clean" by older standards, modern high-reliability assemblies typically require cleaning of RMA residues, especially in aerospace, medical, or automotive applications, to prevent potential long-term reliability issues from moisture absorption or electrochemical migration.
- Storage: Like all solder pastes, RMA solder paste requires refrigeration to maintain its viscosity and performance characteristics.
- Application: Applied using stencil printing for high-volume manufacturing or dispensing for prototyping or repair.
In summary, RMA solder paste is a standard material in electronics assembly, defined by its Rosin Mildly Activated flux content, providing a balanced approach to soldering effectiveness and residue characteristics.
