All About That Solder Paste
All About Solder Paste: Types, Alloys, Powder Sizes, and Process Selection
Solder paste is a homogeneous blend of pre-alloyed solder powder and a flux vehicle that simultaneously provides the metal for joint formation, cleans oxide from pad and lead surfaces, and holds components in place before reflow. Choosing the wrong paste — wrong flux type, wrong alloy, wrong powder size for the stencil aperture, or wrong deposition method — is one of the most common root causes of SMT defects including bridging, voiding, Head-In-Pillow, and insufficient solder volume. Solder paste is available in hundreds of alloys, powder sizes from Type 3 through Type 8, and formulations for printing, dispensing, jetting, dipping, and pin transfer. MTE Solutions is an exclusive Indium Corporation distributor, offering their full line of no-clean, water-soluble, and RMA solder pastes with world-class technical support. For a complete guide to solder materials, flux, and rework, visit our Soldering, Desoldering & Rework hub.
Solder Paste Selection Guide
| Attribute | Options | How to Choose |
|---|---|---|
| Flux Type | No-clean (most common), Water-soluble, RMA (Rosin Mildly Activated) | No-clean: residue stays on board; verify compatibility before conformal coating. Water-soluble: must be cleaned after reflow; higher activity for difficult surfaces. RMA: moderate activity; cleaning optional but recommended for high-reliability applications |
| Alloy | SAC305 (lead-free standard), Sn63/Pb37 (eutectic tin-lead), low-temperature bismuth-tin, high-reliability specialty alloys | SAC305 for RoHS-compliant assembly; Sn63/Pb37 for mil/aero/medical exemptions; low-temp alloys for heat-sensitive components or step soldering; specialty alloys for high-reliability or die-attach applications |
| Powder Size (Type) | Type 3 (25–45 μm), Type 4 (20–38 μm), Type 5 (15–25 μm), Type 6 (5–15 μm), Type 7/8 (ultra-fine for jetting) | Match powder size to smallest stencil aperture: aperture width should be ≥5× the largest powder particle. Finer powder (Type 5+) required for fine-pitch and 01005 components; Type 7/8 for jetting applications |
| Deposition Method | Stencil printing (most common), dispensing, jetting, dipping, pin transfer | Stencil printing: highest throughput, requires stencil design. Dispensing/jetting: no stencil, flexible for mixed or low-volume. Dipping: used for through-hole or connector pin applications. Each method requires a paste formulation optimized for that process |
| Viscosity / Rheology | Controlled by rosin content, solvent system, and rheology modifiers | Higher viscosity for fine-pitch printing (better slump resistance); lower viscosity for dispensing or jetting. Verify print performance with your specific stencil thickness and aperture design before production release |
Frequently Asked Questions
What is the difference between no-clean, water-soluble, and RMA solder paste?
No-clean paste leaves a benign flux residue designed to remain on the board without causing reliability issues — it is the dominant choice in modern electronics manufacturing because it eliminates the cleaning step. Water-soluble paste uses higher-activity organic flux that must be cleaned with DI water after reflow; it is preferred for difficult-to-solder surfaces but requires a validated cleaning process. RMA (Rosin Mildly Activated) paste uses a moderate-activity rosin flux; cleaning is optional but recommended for high-reliability or conformal coating applications. Never leave water-soluble flux residue on a board — it is hygroscopic and will cause corrosion and electrical failures.
How do I choose the right powder size (Type 3 vs. Type 4 vs. Type 5) for my process?
The rule of thumb is that the stencil aperture width should be at least 5 times the diameter of the largest powder particle in the paste. Type 3 (25–45 μm) works for standard SMT with apertures down to about 0.2 mm. Type 4 (20–38 μm) is required for fine-pitch components and apertures below 0.2 mm. Type 5 (15–25 μm) is needed for 01005 components and ultra-fine-pitch QFNs. Using too coarse a powder for a fine aperture causes insufficient solder volume and bridging; using too fine a powder unnecessarily increases paste cost and can affect print rheology.
What causes voiding in solder joints and how does paste selection affect it?
Voiding occurs when flux volatiles, moisture, or entrapped gases cannot escape during reflow before the solder solidifies. Paste formulation affects voiding through flux chemistry (activator type and volatilization profile), powder oxide content (higher oxide = more gas generation), and reflow profile compatibility. For BGA and QFN applications where voiding is a reliability concern, use a paste specifically formulated for low voiding — Indium Corporation offers paste grades optimized for <5% void area under BGAs. Reflow profile optimization (adequate soak zone for flux activation, controlled ramp rate) is equally important.
What technical support is available when selecting or troubleshooting solder paste?
As an exclusive Indium Corporation distributor, MTE Solutions provides access to Indium’s world-renowned technical support team for paste selection, process optimization, and defect troubleshooting. Whether you are dealing with warpage-induced defects, voiding, insufficient solder volume, HiP defects, or reliability failures, Indium’s application engineers can recommend the right paste formulation and process parameters for your specific assembly. Contact our team to connect with technical support.
Browse All Solder Paste at MTE Solutions → | Shop All Indium Corporation Products →
Content developed in partnership with Indium Corporation.
