How to Read a Solder Paste Spec Sheet: The Numbers That Actually Matter
A solder paste technical data sheet can look like a wall of alloy codes, percentages, test results, and temperature curves. The key is knowing which entries identify the material, which describe laboratory performance, and which define the process window you need to validate on your own assembly.
This guide explains how to read the numbers without confusing powder size with alloy, shelf life with stencil life, or a recommended reflow curve with a production-ready oven recipe.
Start with the exact formulation and document revision
Before comparing specifications, confirm the paste family, alloy, powder type, metal loading, intended application, and packaging. A familiar product-family name does not mean every available configuration has identical properties.
Check the technical data sheet's revision and confirm that it applies to the material you are ordering. Use the technical data sheet for process guidance, the safety data sheet for safety information, and the container label for lot identification and applicable expiry information. If these appear inconsistent, ask the manufacturer to clarify before use.
The solder paste specifications that matter most
| Specification | What to look for | Why it matters |
|---|---|---|
| Alloy | Full metal composition and applicable melting information | Identifies the solder system and helps establish the thermal process requirements. |
| Powder type | Particle-size range, not just the type number | Affects suitability for stencil apertures and dispensing needles. |
| Metal loading | Weight percentage for the exact configuration | Distinguishes formulations that may share an alloy but have different processing characteristics. |
| Flux classification | Classification code, test standard, and revision | Provides context for the manufacturer's flux test results. |
| Viscosity | Value, units, instrument, temperature, and test conditions | Values are not a fair comparison without comparable measurement conditions. |
| Working life | Separate stencil-life, tack-life, and pause/restart claims | These describe different parts of the production process. |
| Reflow guidance | Profile, alloy, atmosphere, and stated operating window | Provides a starting point for profiling the actual assembly. |
| Storage and shelf life | Temperature, unopened/opened conditions, packaging, and handling instructions | Determines how the material must be managed before production. |
1. Read the alloy before the marketing name
The alloy and flux formulation are different parts of the specification. For example, SAC305 identifies a nominal composition of 96.5% tin, 3.0% silver, and 0.5% copper. It does not identify a particular flux chemistry, powder type, or metal loading.
Two pastes using SAC305 may therefore require different handling or deliver different printing performance. Compare the complete configuration rather than assuming that matching alloy names make products interchangeable.
2. Match powder size to the application
For the common powder types below, a higher type number indicates a finer particle-size range. Indium Corporation's powder-selection guidance lists:
| Powder type | Particle-size range | Selection consideration |
|---|---|---|
| Type 3 | 25–45 µm | Check suitability for the smallest stencil aperture or dispensing passage. |
| Type 4 | 20–38 µm | A finer range than Type 3, but still dependent on stencil geometry and process requirements. |
| Type 5 | 15–25 µm | Consider when smaller features require finer powder, with supplier guidance. |
Finer is not automatically better. Indium notes that smaller powder particles increase surface area and susceptibility to oxide formation. Its guidance recommends choosing the largest powder size that can consistently print through the smallest aperture. Stencil thickness, aperture geometry, and paste release also matter; component pitch alone is not enough to select a powder type.
3. Treat metal loading as a weight percentage
Metal loading describes the weight percentage of solder powder in the paste, not the percentage of the printed deposit's volume that will become solder. Indium's product documentation shows that metal loading can vary with powder type and formulation.
Do not assume that a higher percentage is inherently better or that a jar intended for stencil printing is interchangeable with a dispensing formulation. Confirm the exact combination and its intended application.
4. Read flux classifications with their test revision
A code such as ROL0 is a flux classification, not an alloy designation or a complete process qualification. Read it alongside the referenced test standard and revision. This detail can change how a result should be interpreted: Kester's EM919G data sheet, for example, reports ROL0 under J-STD-004 and ROM0 under J-STD-004B for the same product.
Likewise, no-clean does not mean residue-free or that cleaning can never be required. Kester describes no-clean residues as suitable to remain for typical applications while also providing cleaning guidance when removal is needed. Evaluate residue acceptance and any cleaning steps against your assembly and customer requirements.
5. Compare viscosity only when the test conditions match
A viscosity number without its measurement conditions is incomplete. Look for the instrument or test method, measurement temperature, speed or other stated settings, and units. For example, Kester specifies a Malcom viscometer measurement at 10 rpm and 25°C for EM919G rather than presenting its viscosity as a condition-free value.
Use viscosity as one part of the assessment, together with print or dispense trials. A larger number from a different test setup does not establish that one paste will perform better in your process.
6. Separate shelf life, stencil life, and tack life
Shelf life concerns storage under the specified conditions. Stencil life concerns usable time during the printing process under stated conditions. Tack-life and tack-test information address the paste's ability to retain components before reflow. A pause/restart claim addresses another specific operating situation.
Do not substitute one limit for another. A long refrigerated shelf life does not establish how long paste can remain on a stencil. Record the relevant conditions as well as the advertised duration.
7. Use the reflow curve as a starting point
Read the manufacturer's recommended profile for the exact paste and alloy. Where specified, review heating rate, soak conditions, time above liquidus, peak temperature, cooling guidance, and air or nitrogen requirements.
Kester explicitly describes its EM919G recommended profile as a starting point and notes that the final profile depends on board mass and component combination. Validate the actual assembly rather than copying an oven recipe from a different board. This guide intentionally does not prescribe one universal reflow temperature or time window.
8. Follow formulation-specific storage and warm-up instructions
Do not apply one refrigeration range, shelf life, or warm-up time to every solder paste. Supplier guidance varies by formulation and packaging. Follow the applicable technical data sheet and label, including any instructions about container orientation and opened material.
AIM's handling guidance warns against opening cold paste before it has warmed fully because condensation can introduce moisture. It also advises against force-warming paste. Allow the sealed material to reach the required working temperature naturally, following the supplier's procedure. Do not freeze material unless its documentation permits it, and keep used paste separate from unused paste.
A practical review checklist
- Confirm the exact paste, alloy, powder type, metal loading, and packaging.
- Check the document revision and applicable test-standard revisions.
- Compare viscosity and performance claims only with their test conditions.
- Confirm stencil or dispensing suitability with the supplier.
- Review residue acceptance and any required cleaning process.
- Validate the reflow profile on the actual assembly.
- Document storage, warm-up, opening time, and expiry controls.
- Run a controlled trial before treating an alternative paste as a production replacement.
Frequently asked questions
Is Type 4 solder paste always better than Type 3?
No. Type 4 has a finer particle-size range, but the right choice depends on aperture geometry, dispensing requirements, and process performance. Choose for the application rather than the higher number.
Does SAC305 describe the complete solder paste?
No. It identifies the alloy composition. You still need the flux formulation, powder type, metal loading, packaging, and handling specifications.
Does no-clean mean there is no residue?
No. No-clean formulations can leave residue. Whether it can remain depends on the formulation and the application's requirements.
Can two viscosity values be compared directly?
Only after checking units and measurement conditions. Different instruments, temperatures, or test settings can make a simple number-to-number comparison misleading.
Can I use one reflow profile for every paste?
No. Use the applicable manufacturer's profile as a starting point and validate it for your paste, alloy, board, and components.
How long should refrigerated paste warm before opening?
Use the manufacturer's instructions for the exact material and container. Confirm it has reached the required working temperature; do not rely on a universal waiting period or the container's outside temperature alone.
Featured Solder Pastes: Four Brands, Four Specification Examples
Use these products to practice comparing the complete specification rather than selecting by alloy name alone. All four use SAC305, but their powder, flux, package, and handling details differ. They are examples to evaluate, not interchangeable substitutes or a ranking of performance.
Indium 8.9HF, 600g Cartridge | PASTEOT-800495-600G
View Indium 8.9HF SAC305 Type 4 solder paste. This no-clean cartridge configuration illustrates why powder type, metal loading, and printer delivery hardware belong in the same purchasing review. The listing specifies Type 4 powder and 88.5% metal loading; verify the applicable formulation and lot-handling instructions.
Kester NXG1, 500g Jars, Case of 10 | 7032130810
View Kester NXG1 SAC305 solder paste. This Type 3, 88.5%-metal no-clean paste uses ROL1 flux and is sold as ten 500g jars. It is a useful example for comparing pause/restart claims and their conditions without confusing them with refrigerated shelf life.
MG Chemicals 4900P-25G, 25g Syringe
View MG Chemicals 4900P-25G SAC305 no-clean solder paste. The 25g syringe illustrates the distinction between package weight and volume: the manufacturer lists 3.38 mL for this package. Check its application method, flux characteristics, and material-specific storage guidance rather than assuming every SAC305 syringe has the same specifications.
Chip Quik SMD291SNL250T3, 250g Jar
View Chip Quik SMD291SNL250T3 SAC305 solder paste. This print-grade paste combines Type 3 powder, synthetic REL0 no-clean flux, and 88.5% metal loading. Its humidity-dependent stencil-life guidance shows why a working-life figure should always be read with its temperature and relative-humidity conditions.
Choose with the complete specification in view
Browse our Solder Paste collection, then confirm the exact configuration and applicable technical documentation before ordering. Matching the alloy is only the beginning: powder size, flux, application method, storage requirements, and the validated process window complete the decision.
Technical references
Prepared using manufacturer-published references located October 28, 2026: Indium Corporation, Powder Choice and Stencil Design Guidelines (Form 97742, R4) and Indium8.9HF Pb-Free Solder Paste (Form 98485, R27); Kester, EM919G Solder Paste Technical Data Sheet (February 17, 2021) and NXG1 manufacturer technical and part-number documentation; MG Chemicals, 4900P Technical Data Sheet (October 2, 2025, version 4.0); Chip Quik, SMD291SNL250T3 Data Sheet (revision 1.3); and AIM, Soldering Handbook. These references support the general guidance above, not a claim that every formulation shares the same limits. Obtain the applicable current revision for the exact material before production use.
