A solder paste technical data sheet may list 88%, 89% or 90% metal. The number appears simple, but it is easy to interpret it incorrectly.
In most solder paste documentation, metal content describes the mass percentage of alloy powder in the complete paste. It does not mean that alloy occupies the same percentage of the physical volume.
Metal loading also cannot determine by itself whether one paste will print, dispense or reflow better than another. A valid comparison must also consider the alloy, powder Type, flux vehicle, application method and test basis.
Key point: A 90% metal value usually describes weight. To understand volume or process behavior, density and the complete formulation must also be considered.
Readers who need the material basics first can review what solder paste is and how alloy powder and flux work as one material system.
Quick Answer: What Does 90% Metal Content Mean?
Solder paste metal content by weight is calculated as:
Metal content by weight = mass of recovered solder alloy ÷ original solder paste mass × 100
For example, one kilogram of paste with a nominal metal content of 90% contains approximately:
- 900 grams of alloy powder;
- 100 grams of the complete flux vehicle.
This describes mass only. Because solder alloy is much denser than the organic vehicle, the metal volume percentage will be far below 90%.
A higher weight percentage is not automatically better. The appropriate metal-loading window depends on whether the product is designed for stencil printing, dispensing, jetting, rework or another process.
What Is Solder Paste Metal Content?
Solder paste works through the interaction of alloy powder and a formulated flux vehicle. The vehicle may contain resins, activators, solvents and rheology modifiers that support storage stability, oxide removal, tack, deposition and reflow.
Metal Content by Weight
Metal loading is commonly reported as a weight percentage. A value of 90% by weight means that 90% of the measured paste mass is alloy and the remaining 10% is the complete non-metal vehicle.
The current IPC J-STD-005B requirements for solder pastes include metal content among solder paste characterization topics.
Metal Content by Volume
Metal content by volume describes the physical space occupied by the alloy relative to the calculated total volume of the alloy and vehicle.
It is normally much lower than the weight percentage because the density of solder alloy is substantially higher than that of the flux-containing vehicle.
Metal Content vs. Flux Vehicle and Flux Solids
The percentage remaining after the metal fraction is subtracted represents the complete flux vehicle by weight when both values use the same basis. It should not automatically be described as flux solids.
The vehicle can contain volatile and non-volatile ingredients. "Flux solids" may refer to a separately defined non-volatile fraction, and the exact basis should be confirmed from the applicable specification or product documentation.
The distinction between the complete paste and a separate flux is explained further in the comparison of flux paste and solder paste.
How to Estimate Weight Percent vs. Volume Percent
A weight percentage can be converted into an estimated volume fraction only when suitable density values are available.
Estimated metal volume fraction = (metal weight fraction ÷ metal density) ÷ [(metal weight fraction ÷ metal density) + (vehicle weight fraction ÷ vehicle density)]
The result can then be multiplied by 100 to express a percentage.
Illustrative Calculation
The following values are hypothetical and are used only to demonstrate the calculation. They do not represent a specific commercial formulation.
- metal weight fraction: 0.90;
- vehicle weight fraction: 0.10;
- assumed alloy density: 7.5 g/cm³;
- assumed vehicle density: 1.0 g/cm³.
Calculated metal volume = 0.90 ÷ 7.5 = 0.12
Calculated vehicle volume = 0.10 ÷ 1.0 = 0.10
Estimated metal volume fraction = 0.12 ÷ (0.12 + 0.10) ≈ 54.5%
This example shows why 90% by weight can correspond to a much lower percentage by volume. The actual value changes with alloy density, vehicle density and formulation. The vehicle is also a multi-component system, so this calculation is an estimate rather than a method for reverse-engineering a proprietary paste.
Why Metal Loading Is Product-Specific
| Product Factor | Why It Changes the Interpretation |
|---|---|
| Alloy composition | Different alloys have different densities, melting behavior and process requirements. |
| Powder Type | Particle distribution affects total surface area, packing, viscosity and deposition behavior. |
| Flux vehicle | Resins, solvents, activators and rheology modifiers determine flow, recovery, tack and residue behavior. |
| Application method | Stencil printing, dispensing and jetting require different combinations of flow, stability and particle behavior. |
| Package and conditioning | Jar, syringe and cartridge products can use different handling and delivery systems. |
Different tin-based alloys should therefore be reviewed as complete material systems. The guide to tin solder alloys for PCB assembly provides additional alloy context.
Many formulation families use different metal-loading windows for printing and dispensing, but no single percentage applies to every supplier or alloy. A product designed for jet printing may require another balance because droplet formation, nozzle behavior and fine powder become critical.
How Metal Content Can Affect SMT Performance
| Process Characteristic | Possible Metal-Loading Influence | Other Factors That Must Be Checked |
|---|---|---|
| Viscosity and stencil rolling | The powder-to-vehicle ratio can change apparent flow resistance and paste movement. | Temperature, conditioning, powder shape, rheology and printer settings. |
| Slump and deposit shape | Metal loading can contribute to structural stability after deposition. | Flux vehicle, stencil geometry, deposit height, environment and delay before reflow. |
| Aperture filling and release | The formulation must balance filling under force with separation from the aperture wall. | Powder Type, area ratio, foil condition, board support and separation settings. |
| Dispensing pressure | An unsuitable powder-to-vehicle balance can contribute to high pressure or unstable deposits. | Needle size, powder Type, temperature, equipment and dispensing profile. |
| Wet deposit and reflowed alloy volume | The metal volume fraction helps estimate how much alloy is present in a wet deposit. | Transfer efficiency, coalescence, wetting, voiding, residue and final joint geometry. |
Viscosity, tack and shear behavior should be evaluated together rather than inferred from one percentage. These relationships are discussed in the article on solder paste physical properties.
Wet Paste Volume Is Not the Final Solder Joint Volume
During reflow, the alloy powder coalesces into the metallic joint while volatile vehicle components leave the deposit and other ingredients may remain as residue.
A simplified planning estimate may use:
Estimated alloy volume in the wet deposit = wet deposit volume × estimated metal volume fraction
This is not a complete prediction of final joint geometry. The result can also be affected by stencil transfer, powder packing, coalescence, pad wetting, component termination geometry, voiding and process loss.
Deposit volume and consistency should be verified through solder paste inspection before placement and reflow hide the original printed condition.
Does Higher Metal Content Mean Better Solder Paste?
No universal ranking can be made from the highest number.
A higher metal percentage may support deposit stability in one stencil-printing formulation, while another paste may need a different vehicle balance to dispense through a needle, maintain tack or support a specialized rework process.
The useful question is:
Does the complete formulation deliver the required deposit volume, printing or dispensing stability, wetting and residue performance in the intended process?
Before selecting a product, review the intended solder paste application method rather than transferring a metal-loading target from another process.
Typical, Nominal and Specification Values Are Not the Same
| Value Type | What It Usually Means | How It Should Be Used |
|---|---|---|
| Typical value | A representative result, not necessarily a guaranteed lot limit. | Use for preliminary comparison and process planning. |
| Nominal value | An intended reference or formulation target. | Check whether a tolerance or range is also stated. |
| Target | A desired operating or formulation value. | Do not treat it automatically as an incoming acceptance limit. |
| Specification range | A stated minimum and maximum used for defined acceptance. | Confirm the method, revision and contractual status. |
| Guaranteed value | A supplier commitment under specified conditions. | Confirm how compliance is reported, including whether it appears on the CoA. |
A TDS that lists "90% nominal" does not necessarily mean that every lot must measure exactly 90.00%. Procurement and quality teams should define the required range, test method, reporting frequency and disposition rule in the purchase specification or quality agreement.
How to Compare Metal Content on Two TDS Documents
Before ranking two products, confirm that the values are genuinely comparable.
| Comparison Field | What to Verify |
|---|---|
| Alloy | Exact composition and density basis. |
| Metal content | Weight percent, volume percent or another measurement basis. |
| Value status | Typical, nominal, target, range or guaranteed value. |
| Powder Type | Particle distribution and intended feature size. |
| Application | Stencil printing, dispensing, jetting or rework. |
| Flux system | No-clean, washable or another product-specific chemistry. |
| Viscosity | Test method, temperature, equipment, speed and units. |
| Lot evidence | CoA data, specification tolerance and traceability. |
| Process evidence | Printing, SPI, dispensing and reflow results. |
Illustrative Grey-Area Comparison
The following example is hypothetical and does not represent specific commercial products.
Paste A lists 90% nominal metal content for SAC305 Type 4 stencil printing. Paste B lists an 89% typical value for a different alloy and a dispensing application. Paste A cannot be declared better from the one-percentage-point difference.
The comparison is also incomplete because one value is nominal and the other is typical, the alloys differ and the application methods are not the same. A valid decision requires comparable test bases and process evidence.
Specialized fine-feature processes may require products such as semiconductor solder paste, where powder distribution and process capability matter more than a simple percentage ranking.
How Is Solder Paste Metal Content Verified?
IPC-TM-650 2.2.20, Solder Paste Metal Content by Weight, is an IPC test method for determining the metal percentage by mass.
In simplified terms, a known paste mass is processed so that the alloy can be separated from the non-metal residue. The recovered metal is dried and weighed, and its mass is compared with the original sample mass.
The method should be performed by qualified personnel using the applicable controlled procedure. It should not be treated by itself as a universal contractual acceptance limit; the required value and tolerance still need to be defined by the product specification or purchasing agreement.
Sampling and Measurement Uncertainty
A test result is only as useful as the sample and method behind it.
- Sample representativeness: The sample should represent the conditioned product according to the applicable method and supplier instructions.
- Material history: Solvent loss, contamination or uncontrolled exposure can change the condition of an opened sample.
- Separation completeness: Residual vehicle left with the metal, or metal lost during processing, can bias the result.
- Balance and method resolution: Small differences should not be interpreted without understanding test capability and repeatability.
- Replicate results: A single measurement may not describe lot variation or laboratory repeatability.
- Acceptance tolerance: The difference between 89% and 90% is meaningful only when the method uncertainty and contractual range make it meaningful.
For routine control, companies may rely on supplier specifications, lot traceability, a Certificate of Analysis, periodic verification and a defined quality agreement.
Troubleshooting: Is Metal Loading the Real Cause?
| Observed Problem | Metal Loading May Be Relevant | Other Causes to Check |
|---|---|---|
| Paste slumps after printing | Possibly | Flux rheology, temperature, deposit geometry and delay before reflow. |
| Paste rolls poorly | Possibly | Conditioning, storage, viscosity method and printer settings. |
| Poor aperture release | Possibly | Area ratio, powder Type, stencil condition and separation speed. |
| High dispensing pressure | Possibly | Needle size, temperature, equipment profile and formulation. |
| Insufficient solder after reflow | Possibly | Aperture volume, transfer efficiency, coalescence and pad geometry. |
| Excessive residue | Indirectly | Flux chemistry, applied volume and reflow profile. |
| Solder balls or spattering | Indirectly | Moisture, contamination, solvent release and heating conditions. |
Metal loading should remain one diagnostic input rather than the automatic root cause. Broader solder performance evaluation methods can help separate material and process effects.
Metal Content Review Checklist
- confirm the exact alloy;
- confirm whether the percentage is by weight or volume;
- identify whether the value is typical, nominal or a specification range;
- record the powder Type and intended application;
- review the viscosity test conditions;
- confirm whether metal content appears on the CoA;
- define the required tolerance in the purchasing agreement;
- review printing, dispensing or jetting evidence;
- verify deposit volume through the actual process;
- evaluate the complete flux, residue and cleaning system.
Where cleaning is required, the full formulation should be assessed as a system. A purpose-designed washable SMT solder paste should not be compared with a no-clean paste from metal percentage alone.
Relevant formulations can be reviewed in the YIHMA solder paste range.
FAQ
Q: What does 90% metal content mean in solder paste?
A: It usually means that 90% of the paste mass is alloy powder and about 10% is the complete flux vehicle. It does not mean that alloy occupies 90% of the physical volume.
Q: How can weight percent be converted to volume percent?
A: Divide each component's weight fraction by its density, then divide the calculated metal volume by the total calculated component volume. The result is an estimate because the vehicle is a multi-component formulation.
Q: Is higher metal loading better for stencil printing?
A: Not as a universal rule. Many product families use different loading windows for printing and dispensing, but performance depends on the complete alloy, powder and flux system.
Q: Does metal content determine final solder joint volume?
A: It helps estimate the alloy present in the wet deposit, but transfer efficiency, coalescence, wetting, voiding and joint geometry also affect the final result.
Q: Should metal content appear on the CoA?
A: That depends on the supplier and purchasing agreement. Include it as a required lot-reporting item when it is important to incoming acceptance or process control.
Q: Is flux percentage simply 100 minus metal content?
A: It can represent the remaining complete flux vehicle by weight when both values use the same basis. It should not be confused with flux solids content or halide concentration.
Conclusion
Solder paste metal content usually describes alloy mass, not physical volume. Density explains why a value near 90% by weight can correspond to a much lower metal volume fraction.
The number becomes meaningful only when the alloy, powder Type, application, value status and test method are comparable. Final material approval should come from documented specifications and process evidence rather than the highest percentage on a TDS.
For a recommendation based on alloy, powder Type, metal loading, stencil or dispensing requirements and target deposit volume, submit the process details through the YIHMA inquiry page.
