What indicators are usually included in checking the quality of solder paste?

Checking the quality of solder paste usually requires a comprehensive assessment from a number of dimensions such as physical properties, chemical properties, soldering performance, composition compliance and process reliability. The following are specific indicators and testing methods:

Comprehensive assessment system for solder paste quality

I. Physical Properties

Viscosity and thixotropy

Viscosity should be controlled in the range of 80-300 Pa-s (adjusted according to the printing process), and thixotropy is assessed by the viscosity recovery rate when the shear rate changes. The test is carried out by a rotational viscometer (e.g. Malcom) at a reference speed of 10 rpm, and a squeegee test is carried out at the same time to verify the sharpness of the printing edges. Standards are based on JIS-Z-3284 and IPC-J-STD-004.

Particle Size Distribution

Conventional T3 grade particle size requires 25-45 μm, precision T5 grade requires 15-25 μm, particle shape must be true spherical (orthorhombic/elliptical). Laser particle sizer or microscope combined with image analysis software to detect D10/D50/D90 distribution and sphericity, according to J-STD-005 and IPC-TM-650 2.2.14 standards. Be aware of the risk of splashing due to D10<15μm and stencil clogging due to D90>45μm.

Collapsibility

After printing and baking at 150°C, 0.3mm pitch without bridging should be met. Use IPC-A-21 or IPC-A-20 collapse test board simulation observation, the standard follows the IPC-TM-650 2.4.35. This performance and the flux thixotropic agent content is strongly correlated, it is recommended that the step temperature test (1.5 ℃ / s → 3 ℃ / s) to assess the limit of anti-collapse.

II. Chemical Properties

Flux activity

Activity level needs to comply with the IPC-J-STD-004 R/RMA/RA classification. Copper mirror test to observe the degree of copper surface erosion, wetting balance method to measure the spreading power. The RMA grade (slight penetration) is suitable for consumer electronics, and the RA grade (full penetration) is suitable for industrial scenarios.

Halogen Content

Halogen-free solder paste requires total chlorine and bromine <1500ppm, high halogen products need to specify the application scenario. Quantitative detection by ion chromatography, note that XRF cannot detect organic halogens. Standards are based on JIS-Z-3284 and IPC-J-STD-004.

Metal content and purity

Main components (e.g. Sn96.5/Ag3.0/Cu0.5 for SAC305) need to comply with J-STD-006, impurities (Fe/Zn, etc.) <5 ppm. direct reading spectrometer (OES) or XRF to analyze the proportion of the elements, the standard references J-STD-006 and IPC-TM-650 2.2.20.

III. welding performance

Wettability and extensibility

Cu or Ni/Au pad spreading area > 85%, no edge non-wetting or solder ball defects. Optical microscope / AOI detection of the expansion rate, X-ray assisted verification of the void rate. Standards are based on JIS-Z-3197 and IPC-A-610. >80% for OSP pads and >75% for ENIG pads.

Voiding rate

Voiding rate is <5% for automotive electronics and <3% for dense solder joints such as BGAs. x-ray detector scanning of the internal structure of the solder joints is based on IPC-7095 and IPC-A-610. the main causes include residual volatiles (which can be improved by prolonged preheating) and oxidization of the solder paste (Nitrogen Protected Reflow is recommended).

Melting Point Compatibility

The melting window (time above the liquid phase line) is controlled at 60-90 seconds. Reflow oven thermometers (e.g. KIC) simulate the soldering profile, following IPC-TM-650 2.4.45.

IV. Composition Compliance

Alloy Composition

The main composition of lead-free solder paste needs to meet J-STD-006, lead content <0.1%. Spark direct reading spectrometer to analyze the composition, standard reference IPC-TM-650 2.2.20.

Flux Residue

No-clean type requires surface insulation resistance (SIR) >100MΩ, residue conductivity <10μS/cm. 85℃/85%RH environment 72 hours SIR test to observe the corrosion of copper foil, according to JIS-Z-3197 and IPC-TM-650 2.6.3.2.

V. Process Reliability

Printing adaptability

Pad volume error <±10% for pads under 0.3mm, no bridges or missing prints. 3D SPI equipment detects printed graphic dimensions according to IPC-TM-650 2.4.35. Printing Window Index (PWI) formula: |measured volume-targeted volume |/tolerance range×100%, over 100% process adjustment is required.

Long-term stability

Unopened solder paste has a viscosity change of <8% after being refrigerated at 0-10°C for 3 months. Simulated by accelerated aging at 50°C for 7 days according to IPC-TM-650 2.2.14. Activation energy Ea>50kJ/mol is guaranteed for 6 months.

Mechanical strength

The shear strength of solder joints should meet the standard, and the strength decreases <10% after aging at 150℃/24 hours. Shear tester combined with SEM to analyze the fracture surface, the standard cited IPC-TM-650 2.4.19. Automotive electronics is recommended to add thermal cycling test (-55 ° C ↔ 125 ° C).

VI. Testing tools and standards

Physical testing: rotational viscometer, laser particle sizer, 3D SPI, collapse test plate

Chemical analysis: XRF spectrometer, ion chromatograph, copper mirror test device, wetting balance meter

Welding verification: X-ray detector, AOI system, shear tester, thermal cycling chamber

Core Standard:

IPC-J-STD-004 (flux), IPC-J-STD-005 (solder paste test)

JIS-Z-3284 (Alloy), IPC-7095 (BGA Void)

IPC-TM-650 Series (Test Methods)

Additional requirements for automotive electronics: Ionic contamination <0.75μg/cm² (VDA 2629)

Implementation points:

Incoming material must be checked for viscosity/metal content/halogen, monthly check for solder joint strength/SIR

Process monitoring with in-line viscometer (Malvern Insitec) and AI vision SPI (Koh Young)

Dynamic control by product level: ±10% tolerance for consumer electronics, tightened to ±5% for automotive electronics.

This system ensures consistent performance and long-term reliability of solder pastes in the SMT process through multi-dimensional synergistic verification. In case of deviation from key indicators (e.g. metal content deviation >0.5%), corrective actions are immediately initiated and the root cause is traced.