Solder performance evaluation methods：solder paste：solder paste

★ Solder performance evaluation methods：solder bar

Solder performance evaluation primarily considers three aspects: physicochemical characteristics, process characteristics, and reliability.

The requirements for the physicochemical characteristics of solder paste mainly refer to the international standard IPC/J-STD-005. The main test items are as follows:

① Determination of metal particle size.
The particle size must be determined by a test method agreed upon by the user and the supplier.
② Requirements for metal particle size.
The principle for selecting the alloy powder particle size is that the solder paste prepared with this alloy powder can meet and achieve good printing on the printed circuit board, and there is no phenomenon of unrecoverable solder at the edges after welding.

The principles for selecting alloy powder particle size are as follows: the solder paste formulated with the alloy powder should be able to meet and achieve good printing on printed circuit boards, and there should be no unrecoverable solder at the edges after soldering.
Unless otherwise specified, for system boards with larger components, the alloy powder particle size should be between 25 and 45 μm (i.e., No. 3 powder); for mobile phone PCBs with smaller components, the alloy powder particle size should be between 20 and 45 μm (i.e., both No. 3 and No. 4 powders are acceptable).
③ Metal particle size.
When measuring solder particle size, it should be classified according to the standard sieves or the densest sieves specified in Tables 2.4 and 2.5.

Type	Not greater than	Less than 1% particles larger than	At least 80% particles in the range	At most 10% particles smaller than
1	180 μm	150 μm	150–75 μm	20 μm
2	90 μm	75 μm	75–45 μm	20 μm
3	53 μm	45 μm	45–25 μm	20 μm

Type	Not greater than	Less than 1% particles larger than	At least 90% particles in the range	At most 10% particles smaller than
4	45 μm	38 μm	38–20 μm	20 μm
5	32 μm	25 μm	25–15 μm	15 μm
6	25 μm	15 μm	15–5 μm	5 μm

④ Maximum Powder Size (Grinding Fineness).
The maximum particle size shall be determined according to ASTM D-1210 or IPC-TM-650 Test Method 2.2.14.3, requiring the use of a Hegman gauge (model CMA185) or equivalent instrument.
Solder Powder Size Distribution
The powder particle size distribution shall be determined according to IPC-TM-650 Test Methods 2.2.14, 2.2.14.1, and 2.2.14.2.

⑤ Solder Powder Particle Shape.
The solder powder particles should be spherical, with a maximum aspect ratio of 1.5. The measurement method follows IPC-TM-650 test methods 3.3.3.1.1 and 3.3.3.1.2. Other shapes are acceptable if agreed upon by the user and supplier.

Solder Powder Particle Shape Measurement Method
The solder powder particle shape is determined by visual inspection using a binocular microscope with sufficient magnification to determine whether the particles are spherical (aspect ratio less than 1.5) or elliptical. If more than 90% of the particles are spherical, they are classified as spherical; otherwise, they are classified as non-spherical.

Solder Powder Particle Roundness
The roundness of the solder powder particles is tested using a light scattering instrument. A value between 1.0 (perfect sphere) and 1.07 is considered spherical; values greater than 1.07 are considered non-spherical.

Measured according to IPC-TM-650 test method 2.2.20, the alloy content must be between 65% and 96% (by weight). The metal content percentage must remain within ±1% deviation of the nominal value specified in the order.

① Viscosity Measurement Requirements.
The measured viscosity must be within ±10% deviation of the value required in the order. Measurement and test conditions are in accordance with 4.2.3.1.
② Viscosity Measurement Method. Solder paste with a viscosity range of 300,000 to 1,600,000 centipoise should be measured according to IPC-TM-650 test method 2.4.34 or 2.4.34.2; while solder paste with a viscosity range of 50,000 to 300,000 centipoise should be measured according to IPC-TM-650 test method 2.4.34.1 or 2.4.34.3.

Unless otherwise specified in the contract or purchase order, the slump test shall be performed using two different stencil thicknesses and three different pad opening sizes.

① Test using a 0.2mm thick stencil.

See Figure 2.19 (pattern in IPC-A-21). When tested according to section 5.2.1 of IPC-TM-650 test method 2.4.35, there shall be no short circuits between 0.63mm x 2.03mm pads with a spacing of 0.56mm or greater; when tested according to section 5.2.2 of IPC-TM-650 test method 2.4.35, there shall be no short circuits between 0.63mm x 2.03mm pads with a spacing of 0.63mm or greater; when tested according to section 5.2.1 of IPC-TM-650 test method 2.4.35, there shall be no short circuits between 0.33mm x 2.03mm pads with a spacing of 0.25mm or greater; when tested according to section 5.2.2 of IPC-TM-650 test method 2.4.35, there shall be no short circuits between 0.33mm x 2.03mm pads with a spacing of 0.30mm or greater.

② Tested using a 0.1mm thick steel stencil.

See Figure 2.20 (pattern in IPC-A-21). When tested according to section 5.2.1 of IPC-TM-650 test method 2.4.35, there should be no short circuits between 0.33mm x 2.03mm pads with a spacing of 0.25mm or greater; when tested according to section 5.2.2 of IPC-TM-650 test method 2.4.35, there should be no short circuits between 0.33mm x 2.03mm pads with a spacing of 0.30mm or greater; when tested according to section 5.2.1 of IPC-TM-650 test method 2.4.35, there should be no short circuits between 0.2mm x 2.03mm pads with a spacing of 0.175mm or greater; when tested according to section 5.2.2 of IPC-TM-650 test method 2.4.35, there should be no short circuits between 0.2mm x 2.03mm pads with a spacing of 0.20mm or greater.

When the solder paste is tested according to the following available test methods, it must meet the specified requirements.

① Powder Grades 1-4:

Powder grades 1-4, classified according to IPC-TM-650 test method 2.4.43, must meet the acceptance criteria shown in Figure 2.21 during testing. Furthermore, during the evaluation process, no single solder ball larger than 75 microns should appear in more than one of the three test patterns.
② Powder Grades 5-6:
Powder grades 5-6, classified according to IPC-TM-650 test method 2.4.43, must meet the acceptance criteria shown in Figure 3 during testing. Furthermore, during the evaluation process, no solder ball larger than 50 microns should appear in more than one of the three test patterns.

The test shall be conducted according to IPC-TM650 test method 2.4.44. The user and supplier must agree on the minimum adhesion strength and time.

When tested according to IPC-TM-650 test method 2.4.45, the solder paste shall uniformly wet the copper pads without any signs of poor wetting, non-wetting, or dewetting.

Storage Requirements
Solder paste is generally stored at low temperatures, typically between 0 and 10°C.
Usage Requirements
Temperature equilibration: Solder paste stored at low temperatures can be brought to room temperature within 4 hours. Rheological properties of solder paste: The chemical reaction between the flux and the solder alloy in the solder paste affects its rheological properties, which are crucial for the printing performance of the solder paste. After determining the solder alloy composition, the key to the printability and solderability of the solder paste lies in the flux. Proper use of flux can improve solderability and reduce welding defects. Solder paste with good wettability will not cause tombstoning after soldering, while solder paste with poor wettability will result in more resistance and capacitance displacement after soldering. Therefore, process testing is necessary when selecting solder paste to determine if the printing performance meets the actual application requirements and what the post-soldering quality is like. Analyze the rolling, filling, and demolding performance of the solder paste during printing, observe whether there are any changes in printing quality every hour, and test the viscosity changes over 1 to 8 hours. Select the solder paste that suits your product and process requirements based on the experimental results. Stirring before use: Manual or machine stirring for 1-2 minutes before use can stabilize the viscosity. Solder paste stencil life: Any solder paste must print well when first placed on the stencil, and it must also maintain good printability after several hours on the stencil. Over time, as the material is exposed to air, the viscosity of the solder paste will gradually increase due to the volatilization of the solvent in the solder paste. Based on this, measuring the change in solder paste viscosity over time will yield the actual stencil life of the solder paste. If the stencil life of the solder paste is short, then the printing parameters need to be adjusted frequently to ensure good printing results.

Mixing Before Use:

Manual or machine mixing for 1-2 minutes before use will stabilize the viscosity.

Solder Paste Life on the Stencil:

Any solder paste, when initially placed on the stencil, must print well. Similarly, the solder paste must maintain good printability even after several hours on the stencil. Over time, as the material is exposed to air, the viscosity of the solder paste will gradually increase due to the evaporation of solvents. Based on this, measuring the change in solder paste viscosity over time will determine the actual stencil life of the solder paste. If the stencil life is short, printing parameters will need to be adjusted frequently to ensure good printing results.

Solder Paste "Pause Response" Performance:

Typically in a manufacturing environment, equipment requires maintenance or the pick-and-place machine needs to be stopped for feeding, resulting in a minimum downtime of 0.5 hours. During this time, the solder paste performance must not degrade. The ability of the solder paste to adapt to this downtime and maintain good performance after printing resumes is defined as its "pause response" performance.

Solder Paste Anti-"Shear Thinning" Performance:

Solder paste should be designed to exhibit shear thinning (viscosity reduction) during printing. This characteristic ensures that the solder paste better fills the stencil openings. At the end of the printing stroke, the viscosity of the solder paste returns to its original value. In the next printing stroke, the viscosity will decrease again. In continuous printing, some solder pastes do not recover their viscosity between printing strokes, meaning that the viscosity will continuously decrease over time, which may eventually lead to collapse and bridging in fine-pitch printing.

Solder Paste Component Positioning Performance:

After component placement, the solder paste provides a certain degree of fixation to the components to prevent displacement due to acceleration and vibration during the conveying process.

Adaptability to Different Reflow Temperature Profiles:

The same type of solder paste can be used on various products, meeting various reflow temperature requirements.

Adaptability to Nitrogen and Air Reflow:

The solder paste is suitable for both air and nitrogen reflow environments.

Solder Joint Appearance:

After soldering, the solder joints have a metallic luster.

Mechanical properties: The solder joints after welding must meet the product's connection strength requirements.
Conductivity: The solder joints after welding must meet the product's signal transmission requirements.

Corrosiveness: The residues after soldering should not be corrosive to the solder joints, components, and PCB.
Insulation: The impedance value of the residues after soldering should be greater than 1 x 10Ω (as required by J-STD-004).
Stability: The performance of the residues after soldering should be stable, not easily hydrolyzed, and not easily decomposed at high temperatures.