Analysis of common problems in Stencil testing

In the steel mesh test, the performance of solder paste is like a mirror, which clearly reflects its performance shortcomings and process adaptability. The following R&D engineers of Aoniu Technology help newcomers in the industry to quickly grasp the logic of problem investigation from the high-frequency test problems, the causes behind them and targeted solutions when customers use solder paste.

1. Missing print and tin: How is the "blank area" formed on the pad?
Missed printing shows that the pad is partially or completely uncovered by solder paste, especially in the pad with fine pitch (< ＜0.5mm). This is usually because the viscosity of solder paste is too high (> ＞150Pa・s), which blocks the mesh like solidified honey, resulting in poor tin discharge. Or the tin powder particles are too coarse-for example, the T4 grade (20-38μm) is misused to replace the T5 grade (15-25μm), and the particles above 20μm are easy to get stuck in the 100μm mesh (the particle diameter should be less than 1/3 of the mesh size, that is, < 33 μ m). In addition, the roughness of the inner wall of the steel mesh opening (Ra > 1 μ m) will increase the demoulding resistance of the solder paste, especially when the flux activity is insufficient, the solder paste is easy to stick to the mesh wall and form "hanging wall" residue.

solution:
First, use a viscometer to detect the viscosity of the solder paste. If it exceeds the target value by 10%, it is necessary to replace the batch with the same model but suitable viscosity (fine spacing is recommended to be 100-120 PAS), and use T5 solder paste (15-25μm). Confirm the particle size distribution by laser particle size analyzer, ensure that D90(90% of particles) is less than 25 μ m, and avoid T4 solder paste being used for spacing less than 0.4 mm.. Ultrasonic cleaning and polishing the inner wall (Ra < 0.5 μ m) of the steel mesh with frequent missing printing, and applying release agent to reduce friction if necessary.

2. Printing collapse: the root of solder paste "collapse" flowing out of the pad.
The collapse is characterized by fuzzy edges of solder paste, falling height and even flowing to adjacent pads. The essence is that the thixotropy of solder paste is insufficient or the particles are too fine. For example, the specific surface area of T6 grade (5-15μm) tin powder is large, and if the viscosity is too low (< ＜80Pa・s), the surface tension will decrease, resulting in excess liquidity. When the ambient humidity is higher than ＞60% RH, the solder paste will be softened by absorbing moisture, which will aggravate the collapse, while the opening of steel mesh is too large (> 95% of the pad), which will lead to the excessive tin content and the excess solder flowing under gravity.

coping strategy:
The granularity is adjusted and matched, and T4-grade solder paste is selected for the regular spacing (0.5-0.65mm) to avoid excessive flow caused by T6-grade solder paste. The thixotropy was optimized, and the formula containing nano-silica thickener was selected first through cyclic viscosity test (the viscosity recovery rate after high-speed shearing was more than 90%). Optimize the environment and opening design, control the humidity of the workshop at 40%-50% RH, and start the dehumidifier if necessary. Adjust the opening size of steel mesh to 90%-95% of the pad, and design 50μm chamfer to reduce stress concentration.

3. Edge tailing: Solder paste "wiredrawing" exposes the thixotropic short board.
After printing, there is a slender "tail" on the edge of the solder paste, or there is a residual filiform solder paste between the meshes, which is a direct manifestation of insufficient thixotropy. When the solder paste thins under the shearing force of the scraper, it can't recover its viscosity quickly (the ideal recovery time is less than 10 seconds), and it will be "pulled" by the edge of the mesh when demoulding. In addition, the failure of thickener and thixotropic agent in flux or the decrease of surface tension caused by tin powder oxidation will also aggravate the tailing phenomenon.

Improvement measures:
Cyclic test was carried out by rotating viscometer to observe the viscosity recovery rate (ideal recovery rate > 90%) after high-speed shearing. If it is not up to standard, solder paste with better thixotropy should be replaced. Check the storage conditions of solder paste to avoid the flux volatilization caused by exposure for more than 4 hours after opening. As for the surface treatment of steel mesh, electro-polishing treatment (Ra < 0.3 μ m) or spraying fluorine-based coating to enhance the demoulding property is carried out on the mesh with frequent tailing.

4. Uneven thickness: the driving force behind the "different heights" of solder joints.
3D SPI detection found that the thickness difference in different areas of the same pad was > 10%, or the standard deviation of the whole solder paste thickness was > 5%, which was usually due to uneven printing pressure (such as insufficient local pressure caused by scraper wear), unbalanced tension of steel mesh, or alloy powder settling in solder paste (uneven particle distribution caused by long-term non-stirring). In addition, the bending of PCB substrate (> ＞0.3mm) will make the steel mesh and pad not closely attached, resulting in abnormal tin content in some areas.

Screening steps:
First, use a tensiometer to detect the tension of the steel mesh to ensure that the tension difference of the whole plate is less than 5 N/cm, and tension the area with insufficient tension or replace the mesh plate. Use the pressure sensor to calibrate the scraper of the printing machine to ensure uniform pressure (5-8N/mm). Before using the solder paste, it needs to be mechanically stirred for 3-5 minutes to prevent the particles from settling. Vacuum support columns are added to the curved substrate to ensure that the flatness error is less than 50μ m during printing.

5. Mesh blockage: the "number one natural enemy" of fine-pitch printing
In the ultra-fine spacing mesh below 0.3mm, some meshes are completely blocked due to the accumulation of solder paste, and solidified solder paste particles can be found in the mesh under microscope. This is mainly because the tin powder is seriously oxidized (black oxide film is formed on the surface), or the flux is not active enough to wet the particle surface, which leads to powder agglomeration. In addition, if the steel mesh is not cleaned in time after frequent shutdown, the residual solder paste will dry up in the mesh, which will also cause blockage.

Prevention program:
The oxidation degree (particle surface oxide < 1%) of the new batch of solder paste should be tested before entering the factory, and the batch with serious oxidation should be rejected directly. During the printing process, clean the mesh surface with steel mesh paper every hour. If the machine is stopped for more than 30 minutes, the mesh plate should be removed and cleaned with cleaning agent by ultrasonic. For ultra-fine mesh spacing, solder paste with strong oxidation resistance (such as adding 0.5% nano-silver coated particles) is preferred, and the printing speed is reduced to 20mm/s to reduce the oxidation caused by heat generated by particle friction.

6. Environmental factors: the hidden influence of temperature and humidity and its precise control.
When the workshop temperature > 30℃, the flux solvent in the solder paste volatilizes rapidly, and the viscosity may increase by 20% within 2 hours, resulting in network blockage. When the humidity is less than ＜30% RH, the solder paste is easy to lose water and dry, and it is "incomplete" when demoulding. In addition, the solder paste is used immediately after it is taken out of the refrigerator at 5℃, and the surface condensed water is mixed with the paste, which will lead to tin explosion during reflow soldering. Although it is not directly expressed in the test, it will affect the subsequent welding quality.

Key points of environmental control:
Establish a real-time temperature and humidity monitoring system (accuracy 1℃, 5% RH) to ensure that the test environment is stable at 25 3℃ and 40%-50% RH. The reflow of solder paste strictly follows the principle of "standing at room temperature for 3 hours", and the dew point of the surrounding environment is detected by humidity card < 10℃ to avoid condensation water. In summer, the temperature and humidity meter is used for inspection every hour, and the ultrasonic humidifier is turned on in winter when it is dry, so as to maintain a constant dew point temperature and reduce the influence of environmental fluctuations on the state of solder paste.

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