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How does the viscosity of solder paste affect SMT soldering quality?

2026-05-12

In modern SMT processes, solder paste not only serves as an electrical and mechanical bonding material but also directly determines the stability of the entire stencil printing, component placement, and reflow soldering stages. Among these factors, viscosity is considered one of the most critical process parameters, as it directly affects solder paste printability, the accuracy of solder deposition, bridging, tombstoning, open joints, and the long-term reliability of the PCB. International research on the rheology of solder paste consistently demonstrates that even minor changes in viscosity can significantly impact SMT print quality and the defect rate in mass production.

What is the viscosity of welding cream?

The viscosity of solder paste is a parameter that characterizes the resistance to flow of the mixture composed of tin alloy powder and flux. In SMT, solder paste is not a typical Newtonian fluid; rather, it exhibits non-Newtonian behavior and thixotropic properties. This means that when subjected to shear stress from the squeegee during stencil printing, its viscosity decreases, allowing the paste to readily pass through the stencil apertures; once printing is complete, the viscosity rapidly recovers, maintaining the desired pattern on the PCB pads.

This is the fundamental mechanism that enables SMT to accurately print ultra‑small pads on today’s high‑density electronic circuit boards, such as BGA, QFN, CSP, and Mini LED PCBs.

According to numerous international technical documents and IPC standards, solder paste viscosity is typically measured in Pa·s or cP at the standard temperature of 25°C. The appropriate viscosity value varies depending on the type of solder paste, tin particle size, and the specific SMT application.

Solder paste viscosity in SMT applications

Why does viscosity directly affect SMT soldering quality?

In the SMT process, solder paste printing is the step that has the greatest impact on the overall defect rate. Numerous studies in the electronics industry indicate that more than 50% of SMT defects originate during the stencil‑based solder paste printing stage. When the viscosity is inappropriate, the entire process stability—from paste printing and component placement to reflow—can be compromised.

1. Excessive viscosity causes insufficient tin and incomplete printing.

When the solder paste has excessively high viscosity, its ability to flow through the stencil apertures is significantly reduced. This prevents the paste from fully filling the stencil openings, particularly in fine-pitch or micro‑pad designs. As a result, insufficient solder paste deposition occurs, leading to defects such as:

Solder shortage
Open circuit
Cold weld
Waterproof
Weak mechanical weld

Additionally, overly thick solder paste can exacerbate stencil adhesion, reduce transfer efficiency, and cause variations in solder volume across pads. In high‑speed SMT production, this leads to a significant deterioration in solder joint uniformity.

For high-density PCBs using 0201, 01005 components, or fine-pitch BGAs, excessively high viscosity can also cause stencil clogging, reducing throughput and increasing the rework rate.

2. Viscosity that is too low causes solder paste flow and bridging.

Conversely, when the viscosity is too low, the solder paste tends to spread excessively after printing. This phenomenon is known as solder paste slump. When the solder paste loses its ability to maintain its shape, adjacent pads may bridge during reflow, leading to defects:

Short-circuit weld
Short circuit
Weld bead

This is a particularly hazardous defect in fine-pitch IC or high‑pin-count BGA designs. When pad pitches are only a few hundred microns, even a slight reduction in viscosity due to ambient temperature changes or flux evaporation can significantly increase the risk of defects.

Some studies on the viscosity of paste solder have also shown that low viscosity reduces the self‑alignment capability of the solder paste after printing, leading to blurred print edges and decreased print resolution. This significantly impacts SMT quality in high‑precision electronics manufacturing.

3. Impact on SMT component placement capability

In addition to forming the solder joint, solder paste must also keep components securely in place after the pick-and-place machine has completed placement. If the viscosity is unstable, the holding force on the components will vary, increasing the likelihood of component shift or misalignment prior to reflow.

For ultra‑small components such as 0201 or CSP packages, even minor changes in the solder paste’s rheological properties can significantly misalign the component. This increases the risk of tombstoning, skew, or BGA misalignment.

In actual production, modern SPI (Solder Paste Inspection) systems typically monitor the height, volume, and uniformity of solder paste to detect early signs of abnormal viscosity changes, thereby reducing placement and reflow defects.

4. Impact on the long-term reliability of welds

Viscosity not only affects the weld geometry but also influences the microstructure after reflow. When the amount of solder paste is uneven, the resulting intermetallic compound (IMC) layer becomes unstable, thereby reducing the thermomechanical fatigue life of the solder joint.

In industrial electronics, automotive electronics, or medical devices, instability in solder volume caused by improper viscosity can lead to solder joint cracking after numerous thermal cycles. This directly impacts the durability and reliability of the PCBA.

Factors that affect the viscosity of welding flux

Ambient temperature

Temperature is the most significant factor affecting the viscosity of solder paste. As temperature rises, viscosity decreases rapidly, causing the solder paste to spread excessively. Therefore, SMT environments are typically maintained at around 23 ± 3°C to ensure process stability during printing.

Tin particle size

Type 4, Type 5, or Type 6 solder pastes use smaller tin particles to meet the demands of fine-pitch SMT. Smaller particles improve fine‑feature printing but also alter rheological properties and increase the risk of oxidation.

Metal content in solder paste

Metal loading directly affects the viscosity of solder paste. A high tin powder content typically increases viscosity while improving the mechanical strength and electrical conductivity of the solder joint.

Shelf life and storage conditions

Solder paste will change its viscosity after opening due to the evaporation of the flux solvent. If it exceeds the allowable working life, the solder paste will lose its rheological stability, significantly increasing the SMT defect rate.

Methods for controlling the viscosity of solder paste in SMT production

To maintain consistent SMT soldering quality, modern electronics manufacturers typically implement multiple concurrent control measures:

SMT room temperature and humidity control
Use a specialized viscosity meter.
Comply with the pre‑production thermal recovery time for solder paste.
Stir the solder paste according to the proper procedure.
Control of the service life of welding consumables on the production line
Optimize stencil thickness and aperture design
Use SPI to inspect the solder paste volume after printing.
Choose the correct type of solder paste according to the component pitch.

In addition, for high-speed SMT lines or automotive PCBA production, selecting a solder paste with stable thixotropic properties is critically important to ensure print repeatability and minimize variation between stencil-printing cycles. A study on Type 4 solder paste showed that viscosity can decrease by approximately 25% after just the first few printing cycles before reaching a steady state.

The MALCOM solder paste viscosity meter is widely used in SMT production.

In modern SMT production, precise control of solder paste viscosity is a critical factor for maintaining stencil printing quality and minimizing soldering defects on PCBs. MALCOM viscosity meter series by Hapoin Vietnam provide as PCU-203, PCU-205 or PCU-285 It is specifically designed to measure and analyze the rheological properties of solder paste in accordance with JIS Z3284, enabling accurate assessment of viscosity, thixotropic index (TI), and the stability of solder paste during SMT processes.

Thanks to its spiral pump sensor technology and the ability to simulate real-world shear forces during stencil printing, the MALCOM machine enables manufacturers to monitor viscosity changes in real time, promptly detecting risks of bridging, tin deficiency, or solder slump before they lead to mass defects on the production line. This solution is widely adopted by electronics, EMS, and automotive electronics manufacturers to improve SMT yield, reduce rework, and optimize the stability of PCBA manufacturing processes.

**Genuine MALCOM PCU-285 solder paste viscosity meter at Hapoin**

Conclusion

The viscosity of solder paste is a critical parameter that determines stencil printing quality and the overall stability of the SMT process. If the viscosity is too high, the solder paste struggles to pass through the stencil, leading to insufficient solder deposition and open joints; conversely, if the viscosity is too low, it can cause spreading, bridging, and short circuits. As the electronics industry continues to advance toward miniaturization and high-density assembly, precise control of solder paste rheology has become an essential requirement to ensure PCBA yield, reliability, and longevity.

For professional SMT manufacturers, optimizing solder paste viscosity is no longer merely a material‑related issue; it has become a key strategy in modern electronic manufacturing quality control.

HAPOIN Group is the exclusive distributor of MALCOM products in Vietnam, representing numerous product lines. Viscometer from MALCOM.

We guarantee that all products are genuine imports from Japan, shipped directly from our warehouse, and come with free technical support.

For more information, please scan the Zalo QR code on the right or call 0398848969 to speak directly with us.

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