"Sticking" to Quality:
Why is Solder Paste Viscosity the "Invisible Switch" for Printing Effect?
?? SMT Process Analysis · Printing Defect Troubleshooting · Earlysun Tech Post
In the Surface Mount Technology (SMT) process chain, printing is the first key line of defense determining soldering quality. Industry data shows that over 60% of soldering defects originate from the printing stage. Many engineers have fallen into the "parameter debugging loop", but in fact, the key to the problem may lie in the subtle difference of whether the solder paste "sticks or not". Today, we combine industry standards and process principles to deeply analyze the underlying impact of viscosity on printing effects.
First: Understand the Scientific Definition and Core Characteristics of Solder Paste Viscosity
Viscosity is essentially a physical quantity measuring internal friction, characterizing the solder paste's ability to resist flow. It must be clarified that solder paste is not a Newtonian fluid; its viscosity exhibits typical "shear thinning" characteristics—viscosity decreases as shear rate increases during squeegee movement to facilitate filling; and viscosity rises back after printing when shear force disappears to maintain shape integrity.
?? Three Core Variables Affecting Viscosity:
- ? Raw Material Characteristics: The higher the metal powder content and the finer the particle size, the higher the viscosity.
- ? Environmental Conditions: For every 5°C increase in temperature, viscosity drops by about 15%-20%; excessive humidity easily leads to moisture absorption.
- ? Process Operation: Excessive stirring leads to irreversible viscosity drop, while prolonged resting causes viscosity to rise.
Key Point: Printing Defects and Mechanisms Caused by Viscosity Abnormalities
Solder paste viscosity directly determines the quality of the entire "filling-release-shaping" process of stencil apertures. Any fluctuation deviating from the optimal range will trigger chain defects:
(1) High Viscosity: "Starved Joint Trap" Caused by Insufficient Filling
The squeegee struggles to overcome internal resistance to fill the stencil apertures, leading to insufficient solder volume on pads (risk of cold solder joints) or adhesion to stencil walls during release, forming "peaking/trailing".
(2) Low Viscosity: "Slumping Crisis" Triggered by Excessive Fluidity
Excessive fluidity causes the solder paste to spread along the pad edges after printing, forming "slumping". For fine-pitch components, this easily leads to "hidden bridging channels" between adjacent pads.
(3) Viscosity Fluctuation: "Stability Killer" in Mass Production
If viscosity within the same batch fluctuates by more than ±10%, printing quality will be inconsistent, and the defect rate in mass production can soar from 0.8% to 12.5%.
Earlysun Technology: Precise Viscosity Control Scheme Based on Process Adaptation
As a company deeply rooted in the solder paste field, Earlysun Technology always takes the IPC-TM-650 standard as the core, establishing a viscosity control system from raw materials to the entire production process. On the raw material side, metal powder characteristics are strictly controlled to reduce viscosity fluctuation risks from the source; during production, finished product viscosity is ensured to precisely match customer process parameters.
The control of solder paste viscosity is essentially the refined management of process adaptation. It is neither simply pursuing "high viscosity" nor "low viscosity is better", but rather forming a dynamic match with stencil parameters, component types, and environmental conditions.
