{"id":2303,"date":"2026-08-07T18:36:08","date_gmt":"2026-08-08T00:36:08","guid":{"rendered":"https:\/\/sbcgroup.com.mx\/?p=2303"},"modified":"2026-08-07T18:36:10","modified_gmt":"2026-08-08T00:36:10","slug":"reologia-de-la-pasta-de-soldadura-impacto-de-la-viscosidad-en-smt","status":"publish","type":"post","link":"https:\/\/sbcgroup.com.mx\/en\/2026\/08\/07\/reologia-de-la-pasta-de-soldadura-impacto-de-la-viscosidad-en-smt\/","title":{"rendered":"Solder Paste Rheology: Impact of Viscosity in SMT"},"content":{"rendered":"

Understanding Viscosity and Thixotropy in Solder Paste Impression<\/h1>\n\n\n\n

In surface mount electronics manufacturing (SMT), the solder paste printing process is responsible for over 60% of defects on the production line. For process engineers, mastering the rheology of soldering paste<\/strong> y su comportamiento bajo estr\u00e9s es fundamental para garantizar un rendimiento consistente. A menudo, el t\u00e9rmino “viscosidad” se utiliza de manera simplificada para describir c\u00f3mo fluye la pasta, pero esta m\u00e9trica por s\u00ed sola es insuficiente para predecir su comportamiento real durante la impresi\u00f3n a alta velocidad a trav\u00e9s de un stencil.<\/p>\n\n\n\n

Solder paste is a complex material composed of metal powder suspended in a flux vehicle. Its behavior cannot be described with a single number because it is not a simple Newtonian fluid like water or oil. Instead, it is a non-Newtonian fluid with thixotropic properties highly dependent on time, temperature, and applied mechanical forces. Understanding the interaction between shear stress, thixotropy, and particle size is the first step in solving chronic problems such as slumping<\/em>, solder bridges (bridging) and poor stencil release.<\/p>\n\n\n\n

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Fundamental Concepts: Rheology, Shear Stress, and Thixotropy<\/h2>\n\n\n\n

Rheology is the branch of physics that studies the deformation and flow of matter. In the context of SMT manufacturing, rheology explains how solder paste responds to the mechanical forces applied by the squeegee during the printing cycle. To understand this behavior, it is necessary to differentiate between Newtonian and non-Newtonian fluids.<\/p>\n\n\n\n

In a Newtonian fluid, viscosity remains constant regardless of the applied force. Water, for example, has the same resistance to flow whether it is at rest or being vigorously agitated. In contrast, solder paste is a non-Newtonian fluid whose viscosity changes dramatically when subjected to shear stress. A fresh, at-rest solder paste typically exhibits an extremely high viscosity, in the range of 500,000 to 700,000 centipoise (cps), making it almost solid. To put this into perspective, water has a viscosity of 1\u20135 cps, honey around 2,000\u20133,000 cps, and peanut butter approaches 500,000 cps.<\/p>\n\n\n\n

The thixotropy<\/strong> This is the critical property that makes solder paste printing possible. A thixotropic material is one that exhibits high viscosity at rest, but whose viscosity decreases significantly when subjected to continuous shear stress, gradually recovering its original viscosity once the force ceases. Everyday examples of thixotropic fluids include wall paint (which flows easily under the brush but doesn't drip down the wall), ketchup, and human blood.<\/p>\n\n\n\n

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El Comportamiento “Shear-Thinning” en el Ciclo de Impresi\u00f3n<\/h2>\n\n\n\n

The specific phenomenon that allows solder paste to pass through the tiny openings of a stencil is known as shear-thinning<\/em> (shear thinning). This behavior is the practical manifestation of thixotropy during the SMT printing process and occurs in a highly choreographed sequence of physical events.<\/p>\n\n\n\n

As the squeegee begins to move through the stencil, it pushes the roll of solder paste, applying immense hydrodynamic pressure and shear stress. Under this force, the internal rheological network of the flux vehicle temporarily breaks down. The paste's viscosity plummets, transforming it from a thick mass into a highly mobile fluid. This low-viscosity state allows the paste to flow smoothly and completely fill the stencil openings, even those designed for ultra-fine pitch components.<\/p>\n\n\n\n

El momento cr\u00edtico ocurre inmediatamente despu\u00e9s de que el squeegee pasa y el stencil se separa de la placa de circuito impreso (PCB). En este instante, el esfuerzo cortante desaparece por completo. Gracias a su naturaleza tixotr\u00f3pica, la red reol\u00f3gica de la pasta comienza a reconstruirse inmediatamente, y su viscosidad se dispara de vuelta a niveles cercanos a los 500,000 cps. Esta r\u00e1pida recuperaci\u00f3n es lo que permite que el dep\u00f3sito de pasta mantenga su forma rectangular perfecta (conocida como forma de “ladrillo”) sin colapsar ni extenderse sobre los pads adyacentes.<\/p>\n\n\n\n

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Impact of Temperature and Humidity on Rheology<\/h2>\n\n\n\n

The rheology of solder paste is extremely sensitive to the ambient conditions on the production floor. Strict climate control is not a luxury in SMT manufacturing, but a fundamental requirement for maintaining consistency in the printing process.<\/p>\n\n\n\n

The temperature<\/strong> It has an inversely proportional relationship with viscosity. As ambient temperature increases, thermal energy increases the mobility of molecules in the flux vehicle, reducing the base viscosity of the paste. If the temperature in the printing area exceeds 25\u00b0C (77\u00b0F), the paste can become too fluid, losing its ability to recover its brick shape after printing, resulting in slumping<\/em> (crumbling). Conversely, if the temperature is too low (below 20\u00b0C), the initial viscosity will be excessively high. The paste will require a much greater shear stress to reach the desired state. shear-thinning<\/em>, which often results in incomplete filling of the openings and severe stencil release problems.<\/p>\n\n\n\n

The relative humidity (RH)<\/strong> It also plays a crucial role, particularly in the paste's behavior over time. Soldering paste is hygroscopic, meaning it absorbs moisture from the air. High relative humidity (above 60%) accelerates this absorption, degrading flux activators and drastically altering the rheology. The absorbed moisture acts as a plasticizer, reducing viscosity and dramatically increasing susceptibility to [unspecified problem]. slumping<\/em>, especially during the preheating phase in the reflow oven (hot slump). On the other hand, extremely low moisture (below 30%) can cause premature evaporation of volatile solvents in the flux, drying out the paste and increasing its viscosity to the point of clogging the stencil.<\/p>\n\n\n\n

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Relationship between Particle Size and Flow Behavior<\/h2>\n\n\n\n

La reolog\u00eda de la pasta no solo depende del veh\u00edculo de flux, sino tambi\u00e9n de la interacci\u00f3n f\u00edsica y qu\u00edmica entre el flux y el polvo de aleaci\u00f3n met\u00e1lica. El est\u00e1ndar IPC J-STD-005A clasifica el polvo de soldadura en diferentes “Tipos” basados en la distribuci\u00f3n del tama\u00f1o de part\u00edcula. La selecci\u00f3n del Tipo de polvo tiene un impacto profundo en la viscosidad, la vida \u00fatil (shelf life) y el comportamiento de impresi\u00f3n.<\/p>\n\n\n\n

Dust Type (IPC)<\/th>Main Size Range (\u00b5m)<\/th>Typical SMT Application<\/th><\/tr><\/thead>
Type 3<\/td>25 – 45 \u00b5m<\/td>Standard, components \u22650402 imperial<\/td><\/tr>
Type 4<\/td>20 – 38 \u00b5m<\/td>0201 imperial, micro-BGA, fine pitch<\/td><\/tr>
Type 5<\/td>15 – 25 \u00b5m<\/td>Ultra-fine pitch, compact RF modules<\/td><\/tr>
Type 6<\/td>5 – 15 \u00b5m<\/td>Ultra-miniature components, SiPs<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

A medida que la industria avanza hacia la miniaturizaci\u00f3n extrema, los ingenieros de proceso se ven obligados a migrar de pastas Tipo 3 a Tipo 4 o Tipo 5 para cumplir con la “Regla de las 5 Bolas” (la apertura m\u00e1s peque\u00f1a del stencil debe poder acomodar al menos 5 part\u00edculas de polvo a lo ancho). Sin embargo, esta reducci\u00f3n en el tama\u00f1o de part\u00edcula altera significativamente la reolog\u00eda.<\/p>\n\n\n\n

For a given mass of soldering powder, reducing the particle size exponentially increases the total surface area exposed to the flux. Type 4 powder has approximately 20% more surface area than Type 3, and Type 5 has a staggering 75% more surface area. This larger powder-flux interface increases internal friction, which generally results in higher apparent viscosity. Furthermore, the increased surface area accelerates chemical reactions between the metal oxides and the flux activators, which can cause premature thickening of the paste in the stencil and drastically reduce its operational life.<\/p>\n\n\n\n

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Viscosity Tests: Brookfield vs Malcolm Viscometers<\/h2>\n\n\n\n

To control and verify the rheology of solder paste, the industry relies on two main viscosity measurement methodologies, both recognized by the IPC standard J-STD-005A. Understanding the difference between these methods is vital for correctly interpreting manufacturers' technical data sheets (TDS).<\/p>\n\n\n\n

He Brookfield method<\/strong> It uses a traditional rotational viscometer. A T-shaped spindle rotates continuously inside a pot of solder paste at specific speeds (typically 5 RPM). The equipment measures the torque required to maintain that rotational speed and calculates the viscosity in centipoise (cps). While an excellent industry standard for incoming quality control (QC), the Brookfield method has a fundamental limitation: the continuous rotational motion does not accurately simulate the intermittent shear dynamics that occur during the squeegee stroke in an actual SMT printer.<\/p>\n\n\n\n

He Malcolm X method<\/strong> (t\u00edpicamente utilizando el modelo PCU-205) emplea un enfoque diferente basado en un cilindro conc\u00e9ntrico con un rotor en espiral. En lugar de una rotaci\u00f3n simple, el viscos\u00edmetro Malcom aplica un movimiento de cizalla que simula mucho m\u00e1s de cerca la acci\u00f3n de “amasado” y el esfuerzo cortante variable del proceso de impresi\u00f3n real. Este equipo puede medir la viscosidad din\u00e1mica a diferentes tasas de cizalla y calcular el Thixotropic Index<\/em> (the ratio between low shear viscosity and high shear viscosity). A higher thixotropic index generally correlates with better printing efficiency (Cp) and faster recovery of the deposit shape.<\/p>\n\n\n\n

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Troubleshooting: Slumping, Bridging, and Stencil Release<\/h2>\n\n\n\n

When the rheology of the solder paste fails or degrades, the results are immediately apparent in the solder paste inspection (SPI) in the form of three main defects.<\/p>\n\n\n\n

He Slumping<\/strong> This occurs when the paste loses its ability to maintain its brick shape after printing. Cold Slump<\/em> This occurs at room temperature immediately after printing, generally caused by insufficient initial viscosity, degradation of thixotropic agents due to over-kneading, or ambient temperatures above 25\u00b0C. Hot Slump<\/em> ocurre dentro del horno de reflujo durante la fase de precalentamiento (120\u00b0C – 150\u00b0C), cuando el calor reduce dr\u00e1sticamente la viscosidad del flux antes de que los solventes se evaporen por completo. Las pastas solubles en agua (Water-Soluble) son notoriamente m\u00e1s susceptibles al hot slump que las formulaciones No-Clean.<\/p>\n\n\n\n

He Bridging (Welding Bridges)<\/strong> es a menudo la consecuencia directa de un slumping severo. Cuando los dep\u00f3sitos de pasta se desmoronan y se expanden lateralmente, la pasta de pads adyacentes entra en contacto. Durante el reflujo, la tensi\u00f3n superficial de la soldadura fundida consolida esta conexi\u00f3n, creando un cortocircuito el\u00e9ctrico. Este defecto es particularmente cr\u00edtico en componentes de paso fino como QFP, QFN y micro-BGA. Si se observa bridging consistente, las acciones correctivas incluyen verificar la temperatura ambiente, reducir la presi\u00f3n del squeegee (que puede estar forzando demasiada pasta o causando “bleeding” debajo del stencil), o evaluar si la pasta ha excedido su tiempo de vida en el stencil.<\/p>\n\n\n\n

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The Stencil Release Problems (Poor Release)<\/strong> se manifiestan como dep\u00f3sitos de pasta incompletos, con bordes irregulares o con forma de “orejas de perro”. Esto ocurre cuando la fuerza de adhesi\u00f3n entre la pasta y las paredes de la apertura del stencil es mayor que la fuerza de cohesi\u00f3n interna de la pasta y su adhesi\u00f3n al pad de la PCB. Reol\u00f3gicamente, esto indica una viscosidad excesivamente alta o una falta de comportamiento shear-thinning<\/em> proper. Common causes include cold paste (not properly conditioned to room temperature), solvent evaporation from prolonged exposure to the stencil, or the use of a Type 3 powder in openings designed for Type 4, violating the 5-Ball Rule.<\/p>\n\n\n\n

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Learn more<\/h2>\n\n\n\n

To delve deeper into SMT process optimization and material selection, we recommend exploring the following technical resources:<\/p>\n\n\n\n