Optimize Electronics Dispensing and Flow for Higher Throughput with Rheology

Dispensing and flow are important for a variety of electronics manufacturing techniques and materials. From the precise application of solder pastes and adhesives to new developments in flexible electronics and additive manufacturing methods, rheology plays a key role in improving processability, product quality, and manufacturing consistency.

Rheology is the study of deformation and flow when a force is applied, making it crucial for optimizing electronics manufacturing. Rheology answers critical questions across various electronics applications such as:

• How does my solder paste change during storage, and how can I predict its shelf life?
• What is the ideal viscosity of my inkjet-printed electronic circuits?
• Will my thin film or epoxy hold its shape or slump before drying/curing?
• Will my material flow easily or clog in my syringe?

These behaviors are shear-dependent, meaning the material’s viscosity (resistance to flow) changes depending on the forces it experiences during mixing, dispensing, storage, or curing. Therefore, these materials must be assessed over a wide range of shear rates – which is possible on a rheometer but not a viscometer. Rheometers are also capable of measuring yield stress in time-dependent materials for predicting long-term behavior.

Researchers and manufacturers around the world use Waters Rheometers to study their electronics materials’ dispensing and flow. Learn from four published articles to see how rheology improved their materials and processes in the sections below.

Solder paste streamlines electronics mass production but is a reactive mixture that requires careful handling and storage. Solder paste’s powder and flux components “start chemical reactions immediately after blending,” meaning that solder paste changes continuously throughout manufacturing, transportation, storage, and usage, all influenced by temperature and humidity.¹ A rheometer’s ability to control over shear rate and deformation paired with torque sensitivity make it “one of the best methods to quantify the change of solder paste.”

Researchers from Kester Inc, a solder paste manufacturer, studied solder paste shelf life and properties using a Discovery Hybrid Rheometer.¹ They developed a method for accelerated shelf-life prediction using several rheological test methods, revealing that storage temperature affects end-use viscosity more than storage time.

Inkjet printing is a key technology enabling printed electronics including flexible, wearable electronics. Researchers from the University of Cambridge, Jiangnan University, and the Polytechnic University of Milan created inkjet-printed flexible and washable field-effect transistors (FETs) on textiles, which enable fully inkjet-printed electronic circuits.² The Discovery Hybrid Rheometer was used to measure viscosity as a function of shear rate for two ink formulations, which helped them calculate printability metrics such as the inverse Ohnesorge number (Z) range for optimal inkjet printing.

Another study used non-contact microjet printing to create personalized electroencephalography e-tattoos.³ Researchers from the University of Texas and University of California, Los Angeles developed low-viscosity biocompatible inks that are jetted onto the scalp where they “self-dry into soft, stretchable, and conductive thin films.” Using viscosity measurements from a Discovery Hybrid Rheometer 30, they observed that their ink’s tunable viscosity and shear thinning supported stable microjet printing.

Additive manufacturing is a growing technique for printing complex electronics structures including wearable devices, soft robotics, and customized energy storage. Researchers from the University of Alberta developed a novel method for direct ink writing (a form of additive manufacturing) of capacitive pressure sensors using a water-based ink.⁴ Using the ARES rheometer, they confirmed that their ink formulation had desirable shear thinning behavior: able to flow freely while passing through the narrow syringe, then maintain higher viscosity once it’s dispensed onto the substrate. The ink’s shear thinning behavior also means extrusion requires less pressure, thus “facilitating smooth extrusion through small nozzles resulting in continuous patterns.”

Electronics manufacturing and design is made possible through rheology, as demonstrated in the articles above. From shelf life to dispensing and drying, each stage of electronics manufacturing is influenced by rheological properties. Quantifying these properties is the first step toward designing more efficient processes and innovative products.

If you think rheology is either too complicated or irrelevant for your lab, think again. Leading electronics labs are already embracing rheology to make informed decisions about material formulations, processing conditions, and shelf-life estimation. Read our blog Dispelling 5 Myths About Rheology to learn why rheology is relevant and accessible for users of all levels.