Conductive Silver Paste Rheology for Screen Printing

Source and scope: This English article combines two copies of the same October 2021 company-engineer note. It keeps the original four-stage printing logic and adds the measurement conditions needed to use rheology data correctly. The values for one paste, spindle, speed, or temperature do not define another formulation.

A single viscosity number cannot describe a screen-printing paste. The material sits in a container at low shear, moves under the flood bar, accelerates beneath the squeegee, deforms through mesh openings, separates from the screen, and then rebuilds while the printed feature levels. Each stage asks for a different response. Good printability comes from the complete flow curve and time-dependent recovery, not from making the paste simply “thick” or “thin.”

At rest: the paste must resist settling and uncontrolled flow

In storage and on the screen between cycles, the paste is under low stress. A structured suspension uses particle interactions, polymer, and additives to resist settling and edge flow. The relevant properties include low-shear viscosity, yield stress, elastic response, and recovery after previous shear.

Too little structure can allow solids to settle, liquid to separate, or printed edges to spread. Too much structure can make hand mixing difficult, prevent complete mesh filling, retain squeegee marks, or cause incomplete transfer. The practical target is a controlled structure that breaks down during printing and rebuilds afterward.

Under the squeegee: shear thinning enables transfer

As the squeegee travels, stress rises and apparent viscosity usually falls. This shear-thinning behaviour lets a high-solids paste move across the stencil and through the mesh. Yield stress must be exceeded, and the material must wet the approved substrate without flooding beyond the image.

Pressure and speed cannot be considered separately from rheology. Higher pressure may increase transfer but also distort the screen, widen a line, or force paste under the stencil. Speed changes the time and shear history experienced by the material. Squeegee edge, angle, hardness, snap-off, mesh, emulsion, and print gap add further interactions.

Through the mesh: elasticity and separation affect the image

Paste does not pass through a screen as an ideal Newtonian liquid. It stretches, changes structure, contacts the substrate, and separates from mesh and stencil walls. Viscoelastic response and wall slip can influence whether an opening fills, whether a fine line remains continuous, and how cleanly the screen releases.

Research on screen-printing pastes has linked yield stress, viscosity at different shear rates, thixotropy, and viscoelasticity with print behaviour. Lin and colleagues measured commercially available and model pastes using steady and dynamic methods rather than a single spindle reading. Their work also notes that shear rates can become high as paste passes through the mesh. The exact shear history in a production print still depends on the equipment and image.

After release: recovery competes with levelling

Once the screen lifts, the paste needs enough flow to close mesh marks and form a coherent surface. It also needs to rebuild quickly enough to retain line width, spacing, and deposit height. If recovery is too slow, edges slump and adjacent features may bridge. If it is too fast, mesh texture, peaks, or an uneven surface may remain.

This is why “higher thixotropy is better” is not a reliable rule. A recovery profile that produces a high-aspect-ratio solar-cell finger may not be the best profile for a broad dielectric layer, a via fill, or a polymer sensor track. The required geometry chooses the useful rheology.

Report the method with every rheology result

Viscosity without temperature, spindle or geometry, speed or shear rate, sample conditioning, rest time, and measurement sequence is not reproducible. DuPont’s 7095 data sheet, for example, states viscosity at 25 °C using a specified Brookfield instrument and speed. Another data sheet may use Pa·s at a defined shear rate. The numbers cannot be compared as though the test methods were identical.

Paste age and working history matter too. Solvent loss on an open screen, repeated shear, contamination, temperature rise, or unauthorised thinner can change the response over a shift. A useful control plan therefore combines incoming or pre-use checks with print-coupon evidence and finished-film measurements.

Diagnose defects by stage, not by viscosity alone

• Settling or liquid separation: review storage, dispersion, low-shear structure, age, temperature, and contamination.
• Incomplete image or skips: review mixing, mesh opening, paste temperature, high-shear flow, screen wetting, snap-off, pressure, and substrate support.
• Bridging or wide lines: review over-pressure, stencil contact, substrate wetting, solvent balance, yield stress, and post-print recovery.
• Mesh marks or peaks: review levelling time, recovery rate, screen release, deposit volume, and drying onset.
• Thickness drift during a run: review solvent evaporation, paste replenishment, screen residence time, screen blockage, environment, and print sequence.

Do not correct a defect by adding thinner until the cause is understood and the material procedure authorises the adjustment. Thinner can improve transfer while worsening edge retention, dried thickness, solids distribution, cure, adhesion, or final resistance.

Build a production correlation

The most useful rheology limit is one tied to acceptable parts. Record the paste lot and conditioning, room and paste temperature, screen and squeegee, print settings, elapsed time on screen, deposit geometry, dried or fired thickness, electrical result, and defect rate. Over several controlled lots, this establishes which measurements predict the real process and which merely add inspection work.

Primary references

1. Lin, Chang, Hwu and Ger, “The rheological behaviors of screen-printing pastes” — experimental treatment of viscosity, yield stress, thixotropy, and viscoelasticity in printing pastes.
2. “Performances of screen-printing silver thick films: Rheology, morphology, mechanical and electronic properties” — research connecting dispersion and rheological fingerprints with printed-film outcomes.
3. DuPont 7095 Silver Conductor data sheet — an example of product-specific viscosity method, thinner, screen, drying, and firing guidance.