Thixotropy in Solder Mask Ink for Screen Printing

Ink is one of the essential consumables of board fabrication, and how it behaves decides the technical and quality parameters of the finished product. Manufacturers therefore pay close attention to ink performance — and almost all of that attention goes to viscosity. Viscosity is well understood and carefully documented; thixotropy is largely ignored, even though it has a substantial effect on how well the printing process works.

The Screen

Screen printing depends on the screen, and the screen is a woven fabric — nearly always a textile, though non-textile materials exist.

Screens are classified in several ways: by material, usually nylon, polyester or stainless steel; by weave, plain or twill; by thread structure, single or multi-filament; by mesh grade, from thin through medium to heavy; and by mesh count, from coarse to fine. In board production the medium grade is the common choice; the thin and heavy grades are reserved for special requirements.

Three parameters matter most for ink transfer.

Mesh thickness. This is measured as the thickness of the screen at rest and without tension, expressed as a statistical average derived from measurement. It is determined by the diameter of the threads that make up the mesh, and it directly governs how much ink can pass through.

Open area ratio. The ratio of mesh opening area to total mesh area, expressed as a percentage. The higher the value, the more open the mesh.

Theoretical ink passage. The volume the mesh can deliver in theory. In production, actual passage is affected by the mesh material, its properties and specification, the ink’s viscosity, the fineness of its pigment, the ink’s thixotropy, the hardness of the squeegee, the printing pressure, the printing speed and the snap-off distance.

The Ink

Printing ink is a coloured gel-like material, typically composed of synthetic resin, volatile solvent, oils, fillers, drying agents, pigments and thinners.

Resin forms the film and determines the ink’s properties. It governs how the ink handles, its gloss, its adhesion, its hardness and its resistance to water, solvents, acids, alkalis and heat. When an ink is praised for its performance, the resin is usually the reason.

Solvent dissolves the resin so that it forms a proper vehicle, giving the ink the consistency needed for printing.

Among the many properties an ink is specified by, four interact directly with the printing process.

Thixotropy is the property by which the ink behaves as a gel at rest and changes viscosity when disturbed. It is also described as shear-thinning behaviour or resistance to sagging.

Flow describes how far the ink spreads under an applied force. Flow is the inverse of viscosity, and it depends on the ink’s plasticity and thixotropy: high plasticity and thixotropy give high flow, and high flow tends to enlarge the printed image. Low flow causes the ink to fail to close up, leaving mesh marks — the pattern of the screen reproduced in the printed film.

Viscoelasticity describes how the ink springs back after the squeegee has sheared and broken it. Good printing requires the ink to deform quickly and to rebound quickly.

Drying behaviour has two opposite requirements: the ink should dry as slowly as possible on the screen, and as quickly as possible once it has transferred to the board.

Why Viscosity Alone Is Not Enough

Viscosity is the property most closely tied to the operator’s experience of the process, and the one most carefully specified in technical documents and quality reports, including the conditions and instrument used to measure it. If the ink viscosity is too high, the ink passes through the mesh with difficulty and the edges of the printed image come out jagged, so thinner is added to bring the viscosity into range.

Yet there are many situations in which the target resolution simply cannot be achieved at any viscosity. Viscosity is an important factor, but it is not the only one — thixotropy influences printing accuracy as well.

What Thixotropy Actually Is

Viscosity and thixotropy are different physical concepts. Thixotropy is a characteristic of how viscosity changes.

Assume the ink is held at constant temperature and its solvent does not evaporate quickly. Under those conditions the viscosity does not change: it is independent of time, a constant rather than a variable.

Apply an external force — stirring, for example — and the viscosity changes. As the force continues, the viscosity keeps falling, but not without limit; it stops at a floor. When the force is removed and the ink stands for a period, it gradually returns to its original state.

That reversible, time-dependent behaviour — viscosity falling while a force acts and recovering after it is removed — is thixotropy. If a short application of force produces a large drop in viscosity, the ink is described as highly thixotropic; if the drop is small, its thixotropy is low.

The Mechanism, and How It Is Controlled

The behaviour has a physical explanation, and it is not a chemical reaction.

Two conditions are necessary for thixotropy: a resin with tack, and filler and pigment particles present at a certain volume fraction. Resin, filler, pigment and additives are ground together into a uniform mixture. Without applied heat or ultraviolet energy, the components exist as irregular ionic groupings. At rest they arrange themselves in an ordered way through mutual attraction, which is why the ink is thick — and no chemical change is involved.

Once an external mechanical force disturbs that ordered arrangement and breaks the attraction chains, the structure becomes disordered and the viscosity falls. That is the visible behaviour of an ink that becomes thinner when worked.

Solid content, particle shape and particle size all determine how thixotropic an ink is. A liquid whose viscosity is inherently very low has no meaningful thixotropy. To give such a material thixotropic behaviour, an additive can be used to raise its viscosity and introduce the property — which is why thixotropy in an ink is controllable rather than accidental.

Using It in Practice

In application, more is not better and less is not better; the aim is the right amount.

A moderate degree of thixotropy suits screen printing particularly well, and makes the operation considerably easier. As the squeegee drives the ink across the screen, the ink rolls and is compressed, and its viscosity falls — which is exactly what allows it to pass through the mesh. After transfer to the board, the viscosity does not recover immediately. That delay provides a short period of levelling, during which the ink flows slowly and the printed pattern settles.

By the time the ink returns to equilibrium, the edges of the printed image have achieved a satisfactory straightness. Too little thixotropy and the ink recovers too quickly to level, leaving mesh marks; too much and the ink resists passing through the screen in the first place, or levels so slowly that the image spreads.

Understanding the property changes what an operator can do about a printing problem. When a defect resists every adjustment of viscosity, thixotropy is the variable to examine — and because it is set by formulation and can be modified by additives, the answer may lie with the ink rather than with the press. The interaction between ink behaviour and the mask it produces is examined further in this discussion of solder mask design.

The same physics governs the paste used for assembly, where the aperture rather than the mesh controls the deposit, as described in this note on solder paste mask openings. And because ink behaviour shows up as defects on the finished panel, it belongs in the verification loop alongside the checks described in this overview of PCB inspection after fabrication.

FAQ

Is thixotropy the same as viscosity? No. Viscosity describes how resistant the ink is to flow under fixed conditions; thixotropy describes how that resistance changes with time while a force is applied, and how it recovers when the force stops.

Why does high thixotropy help screen printing? Because it makes the ink thin while the squeegee is pushing it, which helps it pass through the mesh, and thick again shortly afterwards, which lets the printed image hold its shape and edge definition.

How can thixotropy be changed? Through formulation. Solid content, particle size and particle shape set the behaviour, and a thixotropic additive can be used to raise the viscosity of a low-viscosity material and give it the property.