Overview of thixotropy and leveling of PCB inks

Abstract: This article outlines the theory of thixotropy and leveling of PCB inks, introduces the development of domestic inks used in PCBs, and explains to readers the application of thixotropy and leveling in PCB inks.

Keywords: thixotropy leveling viscosity shear force

Anti-corrosion ink is used in the production of PCB to complete the material of the processing circuit pattern. Solder resist ink is an ink permanently left on the board surface. It not only has thermal shock resistance, corrosion resistance, corrosion resistance, good electrical properties, but also a surface decoration. The marking inks also serve as markers to illustrate and paint the effect of the Dragon Point.

The history of development of printed boards in China for several decades is also the history of the development of printed board inks. The earliest inks used in single-panel circuits are self-made inks, and self-drying or drying inks based on thick paints, fillers, rosins and solvents. . In the mid-1970s, inks with bitumen as the main body appeared again, and the edges of the lines were improved. In the early 1980s, only professional factories used line inks. About solder mask inks, the thermosetting solder mask inks first produced by the Guangzhou Institute of Chemical Industry were marketed from the north to the south.

With the rapid development of PCB technology and the deepening of reform and opening up, China's ink market is already blooming, with imported inks from the United States, Japan, West Germany, and the United Kingdom, while domestically produced Taiwan is predominant. In addition, Beijing, Guangdong, Hangzhou and Wuxi also have production.

Whether it is imported ink or domestic ink must be tested, gradually find out the changes in its viscosity, performance testing when necessary, use its thixotropic and leveling performance to adjust their own operating procedures in order to be well used for production.

Simply stated, the viscosity of the ink is a measure of resistance or resistance to flow. The concept of viscosity is described below using a simple model diagram.

There is a layer of liquid between the two sheets, the upper sheet can move, the lower sheet is the fixed sheet, and the pitch is x. A force of size F acts on the upper movable sheet to make it move in the tangential direction and its speed is relative to At the bottom is v. When moving, the multi-layered liquid between the two sheets also moves, with the highest velocity at the top and the lowest velocity at the bottom, while the intermediate layer has a moderate velocity. The velocity gradient dυ/dx is defined as the shear rate (D) and its unit is l/s. The force acting on the top is divided by the area, that is, the force per unit area (F/A) is called the shear force (r), and its unit is Pa. The viscosity (η) is the ratio of the shearing force to the shearing rate, η=r/D, and its unit is Pa·S. That is, when the shear force is constant, the higher the viscosity of the liquid, the smaller the shear rate, that is, the smaller the internal rate of decline, the total can be obtained by the following three formulas, but these three formulas are also the quantitative relationship of Newtonian flow . It is also the three elements of rheology: Shear force, shear rate and viscosity. The so-called rheology is the science that describes the flow and deformation laws of objects under external forces.
Shear rate (rate) D=dv/dx=u/x(S-1)

Shear force (stress) r = F/A (N/m2 = Pa)

Viscosity = η (shear stress/shear rate) = (F/A)/(V/X) = (PaS)

Where A = area F = force V = speed (m/s)

The most painful operator's headache in the layout of circuit boards is the fine control of the lines and the steepness of the edges of the lines. Honestly speaking, self-made inks in the 1970s could not solve the problem of thickening, thinning, and non-steep edges. (Of course, this is related to the screen-based graphics at the time, and it is also related to the manufacturing level of the screens at that time.) For this reason, the ink experts are based on the above-mentioned related theories. The first is to add a rheological agent to the ink so that it has a unique and stable structure, which can protect the dispersed pigment particles and form a thixotropic ink, which contributes to sagging control and maintains good leveling property and prevents dyes Settlement. When we print on a graphic screen, either a direct etching pattern or an anti-plating pattern requires a theoretical line width or line spacing, which requires the selection of inks with better thixotropic patterns. Its viscosity characteristics are: When the shear rate increases, the viscosity gradually decreases; a rational shear force release viscosity immediately reply. Before using the ink, dilute and stir it as needed. If it is manual mixing, it will be more laborious. As the time increases, it will gradually feel labor-saving. This is because the original structure of the ink has been destroyed when stirring, Increase, the original structure of the ink damage increases.) Stir and rest for a period of time, the structure destroyed in the static process gradually returns to the original value with the increase of the static time, which means that it is more suitable for screen printing at this time. . (usually more than 20 minutes is suitable for stationary) Bubbles with good thixotropic ink screen printing should also disappear within 5”~10”. This is because at any shear rate its viscosity is lower than that of the unsheared original material and structural damage is also temporary. When the ink screen is printed on the surface of the board, the movement of the ink is far (shearing power) not reach the strength at the time of stirring, and the structure destruction is far less than the strength when stirring, so the viscosity of the ink screen when printed on the board It is far less than the drop when stirring, so the width of the printed pattern, although it needs to be still but the change will not be large, the following figure is a comparison of the two thixotropic results.

Therefore, it should be noted that the increase in shear rate and the area covered by the viscosity curve during recovery is a measure of the thixotropic property.

When we use optical imaging plating inks, different brands also have different results, and the same happens. Please see the following two figures for comparison:

Therefore, it is very necessary to add a thixotropic coating additive to the ink to form an ideal thixotropic structure. This is also an important indicator required in PCB inks, but it should be pointed out that this increase in thixotropic performance is not endless but is just right in terms of production practices. In previous years, some brand inks had similar phenomena. Light emphasized the thixotropy of inks, increased the viscosity, scratched and printed holes in screen printing, blocked holes, and the film surface was rough, which affected the accuracy of lines and the normal operation of subsequent processes. .

The property that the ink printed on the substrate is smooth and smooth is called leveling. When the ink is screen printed on the substrate, it will leave a trace of the grid, this trace can be reduced with the flow before undrained, when the leveling of the ink is poor, you can still see the traces of the slight grid with the naked eye, As shown in Figure 5:

It should be noted that: the ink from the uneven flow to the smooth driving force is not the gravity but the surface tension. In the figure, λ is the distance between the two network lines, and ao (wavelength) is the amplitude. When 2ao is less than 1μm, the naked eye cannot see the difference. . Leveling can be expressed by the following formula:


Where γ is the surface tension, ao is the initial amplitude, a1 is the amplitude at t, and Δt is the time required to level it to a1. As can be seen from the formula, when the viscosity of the ink is low, Δt is small, when the X value is high, Δt is small, and a small Δt value indicates that the leveling is good, so the leveling property is good and the viscosity is low.

However, when â–³t is too small, most of the boards we printed are mostly trussed to bake and dry except for some single panels. At this time, the sag phenomenon may occur. See Figure 6. Weighing force causes ink. The phenomenon of flow is called sag. For ink sag, it refers to the tendency of the ink to flow to different degrees after the screen printing truss (standing up), but the viscosity is a measure of resistance to flow, it is a factor to prevent sagging, after analysis by mechanics, sagging The speed formula is as follows:


Where ρ is the density of the ink, η is the viscosity, g is the constant of gravity acceleration, x is the thickness of the coating, so the viscosity is large, the sag speed is small; when the coating is thin, the sag speed is also small, the leveling and sagging of the ink are two Contradictory phenomena, good ink leveling requires that the viscosity be kept to a minimum for a sufficiently long period of time, and that there is sufficient time for the ink to level out and form a flat coating film, so sagging often occurs. On the contrary, it does not require complete sag. The viscosity of the ink is particularly high and it will result in little or no flow. For this purpose, an excellent rheological agent is needed, and the leveling and sagging are properly balanced, that is, the viscosity of the ink is temporarily low under the screen printing conditions, and is maintained at a low viscosity during the hysteresis recovery of the viscosity, showing a good flow Flatness; Once the leveling viscosity gradually returns, this prevents the sag phenomenon.

To sum up, we need the leveling property of ink when we make multi-layer inner layer, single-panel, signage, grid, frame and other products. This leveling property means that the used ink has leveling and sagging. The balance, the requirement is that after screen printing can be quickly leveled, and when the board is erected and can not appear sag. In the case of double-decked apertures, both the directly printed pattern ink and the photoimaging ink must have good thixotropy, and the leveling requirements are much lower than those mentioned above. Therefore, the screen printing ink is generally not diluted. , And after stirring for a long time, make it fully restored. The purpose is to ensure that the graphics are accurate and do not block the ink.

Source: Beijing Li Tuo Da Technology Co., Ltd.