Solder Paste Viscosity and Printing Window Control Guide

Solder paste is a suspension of solder powder in a flux vehicle, and its viscosity is the property that decides how well it prints, how long it stays on the stencil and how much it slumps before reflow. Because the paste is a non Newtonian fluid, a single viscosity number describes it only partially, and understanding the printing window is what separates a stable process from one that needs constant adjustment. Solder paste is also a living material, in the sense that its properties change with time, temperature and handling, so the process has to be controlled around a moving target.

Why Viscosity Matters at the Printer

During printing the paste is rolled by the squeegee, forced through the stencil apertures and then released onto the pad. Each of those stages needs different behaviour: the paste must flow under shear, release cleanly from the aperture wall, and then hold its shape on the pad without slumping.

Those requirements are in tension. A paste that flows too easily slumps and bridges after printing, while one that is too stiff does not fill the aperture properly and leaves a deposit with a ragged or incomplete edge. The printing window is the range of conditions in which both are satisfied. Finding that window takes a day of trials on the printer, and staying inside it takes discipline for the rest of the product life.

What Viscosity Actually Means

Viscosity is the resistance of a fluid to flow, expressed as a ratio between shear stress and shear rate. For a simple liquid the ratio is constant, but solder paste is a suspension and the ratio changes with the shear rate applied to it.

That means a viscosity figure is meaningless unless the shear rate is quoted with it. Two pastes with the same quoted number can behave very differently on a printer, because one may be far more shear sensitive than the other across the range of rates the printer actually applies. The metal content and the powder size distribution also influence the flow behaviour, so two pastes with the same flux can print very differently.

Rheology and Shear Thinning

Solder paste is shear thinning, which means its apparent viscosity falls as the shear rate rises. That property is what allows the paste to be rolled and pushed through an aperture and still stand up as a deposit once the force is removed.

The recovery behaviour matters as much as the thinning. A paste that thins well but recovers slowly keeps flowing after the squeegee has passed, which produces slumping and bridging. The recovery rate is a property of the flux chemistry rather than of the powder. A paste with slow recovery will produce a deposit that looks correct immediately after printing and has slumped by the time it reaches the reflow oven.

Measuring Viscosity

A rotational viscometer with a defined spindle and speed is the standard instrument, and the reading is normally taken at a single shear rate for comparison with a specification. The measurement is sensitive to temperature, so the sample has to be equilibrated first. The spindle, the speed and the sample size should all be fixed in the test method, because a different setup gives a different number for the same paste.

A more informative approach is a viscosity curve across several shear rates, which shows how the paste behaves through the whole printing cycle. Our paste inspection guide describes how the printed result is verified on the line. A printed volume measurement taken immediately after printing is the best evidence that the paste and the printer are still in agreement.

Temperature and Age Effects

Viscosity falls as temperature rises, so a paste that is used straight from a cold store behaves differently from one that has been at room temperature for hours. Warming a paste quickly can also separate the flux from the powder, which changes both the printing and the reflow behaviour. The correct procedure is to allow the jar to reach room temperature in its sealed container before it is opened, which takes a defined number of hours.

Age has a similar effect. The flux vehicle slowly reacts with the powder, the paste thickens and its working life shortens. A paste that is at the end of its open time prints differently from a fresh jar even when the viscosity reading is unchanged. The powder oxidises slowly as well, which reduces the wetting performance of the paste even if the flow behaviour looks acceptable.

Printing Window and Process Speed

The printing window is the combination of squeegee speed, pressure, separation speed and stencil condition in which the deposit is correct. Paste viscosity sets where that window lies, and a change in viscosity moves the window rather than only degrading the result.

A slow squeegee speed gives the paste more time to fill the aperture, while a fast speed relies on the paste thinning quickly. Where the viscosity has drifted upward, slowing the squeegee can recover the deposit, which is why the adjustment is common and why it hides the real cause. Recording the printer settings with the paste lot is what turns that adjustment into a visible trend rather than a habit.

Aperture Area Ratio

The area ratio between the aperture opening and the aperture wall determines how much of the paste transfers to the pad. A low ratio means the paste prefers to stay on the wall rather than release, and no amount of squeegee pressure will fix it. The area ratio is a design parameter, and a stencil that violates it will fail on the line no matter how well the printer is set up.

Viscosity interacts with the area ratio, because a paste that releases well can print through a smaller ratio. Our land pattern guide covers the pad and stencil geometry that the deposit depends on. Where the ratio is marginal, a step stencil or a different aperture shape can sometimes recover the process without changing the layout.

Defects Linked to Paste

The classic paste related defects are insufficient deposit, slumping and bridging, solder balls, and a deposit that sticks to the stencil rather than the pad. Each of them points at a different combination of viscosity, aperture and machine setting. Bridging, for example, usually points at slumping, while insufficient deposit points at a release problem rather than at the paste itself.

Solder balls are often traced to a paste that has absorbed moisture or to a deposit that has been disturbed after printing. Our solder defects guide describes how these conditions are classified. Where a defect appears on one side of the board only, the cause is more likely to be the stencil or the support than the paste.

Process Control Points

The controls are the paste lot and its expiry date, the storage and warming procedure, the jar open time, the room temperature and humidity, the printing parameters and the deposited volume measured on a sample. Humidity matters because the flux absorbs water, which changes both the viscosity and the tendency to form solder balls.

Each of those is measurable, and the printing parameters should be recorded with the paste lot so that a change in deposit volume can be explained. Our quality documentation describes how these results are recorded at gopcb.

FAQ

Why does the viscosity number not predict printing behaviour? Because paste is shear thinning, so a single figure describes only one shear rate. The behaviour across the range of rates the printer applies is what actually matters.

Can a paste that is too thick be used by slowing the printer? It can, and that is why the adjustment is so common. Slowing the squeegee widens the window, but it also hides a drifting paste and lengthens the cycle time.

How long can a jar of paste stay open? It depends on the product and the environment, and the supplier states a figure. Beyond the open time the paste thickens and its printing and reflow behaviour both change. A jar that has been open for a full shift should be treated as a different material from one that was just opened.