Solder Paste Viscosity and Rheology in Stencil Printing

Solder paste is a suspension of metal powder in flux, and its flow behaviour decides how it fills a stencil aperture, how it releases onto the pad and how long the deposit holds its shape. Two pastes with the same alloy and the same particle size can print very differently because their rheology differs, and many printing problems blamed on the printer, the stencil or the operator turn out to be a paste that is outside its working window. Reading paste viscosity as a measured property rather than as a single number is the first step to controlling it.

Why Paste Behaves Like a Complex Fluid

A paste is not a simple liquid. Its paste viscosity falls as the shear rate rises, so it flows readily while the squeegee is pushing it across the stencil and then stiffens once the motion stops. That behaviour, called shear thinning, is what allows a deposit to fill a small aperture under pressure and then stand as a brick afterwards.

The same property is why the numbers on a datasheet must be read with the test method in mind. A figure quoted at one shear rate says little about behaviour at another, and rheology is properly described by a curve rather than by a single point, which is why two suppliers can quote the same value for pastes that behave quite differently.

Measuring Viscosity in Practice

The classic shop measurement uses a spiral viscometer, in which a rotating spindle is lowered into a sample and the torque is recorded. It is quick and it suits a production line, but it reports one point on the curve at one temperature and one shear rate.

A rheometer gives the full picture, sweeping the shear rate and separating the elastic response from the viscous one. That detail matters when a paste is being qualified for a fine pitch product, because the behaviour at very high shear rate governs how the aperture fills. For a production qualification the useful output is a pair of curves taken at the low and the high end of the printing temperature range, so the window the paste tolerates can be read directly.

What Shear Rate Does in the Aperture

Inside a stencil aperture the paste is sheared hardest at the walls, where the squeegee drives it, and least in the middle. The deposit therefore fills from the edges, and a paste that thins readily under shear will fill a small aperture at a lower printing pressure than a stiffer one.

The release step is the reverse of the fill. As the board drops away, the paste must let go of the aperture wall and stay on the pad, and the balance between adhesion to the pad and adhesion to the wall is set by the same rheology together with the area ratio of the aperture.

Metal Loading and Its Effects

Metal loading is the proportion of the paste by weight that is alloy powder, and it drives both the viscosity and the volume of solder left after reflow. Increasing metal loading raises the viscosity and reduces slump, which helps fine pitch printing but makes the paste harder to push through the stencil.

Reducing metal loading has the opposite effect. The paste flows more easily, prints at lower pressure and slumps more, and the resulting joint is thinner because there is less alloy in the deposit. Metal loading should therefore be treated as a fixed property of a qualified paste rather than as something to adjust on the line.

Slump, Tack and Open Time

Slump is the tendency of a printed deposit to spread after the stencil lifts, and it is the reason a paste with poor slump resistance produces bridges between adjacent pads. Tack is the stickiness that holds a component in place before reflow, and it has to survive the placement cycle without becoming so strong that it lifts paste from the pad.

Both properties change with time. Solvent leaves the paste from the moment the jar is opened, so the deposit stiffens, the tack falls and the print quality drifts. The working period that follows is a genuine process limit rather than an operator preference. A paste that has stood in the stencil for an hour is no longer the material that was qualified, even when the jar itself is still inside its date code.

Temperature and Humidity on the Line

Paste viscosity is strongly temperature dependent, so a print room that swings by five degrees will show a measurable change in deposit volume. The usual response is to hold the room within a narrow band and to let a cold jar reach room temperature before it is opened, which also prevents condensation onto the paste.

Humidity acts more slowly. A very dry room dries the paste in the aperture and on the stencil, while a very humid one lets the powder absorb moisture until the paste turns gummy. Both effects appear as print defects that move around the board rather than as a constant fault.

Mixing, Stirring and Storage

Paste is thixotropic, which means it also recovers its structure after being disturbed. A jar that has been stirred or warmed before use will print thinner for a while and then settle, so the habit of stirring paste to make it easier to print trades a short term gain for variability across the shift.

Storage at the recommended temperature keeps the flux chemistry stable, and the jar should be sealed and returned to the refrigerator rather than left on the bench. Reusing paste that has stood on the stencil for hours is the most common source of a print that drifts through the day.

When the Paste Is Blamed for a Print Defect

Incomplete fills, bridges, slumps and missing deposits all have several possible causes, and the paste is only one of them. Stencil tension, aperture wall finish, squeegee wear, board support and pad finish all change the print, and the paste property that matters most is often the one nobody measured.

The practical approach is to check the stencil tension and the tooling before adjusting the paste, because the mechanical variables are cheaper to correct. Where a defect survives those checks, the wider catalogue of solder defects and board failures gives a useful route through the remaining candidates.

Setting a Working Specification

A workable specification for paste on the line includes the storage temperature, the warm up period, the maximum time in the jar after opening, the printing humidity and temperature limits, and an inspection routine for the deposits. The viscosity value belongs in that list as a window rather than as a target.

Once the window is written down, drift becomes visible as a trend instead of a surprise. Print inspection with an SPI system closes the loop by measuring deposit volume on every board, so the effect of a paste that is going out of specification is caught before the reflow oven ever sees it. The same measurement separates a paste problem from a tooling problem, because a paste change moves every aperture together while a squeegee or support fault concentrates on one area of the board.

FAQ

Should paste be stirred before use? Only if the supplier instructs it, and never as a routine habit. Stirring changes the structure of the paste and makes the first prints of a shift different from the rest, which is the opposite of what process control needs.

Does higher viscosity always print better? No. A stiffer paste resists slump and holds fine pitch deposits well, but it needs more pressure to fill the aperture and can starve small openings. The right value depends on the area ratio of the stencil and on the pitch of the product.

How can paste be checked on the line? A spiral viscometer reading at a fixed temperature gives a usable trend if it is taken at the same point in the paste life. Comparing that number against the value recorded when the paste was qualified is more useful than comparing it with a datasheet.