Solder Paste Rheology: Viscosity, Metal Content and Printability

A solder paste is a suspension of metal powder in flux, and the way that suspension flows decides whether the deposit is a clean brick or a smear. Viscosity is the single figure most often quoted, but it is only meaningful with the test method beside it, and the rheology behind it is what actually governs printability.

What the Paste Specification Contains

A paste specification names the alloy and its melting range, the powder size class, the flux type and activity per J-STD-004, and the metal content by weight. Those five items together determine how the paste prints, how it reflows and how much residue is left on the board. The alloy also fixes the profile window, since a lead free alloy melts some 34 degrees higher than the tin lead it replaced and the flux has to activate below that melting range.

The flux type is coded, so a ROL0 paste is rosin based with low activity and halide free, while a REL1 paste is a different combination of the same three attributes. Two pastes with the same alloy and the same powder class can behave completely differently at printing because of that code.

How Viscosity Is Measured

Viscosity is measured with a rotational viscometer using a spindle that is specified in the method, typically a TF spindle at 5 to 10 rpm for one minute at 25 degrees Celsius. A typical printing paste reads in the region of 100 to 200 pascal seconds under those conditions, and the number is meaningless without them.

Changing the spindle, the speed or the temperature changes the reading by more than the difference between two compliant pastes. That is why a viscosity specification is written as a method plus a range, and why a receiving check repeats the method rather than inventing one.

Shear Thinning and Recovery

Solder paste is shear thinning: it is thick at rest and flows when the squeegee moves it. The thixotropic index is the ratio of the viscosity at low speed to the viscosity at ten times that speed, and it describes how strongly the paste behaves this way; printing pastes commonly fall between 0.4 and 0.7.

Recovery after shear is the other half of the behaviour, because the paste has to stop flowing as soon as the squeegee has passed so that the deposit holds its shape. A paste that recovers slowly leaves a rounded deposit that slumps into the neighbouring aperture before reflow, which shows as bridging on fine pitch. The same mechanism appears later as a solder defect rather than as a printing fault, and our board failure notes describe how the two are separated.

Metal Content and Its Effect on Volume

Metal content is quoted by weight, usually between 85 and 90 percent, but the volume fraction is what governs the deposit, and it is closer to 50 percent. That difference is why a printed volume of paste yields roughly half its volume in solder when the flux has burned off.

Raising the metal content reduces slump and improves the definition of the deposit, but it also makes the paste harder to release from a small aperture. A paste at 88 percent metal by weight holds the shape of the deposit through placement, while one at 85 percent spreads under the weight of a large component before the oven sees it. When a fine pitch aperture starts to print short, the metal content is one of the variables worth testing, together with the powder type and the coating on the stencil.

Powder Size Distribution and Type

Powder class is numbered from type 3 to type 6 in J-STD-005, with type 3 at 25 to 45 micrometres, type 4 at 20 to 38, type 5 at 15 to 25 and type 6 at 5 to 15. The rule of thumb is that the largest particles should be no more than about one fifth of the aperture width for reliable deposition.

That rule is why 0.4 mm pitch work is printed with type 4 and 0.3 mm pitch with type 5, and why jetting uses the finer classes as well. Finer powder oxidises faster and ages more quickly, so the change is made for a specific aperture and not as a general improvement.

Slump, Tack and Open Time

Slump is the tendency of the deposit to flow out of shape before reflow, and it is tested by printing a pattern of lines and measuring how much they spread under a defined condition. Tack describes how strongly the deposit holds a placed component, and open time is how long the paste stays usable after the jar is opened.

Those three figures decide the practical window. A paste with a four hour open time on the stencil needs a refresh or a wipe schedule inside that window, and our solder paste handling notes describe how the open time is tracked against the jar rather than against the shift.

Storage and Warm Up

Paste is stored refrigerated between 2 and 10 degrees Celsius and has a shelf life of around six to twelve months from manufacture, quoted unopened. It is warmed up still sealed for two to four hours before use, because opening a cold jar draws condensation into the flux and changes the rheology.

After opening, the jar has its own working life, and paste that has been returned to the refrigerator a second time is normally discarded rather than requalified. Mixing old paste with new, which is common on a busy line, produces a viscosity that matches neither figure.

Testing in Production

A receiving check repeats the viscosity method and prints a coupon with the standard stencil, then measures the deposits. That gives a printability result that a viscosity figure alone cannot provide, because two pastes at the same viscosity can have very different thixotropic indices.

On the line, deposit volume from stencil printing is tracked through inspection, and a downward drift in volume across a shift is the earliest sign that the paste is drying or the stencil is clogging. The trend is more useful than any single reading. The coupon print is also where a change in slump is easiest to see, because the deposit edges are measured on a known pattern rather than on a production board with vias and mask openings around the pads.

When to Change the Paste Specification

The specification is changed for a reason that can be named: a finer aperture, a new alloy, a coating that has altered release, or a reflow profile that demands a different flux. Changing paste to solve a printing problem that belongs to the stencil or the printer usually moves the problem rather than fixing it. The print parameters move with the paste as well, so a paste change is followed by a print study instead of by a copy of the previous settings.

When a change is made, it is recorded with the print parameters and the inspection results for the first build. Without that record, the next engineer cannot tell whether the paste or the printer produced the improvement, and the change gets reverted on the next product by accident.

FAQ

Does a higher viscosity always print better? No. Viscosity at one speed says little about how the paste behaves under the squeegee or how quickly it recovers afterwards. A paste with a lower reading and a stronger thixotropic index can print fine pitch more consistently than a thicker one.

Can paste be requalified after its shelf life? It can be tested, but the useful checks are printability and reflow behaviour rather than viscosity alone, because ageing changes the flux and the powder surface at the same time. Most shops retire the jar instead, since the cost of the paste is small next to the cost of a reflowed panel.

Why does the same paste print differently in summer? Because the room temperature and the humidity in the printer enclosure have changed. Viscosity is strongly temperature dependent, so a line that holds temperature within two degrees prints the same paste at both ends of the year.