Solder Paste Rheology and the Print Window

Paste is a thixotropic fluid: it flows when it is sheared and it recovers when the shear stops. The print window is the range of conditions over which that behaviour gives a good deposit.

What Thixotropy Means Here

When the squeegee rolls the paste, the viscosity falls and the paste flows into the aperture. When the stencil separates, the viscosity recovers and the deposit holds its shape.

Recovery takes time, so a fast separation leaves a deposit that is still fluid and slumps. A slow separation gives a deposit that holds. The separation speed is therefore a paste dependent parameter. Our print notes describe how it is developed.

Shear Rate in the Aperture

The shear rate is set by the squeegee speed and the aperture geometry. A small aperture at a high speed gives a very high shear rate and a paste that flows well; the same paste at a lower speed flows less.

That is why a print that works at one speed can fail when the line is sped up, without any other change. Our area ratio notes describe the aperture geometry that sets the limit.

Recovery and Slump

A paste with a fast recovery holds a tall deposit and resists slump, which suits fine pitch. A paste with a slower recovery fills the aperture more completely and produces a deposit with fewer voids.

The two properties are balanced in the formulation, so a paste choice is a choice about which failure mode is more likely on the product. Our metal load notes describe one of the parameters that shifts the balance.

Temperature and the Window

Viscosity falls as the temperature rises. A room that warms by a few degrees in the afternoon changes the print, and the change is gradual enough to be attributed to something else.

The print area temperature should be controlled and recorded, because it is one of the few variables that affects every aperture at once. Our paste notes describe the working life that the temperature also affects.

Humidity

Humidity affects the flux as well as the metal. A high humidity makes the paste absorb water from the air, which shows up as spatter at reflow and as a deposit that changes through the day.

The relative humidity in the print area should be controlled, and the paste should be exposed to it for the defined working life only.

Measuring the Behaviour

The rheology can be measured with a viscometer to a defined method, and the result compared with the specification for the paste.

The printing result is more directly measured by weighing or inspecting the deposit, which is what the process actually depends on. Our paste inspection notes describe the measurement.

When the Window Is Too Narrow

A paste whose window is narrow for the product forces the process to be controlled tightly, which costs. The alternative is to change the paste or the aperture.

Where the aperture is what makes the window narrow, a thinner stencil or a different powder size is often the more effective answer than a different chemistry.

Process Control and Verification

A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used. Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

Verification and Records

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

FAQ

Does a longer print stroke help? It gives the paste more time and it also generates more heat and more solvent loss. The stroke should be long enough to fill the aperture, not longer.

Can the print window be widened? It can be widened by the aperture, the stencil thickness and the paste, and not by the printer settings alone.

What does gopcb provide for paste rheology control? We provide paste selection by aperture and speed, print parameter development that includes the separation profile, temperature and humidity control in the print area with recorded values, working life limits, and deposit measurement on the product rather than on a test vehicle.