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

On a design of this kind, print window is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

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.

Process Control and Verification

On a design of this kind, print window is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

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.

Process Control and Verification

On a design of this kind, print window is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

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.

Process Control and Verification

On a design of this kind, print window is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

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.