Solder Paste Slump and Its Effect on Bridging

A printed paste deposit is a structure that has to hold its shape from the moment the stencil lifts until the alloy melts. Slump is the loss of that shape, and it is the mechanism behind a family of bridging defects that are wrongly blamed on aperture design. Understanding slump means understanding the time window between printing and reflow, which is a production constraint rather than a material property alone.

What Slump Is

Paste is a suspension of alloy powder in a flux vehicle that behaves as a thixotropic fluid. It flows when sheared during printing and then recovers its viscosity, which is what allows a tall deposit to stand. Slump is the slow flow that continues after the recovery, driven by gravity and by surface tension.

The result is a deposit that becomes wider and lower over time, and on a fine pitch part the widening closes the gap between adjacent deposits. Nothing has to be wrong with the stencil for this to happen; the material simply does what a fluid does given enough time.

Rheology and Its Limits

Viscosity and the recovery rate are the properties that resist slump. A paste with a high recovery rate holds its shape longer, which extends the window between printing and reflow. A paste with a slow recovery spreads sooner, which shortens it.

Those properties are set by the flux chemistry and by the powder loading. Increasing the metal content raises viscosity and reduces slump, but it also reduces tack and can make the paste harder to print. The balance is chosen by the supplier for a target application, which is why paste selection and process window are the same decision. Our article on paste qualification describes how to measure the window rather than assume it.

Print Definition and Aperture Geometry

Print definition is what there is to slump. A deposit with vertical walls and a flat top has further to fall than one that is already rounded, and the volume is identical in both cases. Good definition comes from the stencil, from the release and from the print parameters.

Area ratio is the key geometric factor. A low area ratio produces a deposit with poor definition and irregular walls, and those irregularities are the starting points for slump and for bridging. The relationship between the aperture and the achievable deposit is set out in our article on paste volume and stencil design.

Process Control and Verification

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. 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.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

Hold Time Before Reflow

Hold time is the interval between printing and reflow, and it is the variable that production controls least well. A board printed at the start of a shift may wait in a queue while the line changes over, and the deposit that was sharp at the start may have spread by the time it reaches the oven.

The behaviour is measured against a defined hold time, so the acceptable interval is a tested limit rather than an opinion. The practical response is to control the queue: a maximum number of panels between the printer and the oven, and a rule for what happens to boards that exceed it. Rejecting or reworking is expensive, so the queue limit is worth enforcing.

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.