Solder Paste Slump: 6 Causes and How to Fix Them
Solder paste slump is the tendency of a printed deposit to spread beyond its aperture footprint before the board reaches reflow. A deposit that holds its shape has a defined edge and a flat top; one that slumps develops a rounded shoulder and creeps towards the solder mask. Slump is measured against the aperture, not the pad, because the aperture is the shape the process intended.
It matters because a deposit is only useful if it stays where it was placed. Paste that spreads onto the mask contributes nothing to the joint and becomes a source of solder balls. On a fine-pitch device, two adjacent deposits that spread can close the gap between them, and the result is a bridge after reflow that no profile change will remove.
<img src="https://www.gopcba.com/wp-content/uploads/2026/08/flexible-pcb-assembly-cover.jpg-1.webp" alt="Solder paste slump test coupon with deposits spreading after a hold period” />
What Solder Paste Slump Actually Is
Three properties combine to hold a deposit in shape: the viscosity of the paste, the yield stress of its flux vehicle, and the roughness of the surfaces it touches. Viscosity resists flow, yield stress defines the point at which flow begins, and the pad and mask surfaces add friction that helps the deposit stay put. A paste with a low yield stress spreads under its own weight even when its viscosity is high.
The pad geometry sets the distance the paste has to move before it becomes a defect. A pad with a wide margin of mask around it tolerates more spread than a pad where the mask sits a few tens of micrometres away. This is why the same paste can be acceptable on a coarse-pitch board and a problem on a fine-pitch one, and why the slump test should use the production stencil.
The Slump Test and How to Run It
The solder paste slump test is a printed coupon measured twice, before and after a defined hold. The standard method, published by IPC, defines the coupon geometry, the hold conditions and the measurement. The result is the change in deposit area or in the width of the bridge between adjacent deposits.
A practical version of the solder paste slump check uses the production stencil and paste. Print a panel, measure the deposits with the paste inspection system, hold the panel in the production area for the longest time a board will wait, then measure again. A change of more than about 5 % in deposit area is a warning; movement that touches the mask is a failure.
Cause 1: Paste Temperature and Warm-Up
Paste that is cooler than its rated working temperature is stiffer and prints with a rough surface, and it slumps less. Paste that is warmer than its rated window prints smoothly and then spreads quickly. The transition between the two is sharp, because the flux vehicle softens over a narrow range of a few degrees.
The common error is a jar that was never allowed to reach room temperature. It prints cold in the morning, warms as it sits on the stencil, and the boards printed at midday slump while the boards printed at the start of the shift do not. Measuring the paste temperature at the stencil, not in the jar, exposes this.
Cause 2: Solvent Loss and Viscosity Drift
Solvent leaving the paste raises the viscosity and usually lowers the tendency to slump, so a paste that has been on the stencil for hours is less likely to spread. The exception is the early period, when the paste is warming and its solvent has not yet left, and that is the window in which slump is most likely.
What makes viscosity drift a solder paste slump problem is not the drift itself but the change in print quality that follows it. Operators respond to a stiff, poorly filling paste by reducing squeegee pressure or raising the print speed, and both changes deposit more paste than intended. The extra volume spreads further and adds to the slump.
Cause 3: Aperture and Area Ratio Limits
Apertures with a low area ratio do not fill or release reliably, so the deposit is short and its edges are irregular. Irregular edges are where solder paste slump starts, because the surface tension pulls a ragged boundary into a rounded one. Apertures that are too large for the pad give the paste room to spread before it reaches the mask.
The aperture should therefore be sized for the pad and for the paste, and the area ratio should stay above the practical minimum. Where the design forces a marginal aperture, a thinner stencil foil reduces the deposit height and the driving force for spread. The aperture design rules give the working figures for each paste class.
Cause 4: Stencil Release and Squeegee Setup
A stencil that releases cleanly leaves a deposit with straight sides. One that releases poorly leaves a deposit that has been torn along one edge, and a torn edge is the first place spread begins. Release depends on the wall finish, the foil tension and the snap-off distance, and all three change as the stencil ages.
Squeegee pressure and angle decide how much paste is pushed under the stencil. Excess pressure forces paste between the foil and the mask, so the deposit starts wider than the aperture. The correct pressure is the lowest value that wipes the stencil clean, and it should be re-established whenever the stencil or the paste changes.
Cause 5: Room Humidity and Hold Time
High humidity softens the flux vehicle by absorbing water, and a soft vehicle slumps. Humidity also affects the paste on the stencil, which has a large surface area, far more than the paste in a sealed jar. A room that drifts from 40 % to 65 % relative humidity between shifts will show a corresponding change in solder paste slump.
Hold time is the multiplier. The longer a printed board waits before reflow, the more time the deposit has to spread, and the effect is not linear: most of the movement happens in the first minutes. The practical control is to keep the queue between the printer and the oven short and consistent, rather than to rely on the paste.
Cause 6: Reflow Ramp and Volatile Release
A deposit that has already spread is not fixed by the profile, but the ramp does control how much further it moves. A fast ramp heats the deposit unevenly, so the outer edges liquefy before the flux has had time to gel, and the molten shape follows whatever boundary exists. A slow, even ramp lets the flux set the deposit shape before the alloy flows.
The profile therefore works with the paste rather than on its own. Where solder paste slump is a problem, the first change should be to the paste and the printing, and the ramp should be adjusted to match the paste maker’s recommendation. The reflow profile should be re-measured rather than assumed to have carried over.
Fixing Slump in the Right Order
Work from the cheapest change to the most expensive. Confirm the paste temperature and warm-up first, then the hold time between printing and reflow, then the room humidity. Only after those should the stencil, the squeegee settings or the paste itself be changed, because those changes cost most and take longest to verify.
Where the paste is replaced, keep the paste volume comparison on the same stencil and the same board. A new paste evaluated on a different stencil produces a result that cannot be attributed. Record the deposit area before and after the hold for both pastes, and let the numbers decide.
Additional Considerations for This Build
Practical attention to paste viscosity pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating paste viscosity explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
FAQ
Does a low-viscosity paste always slump more? Not always. Yield stress and the rate at which viscosity recovers after the squeegee passes matter as much as the steady-state viscosity. A paste can be thin at the moment of printing and recover quickly, which gives good filling with little spread.
Can solder paste slump be corrected by lowering the stencil thickness? It reduces the driving force because the deposit is shorter, and it also reduces the volume. Where the joint needs the volume, a thinner foil trades a slump problem for a starved joint, so the aperture area should be increased at the same time.
How long can a printed board wait before reflow? The limit is the paste’s working life, not a fixed number, and slump is the symptom that shows the limit has been passed. Test it on your own line by holding a printed panel and measuring the deposits at intervals.
