Solder Paste Hot Slump and Reflow Preheat Control

Hot slump is the movement of a solder paste deposit after printing but before the alloy melts, and it is caused by the paste softening as it warms rather than by anything mechanical. A deposit that was well formed at the printer can spread far enough during the reflow preheat to touch its neighbour, and the bridge forms before the solder has even liquefied.

The defect is easy to confuse with a printing problem, because the evidence at the end of the line looks the same. Separating the two comes down to the profile: if the same stencil and the same paste produce good deposits when the preheat is slowed, the problem was slump and not printing.

What Hot Slump Is

Paste is a suspension of alloy powder in a flux vehicle, and the vehicle softens as temperature rises. Between roughly 60 and 150 °C the paste loses the structure that kept the deposit in shape, and gravity and surface forces take over until the flux begins to activate and the powder begins to sinter.

The amount of movement depends on how long the deposit spends in that soft range. A slow, shallow ramp gives the paste time to spread, while a fast ramp passes through the range before the deposit has time to flow, which is why profile shape matters as much as the peak value. A profile that moves slowly through the soft range and then accelerates into the spike usually performs better than one with a long flat soak.

The Role of the Reflow Preheat Ramp

Reflow preheat has two jobs: to bring the assembly to a uniform temperature and to activate the flux without boiling off the solvent too early. The ramp rate in the first 60 °C of the profile decides how much the deposit has softened before the assembly is thermally even.

A ramp of 1 to 2 °C per second is a common starting point for lead-free work, with a soak from 150 to 190 °C that allows the flux to work. Where the preheat and soak zone is stretched to equalise a heavy board, the risk of slump increases for every deposit on the board.

Paste Viscosity and Yield Behaviour

Paste viscosity measured at the printing shear rate says little about how the material behaves at low shear during heating. What matters in the preheat is the yield stress, the point below which the paste behaves like a solid and above which it flows.

A paste with a low yield stress slumps readily, and the same paste may print beautifully and then fail on a board with a long preheat. Where the solder paste slump behaviour has been characterised, the profile can be matched to the material rather than the other way round.

Surface Tension and Wetting

Surface tension pulls a melted deposit toward the pad and away from the solder mask, and during the soft phase it also pulls a deposit toward its neighbours. The balance between the two decides whether a marginal deposit spreads onto the mask or bridges the gap beside it.

Mask surface energy therefore plays a part. A mask with a low surface energy repels the flux and limits the spread, which is why a change of solder mask material can change the bridging rate on a product that has not otherwise been altered. Mask roughness plays a smaller part but follows the same principle, since a rough surface holds the flux in place.

Aperture Geometry and Spacing

Slump becomes a defect when a deposit has somewhere to go. The distance between two apertures, measured as the mask web between the pads, sets how much movement is tolerable, and a web below 0.15 mm leaves very little margin on a fine pitch device.

Aperture area also matters, because a large deposit holds more heat and softens earlier than a small one. Where a board mixes power pads and 0.4 mm pitch devices, the large pads will begin to flow while the small ones are still firm, so the profile has to be set by the most vulnerable pair of apertures rather than by the average.

Bridging and Solder Beading

Bridging between adjacent leads is the classic symptom, and on a fine pitch device it often appears on the same two pins across a whole production run. Beading is the second symptom, where a small sphere of solder sits beside the joint after reflow.

Both defects share a common cause in the soft phase. A paste that has spread onto the solder mask cannot be pulled back by surface tension, so the solder that would have formed the fillet ends up as a bead on the mask instead, and the joint itself finishes thin. A bead beside a joint is therefore a useful indicator, because it means the deposit had already moved before the alloy melted.

Profiling to Control Slump

Slump is controlled through the shape of the first part of the profile. Shortening the time spent between 60 and 150 °C, and reducing the amount of soak, both limit the period during which the paste can flow, and the change is usually visible on the next production board.

The trade is with flux activation. A profile that rushes through the soft range may reach the spike with the oxide layer partly intact, which produces poor wetting and a dull fillet. The profile soak and spike balance is therefore set on a board with the finest pitch the product family contains.

Inspection and Measurement

Inspection starts at SPI, because an SPI system can compare the deposit area on the first board of a run against the reference and show whether the paste has spread after printing. A pre-reflow optical check on the fine pitch devices is the second step, and it costs seconds per board.

After reflow, bridging is obvious, but the marginal cases are not. Where a product has a history of beading, the mask beside the pads should be inspected with magnification, since the evidence is a faint solder residue rather than a visible sphere on every board.

Process Limits and Corrective Action

Corrective action follows a fixed order: confirm the profile against the qualified reference, confirm the paste condition against the open time limit, and confirm the stencil and mask dimensions against the drawing. Acting on the printer first wastes the easiest checks.

A paste change is the last resort, because it changes the printing window as well as the slump behaviour. Where a new paste is introduced to solve slump, the print parameters should be re-qualified from scratch, and the transfer efficiency should be measured on the same apertures as before.

FAQ

What causes hot slump in solder paste? The flux vehicle softens as the assembly warms, and the deposit flows under its own weight and surface forces before the alloy melts, which is why profile shape decides the severity.

Does a longer soak always increase slump? It increases the time available for the deposit to flow, so a long soak on a product with narrow mask webs usually makes bridging worse unless the paste has a high yield stress.

Can a stencil change fix hot slump? Reducing the deposit volume and increasing the spacing between apertures helps, but the underlying cause is thermal, so the profile is normally adjusted first.