Selective Soldering Hot Tear: Causes and Fillet Control

A hot tear is a crack or a lifted edge in a solder fillet that forms as the joint cools, and it is found after selective soldering rather than during it. The joint looked sound when the nozzle left it. The tear appears when the solder contracts against a barrel that is still expanding or still moving. The defect is found by the quality team rather than by the machine operator, which is why the settings that caused it are rarely still on the screen.

The defect is mechanical with a thermal cause, which is why it is often mistaken for contamination or for a bad alloy. Fixing it means understanding how the joint cooled, not only what the solder contained. The process window around the nozzle is where the answer usually is. The fix is nearly always a change to cooling rather than to the alloy, and the record is what shows which of the two moved.

What a Hot Tear Is

A hot tear runs along the boundary between the solder and the pad or the barrel wall, and it may be visible only under magnification. In its earliest form it is a hairline crack that grows with thermal cycling in service. Because the fillet surface can still look bright, inspection by eye misses it. A tear that appears on one product and not another usually follows the thermal mass of the board rather than the solder pot.

The tear is created while the solder is in the mushy range, when it has almost no ductility. Any movement of the board or the barrel at that moment pulls the joint apart, and the crack remains after the joint solidifies. That is why the timing of nozzle withdrawal matters as much as the soldering itself.

Why the Fillet Lifts

Fillet lifting usually starts where the solder meets a pad that is poorly bonded to the laminate, or where the mask edge has created a stress riser. The solder then pulls away with the pad rather than staying with it. The defect is reported as a solder problem but begins as a board problem. A preheat that is raised to speed the cycle will reduce the gradient at the joint, and that alone removes many of the tears seen in production.

Differential contraction is the other cause, because solder, copper and laminate shrink at different rates as they cool. A joint that is thick and stiff will resist that movement until it cracks. Increasing the fillet size to look safer can therefore make a hot tear more likely.

Thermal Mass and Heat Balance

Heavy copper planes and large thermal reliefs draw heat away from the joint, so the solder may be solid while the plane is still warm. As the plane cools it contracts and pulls on the joint. Thermal mass therefore has to be considered joint by joint rather than board by board. The nozzle should be checked for alignment and for build up, because a partly blocked opening changes the flow without changing the setting.

Preheat reduces the difference between the joint and its surroundings, which is why it is the first thing to check. A board that enters the nozzle cold will have a large temperature gradient around the joint. That gradient is what drives the movement.

Nozzle Travel and Contact

The nozzle should leave the joint cleanly, without dragging solder or pulling the fillet with it. A worn or badly sized nozzle leaves a bridge of solder that is still liquid when the machine moves on, and that bridge breaks and leaves a torn edge. Sizing is covered in selective solder nozzle design work. Flux that is applied too far ahead of the nozzle has time to dry out, and dry flux does not wet the joint it was meant to protect.

Nozzle dwell, travel speed and the path between joints all set how much heat the joint receives and how long it stays liquid. Programme changes made for cycle time can therefore introduce a hot tear that was not there before. Travel should be validated on the joint, not only on the clock.

Flux and Preheating

Flux has to be active while the solder wets, and it also has to be dry before the nozzle arrives. Liquid flux left on a joint flashes into vapour when the nozzle touches it, and that vapour disturbs the fillet. Preheating dries the flux and reduces the thermal shock at the same time. A joint repaired after a tear has been reheated twice, so the second inspection should be closer than the first rather than lighter.

Too much flux is as bad as too little, because the excess boils under the solder and leaves voids and rough surfaces. Flux volume and spray pattern should be set with the joint in mind, and the limits in selective soldering flux volume work are the reference for that setting.

Solder Alloy and Temperature

Lead-free alloys are stronger than tin-lead and they crack differently, so a tear in a lead-free joint may look sharper than expected. Higher melting points also mean a wide mushy range in some alloys, which lengthens the vulnerable period. Alloy and profile should be considered together. Comparing a board soldered at the start of a shift with one soldered at the end shows whether the machine or the board is drifting.

Pot temperature that is too low makes the joint solidify sooner but also raises the chance of a cold joint, while a high temperature extends the liquid time and increases oxidation. The two risks pull in opposite directions, so the temperature should follow the alloy specification rather than the operator preference.

Preventing and Checking

Prevention starts with holding the board still while the joint solidifies, which means good support and no conveyor movement at that moment. Fixtures should clamp the panel without bowing it. A panel that is free to move will move.

Checking should use magnification and, where a standard applies, the criteria in the published IPC acceptance documents. A joint that has been reheated to repair a tear should be inspected again, because repeated heating can enlarge the crack. Comparing joints between shifts shows whether the change is real or imagined, and the wider sequence is described in selective soldering process control work.

Records and Process Windows

The record should carry the preheat, pot temperature, nozzle size, dwell and travel speed together with the inspection result. That combination is what allows a hot tear to be traced to a specific change. A record of temperature alone cannot explain a mechanical fault.

Over a campaign the record shows which products are sensitive and where the window is narrow. Products with large thermal mass usually need a slower profile and more preheat. Knowing that in advance is what keeps the defect out of production.

FAQ

What does a hot tear look like? A crack or a lifted edge in the fillet, often a hairline under magnification, found after the board has cooled rather than at the nozzle.

Is a hot tear a solder problem or a board problem? Both. It is caused by movement while the joint solidifies, which can come from thermal mass, fixture support or nozzle travel.

Does more solder prevent it? No. A larger fillet is stiffer and can make the tear more likely, so the aim is controlled cooling and a stable board.