Placement Nozzle Selection and Wear in SMT
The nozzle is the only part of a placement machine that touches the component, and it is usually the last item considered when placement accuracy is reviewed. Its diameter decides how the part is held, its material decides how long it holds it the same way, and its condition decides whether the part is placed or dropped. This article covers selection, wear and the measurements that keep the process repeatable.
What the Nozzle Has to Do
A placement nozzle has to pick a component without marking it, hold it against acceleration without slipping and release it without disturbing the paste. Those three requirements pull in different directions, and the nozzle that is chosen is a compromise between them.
The vacuum is the holding force, and it acts on whatever area the nozzle tip covers. A tip that is too large picks up the part but also picks up adjacent parts or covers fiducials, while one that is too small holds the part on a fraction of its area and lets it rotate.
Matching the Nozzle to the Component
For chip components the rule is that the tip should cover between 50 and 80 percent of the part’s top surface, which keeps the part centred while leaving the paste undisturbed at the edges. Below that the part can shift during travel, and above it the nozzle can touch the paste on the neighbouring pads.
For larger devices the tip is replaced by a set of pads or a gripper, and the same principle applies to the contact area. Where a part has a sensitive top surface, a soft tip or a rubber pad is used rather than direct contact with metal.
The tip diameter is recorded against the part number in the machine programme, so a nozzle change is a documented event rather than a maintenance decision made at the machine.
Materials and Their Behaviour
Ceramic tips resist wear and are used for high cycle counts, while tungsten carbide tips last well on abrasive parts and metal tips are used where static dissipation matters. Rubber and urethane tips protect the component but wear faster and change dimensions as they age.
Material choice also affects the release. A tip that has become slightly tacky from flux or from the components themselves will hold the part for a moment after the vacuum is released, which shows up as a placement offset in one direction.
Wear and Its Symptoms
A nozzle wears at the tip face, where the component touches it, and on the bore, where the vacuum passes. Tip wear rounds the edge and reduces the effective contact area, so the holding force falls even though the vacuum reading at the sensor is unchanged.
The symptoms are a rising placement offset, parts that rotate slightly during travel and, at the extreme, dropped components on the conveyor. Because the change is gradual, the trend in placement data is the evidence rather than a single inspection.
The tip is measured under a microscope against a new example, since a worn tip looks correct at a glance and differs by a few tens of micrometres.
Vacuum Level and Measurement
The vacuum is produced by a pump or an ejector and measured at the nozzle with a gauge or through the machine’s own sensor. A reading that is adequate at the pump but low at the tip points to a leak in the line, a worn seal or a partially blocked bore.
Filters in the vacuum line protect the pump from dust and from solder spheres, and a blocked filter shows as a low reading at the tip with a normal reading at the pump. The filter is a consumable and its replacement interval belongs in the maintenance plan.
Cleaning and Contamination
Flux from the paste and from the components accumulates on the tip face, and it changes both the friction and the release. Cleaning is done with a suitable solvent and a soft tool, never with a wire brush, because the tip geometry is what makes the nozzle work.
Contamination inside the bore is harder to see and it reduces the effective vacuum. Where a machine runs several products, the cleaning interval follows the dirtiest product rather than the average one, and the interval is set from measurement of the vacuum at the tip.
Replacement Practice
Nozzles are replaced on a count of picks or on a measured vacuum loss, whichever comes first, and the replacement is recorded against the tool position. A nozzle that is replaced on the basis of appearance is either replaced early or left too long.
The count is set by measuring the vacuum at the tip over a run, and by recording the placement offset for the parts that nozzle handles. Both figures change slowly, and the point at which either leaves its band is the replacement point.
Placement Accuracy and Its Verification
Placement accuracy is verified with a glass plate or an X-ray system on a sample of boards, and the result is expressed as a spread rather than a mean. A change in spread that follows a nozzle change identifies the nozzle rather than the programme. The methods are described in our capability notes.
Where the spread has grown, the first check is the nozzle at the position concerned, followed by the feeder and the vision system. Separating those three on evidence is faster than adjusting the programme on suspicion.
Records and Spares
The record that makes nozzle management workable is the tool position, the part number it serves, the pick count since replacement and the vacuum measured at the last service. With those four items, a placement problem can be traced to a tool rather than to the machine.
Spares are held for the nozzles that actually wear, since a machine stopped for a nozzle that is not in stock costs more than the whole set. The set is reviewed when a new product is introduced, because a new package usually needs a nozzle that is not on the shelf. The judgment applied to the finished placement is discussed in our board quality guide.
The pick and place programme stores the nozzle against the part number, so a change of nozzle is a change to the programme rather than an adjustment at the machine. Where a nozzle is substituted because the correct one is not available, the substitution is recorded, because it changes the holding force and therefore the placement result.
Nozzle height is set in the programme and it decides how far the tip travels before the vacuum is applied. A height that is too low presses the part into the paste, while one that is too high fails to seal against the component top, which appears as a dropped part rather than as a placement offset.
Several nozzles on one head let a machine place different packages without a tool change, but each position has its own offset that has to be calibrated. A head whose nozzles are not individually calibrated places one package correctly and the next with a consistent offset.
Where a component is picked from a tape whose cover does not peel cleanly, the nozzle lifts the tape instead of the part and the pick fails intermittently. That is a feeder problem, and it is worth separating from a nozzle problem before the tool is changed.
A placement machine records failed picks per feeder, and a rise at one position points to the pick height, the tape or the nozzle at that position rather than to the machine as a whole.
FAQ
How often should a nozzle be replaced? On a pick count correlated with the measured vacuum at the tip, rather than on a calendar or on appearance.
Can one nozzle serve a whole product? Only where the components are similar in size; mixed boards need a nozzle set that is selected by the programme for each part.
Why does a part rotate during placement? Usually because the tip covers too little of the part, so the holding force is applied off centre and the part turns under acceleration.
