Placement Nozzle Contact Force and Height Control
The placement nozzle is the last thing to touch a component before it is put on the board, and it is the only part of the machine that contacts the part directly. It picks the component from the tape by vacuum, holds it through the move, places it onto the paste and releases it. Every one of those steps has a height associated with it, and the heights are what decide whether the part is placed cleanly or crushed, shifted or dropped.
What the Nozzle Actually Does
The nozzle contacts the top surface of the component and holds it by vacuum through the body. The contact has to be light enough not to mark the part and firm enough to seal the vacuum against the surface. A body with a rough or domed top needs a soft tip that conforms, while a flat body with a large area needs a tip that covers enough of the surface to generate holding force without touching a neighbouring part.
The same nozzle has to place the part onto a paste deposit and release it without dragging. The release depends on the vacuum being broken quickly, on the tip not adhering to the body, and on the part not being pressed so far into the paste that the deposit spreads onto the mask. All three are matters of the timing and the height rather than of the nozzle material alone.
Pick Height and Placement Height
Pick height is the distance the nozzle descends to before the vacuum is applied. Too high and the vacuum leaks, so the part is not held securely and may be lost during the move; too low and the nozzle presses into the component, marking it or cracking it. The correct value is the one at which the tip just contacts the surface, and it depends on the tape’s pocket depth and on the height of the stack in the feeder.
Placement height is the distance at which the nozzle releases the part. On a machine that measures the board with a laser or a vision system, this height is set relative to the measured board surface rather than to a nominal plane, which compensates for board thickness and warpage. Where the machine does not measure, the height is nominal and the placement varies with the board. Setting the placement height from a measured surface is one of the largest single improvements available for placing parts onto a small paste deposit.
Vacuum, Blow-Off and Release Timing
Vacuum has to be established before the nozzle lifts and maintained until the part is in position. A leak caused by a worn tip, a contaminated nozzle or a part that is not flat reduces the holding force, and the symptom is a part that is dropped or that hangs at an angle. The vacuum level should be monitored by the machine where the sensor exists, and its trend is a useful indicator of nozzle condition.
Release is a short pulse of air, sometimes with a mechanical stripper pin. The pulse has to be long enough to break the vacuum and short enough that it does not blow the part off the paste. A part that moves on release is often described as a placement accuracy problem when it is a release timing problem. The pulse should be adjusted per component type, and the setting should be recorded with the placement program rather than left to the machine default.
Contact Force and Component Damage
The force applied on placement is the product of the nozzle pressure and the area of contact, and on a small component it can be surprisingly large. A force that is too high pushes the component into the paste, so that the paste is displaced onto the mask and the part sits lower than intended, and on a ceramic body it can produce a crack that is invisible at the time. The visible symptom appears later, as a cracked capacitor after thermal cycling.
The force should be set from the component and the paste volume, and it should be measured rather than assumed. Where a machine allows the placement force to be programmed, the value should be established by checking the deposit after placement under magnification: a correct placement leaves the paste fillet around the termination without displacing it onto the mask. Where the paste is squeezed out, the force is too high or the deposit is too large, and both should be checked before the programme is released. The inspection standard should include the deposit condition after placement as a check.
Nozzle Selection by Component Type
Nozzle selection follows the component. A chip component uses a tip that covers most of the body top; a small outline package uses a tip sized to the body; a connector with a long body may need two nozzles or a dedicated gripper. A part with a rounded top needs a compliant tip, and a part with a cavity or a hole on top cannot be picked by vacuum at all and needs a mechanical gripper.
The tip material matters for the same reason. A rubber or urethane tip conforms and grips well but wears and picks up contamination; a ceramic or metal tip is durable but does not conform. Where the component body is soft or has a marking that must survive, the softer tip is preferable even at the cost of frequent replacement. The selection should be recorded per component in the placement program, so that a change of operator does not change the nozzle.
Wear, Contamination and Cleaning
Nozzle tips wear at the contact face, and the wear reduces the seal and increases the vacuum leak. They also collect adhesive from the tape, dust from the room and flux from the machine, and a contaminated tip marks the component and can transfer material to the next one. Cleaning should be on a defined interval rather than only when a problem appears, and the cleaning method should not damage the tip.
The interval should follow the number of picks rather than the elapsed time, because a machine running a large part count contaminates its nozzle faster. A simple record of picks per nozzle, with cleaning at defined multiples, is enough to make the practice consistent. Where the machine has a tool changer, the nozzles should be identified and their history kept individually, since a single bad nozzle in a set of twenty is easy to lose. The preventive maintenance schedule is the right place for the cleaning and the inspection of the tip faces.
Measuring Force and Verifying Setup
Force is measured with a load cell placed on the machine table at the placement position, with the nozzle driven down onto it. The measurement should be taken on the machine that will run the product, because the value depends on the machine’s own settings and on the nozzle fitted. A value that is significantly different from the expected one indicates a worn tip, a clogged vacuum path or a mis-set height.
Height is verified by placing a component onto a glass plate or a piece of tape and looking at the result from the side. A part that is placed too low leaves a visible mark or a flattened tip; one placed too high is not fully in contact with the paste. The check is quick and it should be part of the setup for a new product, along with the first article. Where the machine supports automatic measurement of the board surface, the measurement itself should be verified against a known thickness, because a drifting sensor produces a drifting placement height that no other check would catch.
Defects Attributable to the Nozzle
A part that is rotated on the board is usually a placement issue rather than a nozzle issue, unless the part rotated during the move because the vacuum was marginal. A part that is shifted in one direction, or that appears to have been dragged, points at the release timing or at the nozzle touching down on the part after placement. A part that is missing has either been dropped or never picked, and the two are distinguishable from the machine’s own trace.
A mark on the top of a component is a nozzle signature, and its position identifies which nozzle made it. Collecting those marks and mapping them to the nozzles in use is a direct way to find the one that needs cleaning or replacement. Where the marks appear on a single component type, the tip material or the force for that component should be reviewed rather than the whole machine. The yield record should separate nozzle-related defects from placement accuracy defects, because the two have different corrective actions.
FAQ
Does a softer nozzle tip always reduce component damage? It reduces the risk of marking and cracking, but a soft tip wears faster and conforms less as it ages, so the vacuum seal degrades. The right choice is the softest tip that holds the part reliably over its service interval.
Can placement height be set without measuring the board? It can, and many machines do it, but the placement then varies with the board thickness and the local warpage. Where the paste deposit is small, that variation is the difference between a good joint and a shifted part.
How is a clogged vacuum path found? By the machine’s vacuum sensor where one exists, and otherwise by a drop in the pick success rate or by parts that hang at an angle during the move. Cleaning the nozzle and the vacuum path on an interval prevents most of it.
