Component Placement Force And Z Axis Control

The placement machine presses each component onto its paste deposit with a controlled force, and that force is one of the least discussed and most influential settings in the assembly process. Too little force and the part sits on top of the deposit without contact; too much and the deposit is pushed aside, so the joint forms with less solder than the print delivered.

This article covers how the force is applied, how the z axis and the dwell are set, and how the setting interacts with self alignment.

How The Force Is Applied

A modern placement head moves the nozzle down until the component touches the board, and then continues to a defined position or applies a defined force. The two control modes are position and force, and they behave differently. In position mode the head travels to a programmed height, so the force depends on the stiffness of everything between the nozzle and the board. In force mode the head applies a set load and stops when that load is reached, which is more repeatable on a board of varying thickness.

Whichever mode is used, the force passes through the component body. A fragile part such as a small ceramic capacitor or a thin package can be cracked by a force that a larger part tolerates, so the setting is normally defined per component class rather than globally. The nozzle is the other part of the path: its material and its shape determine how the load is distributed, and a hard nozzle concentrates it at the centre of the body.

Z Axis And Dwell

The z axis defines how far the component is pressed into the deposit. The correct depth is enough that the terminations or balls make contact with paste across their whole area, and not so much that the paste is displaced beyond the pad. The height is measured from the board surface, which means that a board with a different thickness, a stiffener or a coating under the footprint changes the effective depth.

The dwell is the time between the component touching and the nozzle lifting. A short dwell leaves the paste no time to relax, which matters with a thixotropic paste, and a long dwell allows the deposit to spread under the part and the part to settle. For a fine pitch device the dwell also affects how much the paste is pushed up the side of a lead, which influences the fillet. The relation between the deposit and the pad is described under placement order and pad positioning.

Paste Displacement And Its Consequences

Paste displaced by the nozzle does not disappear. It moves outwards from under the component and lands on the mask beside the pad, where it reflows into separate balls rather than joining the joint. It also reduces the volume available to the joint, so the joint that forms is smaller than the print intended. Both effects are worse for a part that presses a large area, which is why a large package is more sensitive to the setting than a small one.

The displacement is visible if the board is inspected after placement and before reflow, and this is the check worth doing when a placement setting is being established. A deposit that has spread visibly beyond the pad boundary is being pressed too hard, and the answer is a higher z stop rather than a change to the paste. The pad and mask geometry that determines how much room the paste has to spread is treated under PCB pad design standards.

Self Alignment

During reflow the surface tension of the molten solder pulls a component towards the centre of its pads, which corrects a small placement error. That mechanism requires the joint to be free to move, and a component that is pressed hard into the paste has more solder under it and is more constrained at the moment the alloy melts. The alignment also depends on both ends of the component melting at the same time, which is a thermal question rather than a mechanical one.

Where self alignment is relied upon, the placement force should be set for contact rather than for maximum contact, and the deposit should be centred on the pad. A part placed with an offset that exceeds what the surface tension can correct stays offset, and the limit of the correction is roughly a fraction of the pad width rather than the full pad width. The defect that results when the alignment fails is described in the material on SMT component shift causes.

Setting And Verifying The Parameters

The parameters are set per component class in the placement programme: the force or the position, the dwell, and the nozzle type. They are verified the same way as a print parameter, by inspecting the deposit after placement on a sample board and by checking the joints after reflow. A useful check is to place a board without reflowing it and inspect the deposits under magnification, which shows the displacement directly and distinguishes it from a reflow defect.

The verification should be repeated whenever the board thickness, the stiffener or the support arrangement changes, because each of those changes the effective depth of the press. It should also be repeated after a machine service, since the z axis calibration can shift. Treating the setting as a fixed number that never needs checking is the mistake that makes the defect appear as an intermittent one. The wider design measures that make the process tolerant are described under design guidelines for manufacturability.

Choosing The Nozzle

The nozzle determines how the force is distributed and how the component is held during transport. A rubber tip conforms to the top of the part and distributes the load, which is the safer choice for a ceramic body. A metal tip gives better placement accuracy and a more defined contact, and it concentrates the load, so it is used where the part can take it. A nozzle that is too small for the part concentrates the load further and can mark or crack the body; one that is too large can touch a neighbouring component.

The nozzle also has to release the part cleanly. A tip that sticks, because of contamination or vacuum leakage, lifts the component slightly on release, which shows as a tilted or a lifted part after reflow. Cleaning the nozzles on a schedule and checking the vacuum level is a small maintenance task that removes a whole class of placement defects.

FAQ

Is position or force control better? Force control is more repeatable where the board thickness varies, because it does not depend on the distance to the board surface. Position control is adequate where the board and the support are consistent.

How can paste displacement be detected before reflow? By inspecting a placed board under magnification before it enters the oven. Paste that has spread beyond the pad boundary or onto the mask is a direct indication that the setting is too aggressive.

Does a higher force improve the joint? It does not. Beyond the point where the component and the paste are fully in contact, additional force only displaces paste and reduces the volume available to the joint.