Selective Soldering Nozzle and Process Control

Selective soldering exists because a board can be too complex for a wave and too populated for a hand iron. A nozzle delivers a small wave to one joint at a time, which gives the process the control of hand soldering and the repeatability of a machine.

The nozzle is also the source of most of the process variation. Its diameter, its height above the board and its condition decide how much heat reaches the joint, and those three variables change with every product that is loaded.

How the Nozzle Delivers Solder

Alloy is pumped through a nozzle that is positioned under the joint, and the solder forms a small fountain that contacts the pad and the lead. The contact is brief, so the heat has to be delivered quickly, which is why preheating from below is part of the process rather than an optional extra.

The nozzle tip geometry determines the flow pattern. A wide flat tip is used for a connector row where several joints are soldered at once, while a narrow tip is used for a single pin, and the wrong choice produces either a cold joint or a flooded one.

Nozzle Diameter and Joint Size

The nozzle should be large enough to cover the joint and the surrounding pad, and small enough that it does not touch a neighbouring joint. Where the pitch is tight, the nozzle is chosen for the individual joint and the neighbouring pins are protected by a mask or by the geometry of the fountain.

The relationship is easiest to manage when the footprint has been designed with selective soldering in mind, since a pad with a generous annular ring tolerates a nozzle that is slightly oversized. This is one of the points where an assembly process requirement feeds back into the design, as described for pad design standards.

Height, Contact and Dwell

The nozzle height sets how much of the fountain touches the board, and it is measured from the tip to the underside of the laminate. Too high and the solder does not reach the joint, too low and the fountain floods the area around it.

Dwell time then determines how much heat the joint receives. A short dwell leaves a cold joint on a heavy copper plane, while a long one overheats the laminate and can damage a nearby component. The two parameters are set together for each product and recorded with the programme.

Preheat and Thermal Balance

Preheating from below raises the board close to the soldering temperature, so that the nozzle only has to supply the final increment. Without it, the nozzle dwell becomes long enough to damage the board, and the joints at the end of a connector row cool before the fountain reaches them.

The preheat profile should follow the mass of the assembly rather than a fixed setting. A board with a heavy ground plane needs more preheat than a signal board, and the difference is visible in the fillet shape before it is visible in a temperature reading.

Flux Application

Flux is applied to the joint before soldering, either by spray, by a jetting head or by a pin transfer. The volume has to be enough to clean the surface and little enough that the residue does not have to be cleaned afterwards, since selective soldering is often chosen to avoid a cleaning step.

Where a no clean process is required, the flux chemistry has to be qualified for the residue it leaves and for the electrical environment the board will see. Choosing the flux for the solderability alone is a common mistake.

Nozzle Maintenance and Condition

The nozzle erodes and collects dross like any other solder contact, and a worn nozzle produces a fountain with a different shape from the one the programme was written for. Replacement on a schedule rather than on failure keeps the process repeatable.

Cleaning between cycles, checking the tip for build up and verifying the flow rate are quick routines that prevent the majority of nozzle related defects. A nozzle with a partial blockage behaves like a smaller nozzle, which produces joints that are sometimes good and sometimes not.

Programming the Path

A selective soldering programme is a sequence of positions, heights and dwell times, and the sequence has to respect the thermal history of the board. Soldering the heaviest joints first gives the board time to reach temperature, while soldering them last risks a lack of heat after the nozzle has been idle.

The programme should also account for the order of the neighbouring joints, since soldering one joint can disturb a joint that has just solidified if the nozzle passes too close. Spacing the sequence is part of the programming skill rather than a machine setting.

Common Defects and Their Causes

A cold joint usually points to insufficient preheat or a dwell that is too short, while a bridging defect points to a nozzle that is too wide or a height that is too low. Solder balls around the joint indicate that the flux was applied too generously or that the nozzle was withdrawn too quickly.

Each of those symptoms is diagnosable from the machine record, provided the record includes the height, the dwell and the flux volume for the joints in question. Without it, the investigation starts from scratch every time.

Where Selective Soldering Fits

It suits boards with a small number of through hole parts, with connectors that cannot be reflowed and with mixed technology where a wave pallet would be complex. It is slower than a wave per joint, so it is chosen for the joint count rather than for the volume.

The economics change when the alternative is hand soldering, because the machine removes the variation that comes from an operator working on a dense board. Consistency is the benefit that is bought, and it is usually worth more than the cycle time.

Process Control and Verification

On a design of this kind, nozzle height is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

A first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can selective soldering replace wave soldering? For a board with a few through hole joints it can, while a board with many holes remains faster on a wave with a pallet.

How often should a nozzle be replaced? On a schedule based on the hours it has run and on the number of cycles, with a visual check for erosion at each shift.

Is cleaning required after selective soldering? Only if the flux residue is not acceptable for the application, which is why the flux is usually chosen for a no clean classification.

Does the process suit lead free alloy? Yes, but the higher melting point requires more preheat and a shorter dwell, so the programme has to be developed for the alloy rather than converted from a tin lead version.