Selective Soldering Nozzle Choice: 6 Rules for Clean Joints

Selective soldering uses a small pumped fountain of molten solder to join individual through-hole pins, one joint or one group at a time. The nozzle is the part that shapes the process: its diameter, its inner geometry and its height above the board set how much solder the joint sees and how long the joint stays in contact with it.

Where a wave machine treats the whole panel at once, selective soldering works locally and therefore depends on local decisions. That is its advantage on mixed-technology boards and its difficulty, because the process window is defined joint by joint rather than for the panel as a whole.

How Selective Soldering Works

A pump drives solder through a nozzle to form a stable, low fountain. The board is positioned so that the target pins pass through the fountain, the flux has already been applied and activated, and the joint heats by contact with the flowing alloy rather than by radiation from below.

The fountain must be stable, because an unsteady wave gives inconsistent contact and produces joints that vary across the same panel. Pump condition, nozzle cleanliness and the solder level in the pot all feed into that stability, and all three are maintenance items rather than settings.

Nozzle Diameter and the Solder Fountain

Nozzle diameter is chosen from the geometry of the joint, not from the pin alone. A nozzle that is too small cannot deliver enough heat and solder to fill the barrel, while one that is too large wets neighbouring pads and floods the area around the pin with excess alloy.

The working rule is to cover the pad and the barrel with a margin while staying clear of the nearest neighbour. On dense connectors that margin disappears, and the answer is usually a smaller nozzle with a longer dwell rather than a larger one with a shorter contact.

Wave Height and Contact Depth

The height of the fountain above the nozzle rim determines how deeply the board is immersed. Deeper contact transfers more heat but also pushes solder onto the mask and around the pad, while shallow contact risks a joint that never reaches the required temperature.

Contact depth should be expressed as a measured value rather than an operator judgement, and it should be rechecked after any nozzle change. Because the fountain height depends on pump speed and solder level, a pot that is partially empty will behave differently from a full one.

Flux Application and Activation

Flux is applied to the target area by spray or by drop, and it has to reach the barrel before the solder does. Too little flux leaves oxide on the pin and produces a joint that is slow to wet; too much leaves residue that is difficult to remove and can leave a sticky surface under a component.

Activation happens during preheat, so flux volume and preheat temperature are coupled. A drop that sits on a cold pad will not activate, while the same drop on an over-heated pad is already spent before the fountain arrives. The two settings belong in one recipe.

Preheat and Thermal Mass

Preheat brings the board and the pins close to soldering temperature so that the fountain only has to supply the final increment. It can come from below with infrared or convection, or from the top with hot gas, and the choice depends on the assembly and its component mix.

Thermal mass is what makes the recipe board-specific. A heavy ground plane connected to a pin draws heat away quickly, so the same nozzle and dwell that fills a signal pin will leave a ground pin cold. Where a board mixes both, the recipe has to be built around the hardest joint.

Dwell Time and the Fillet It Produces

Dwell time is how long the joint stays in contact with the fountain. Too short and the barrel does not fill, producing an incomplete or concave fillet; too long and the flux is consumed, the mask is stressed and intermetallic growth thickens at the joint interface.

The visible result is a good guide when it is read correctly. A full, slightly concave fillet with a visible lead outline usually means the dwell was adequate, while a flat top or a visible hole in the joint means it was not. Judging by time alone without looking at the joint wastes the information the joint provides.

Mini-Wave, Dip and Point Nozzles Compared

Mini-wave nozzles form a small standing wave and suit single pins and short rows. Dip nozzles cover several pins at once and shorten the cycle for connectors, at the cost of more heat into the assembly. Point nozzles with a very small aperture suit fine connectors but have the narrowest process window of the three.

Selection follows the board, which is why the through-hole opening window on the solder mask window and the pin pitch should be reviewed together. A nozzle that fits the pitch but not the opening will flood the mask and leave solder where it is not wanted.

Nozzle Maintenance and Wear

Nozzles erode. The alloy dissolves the nozzle material slowly, and the internal geometry changes long before the part fails visibly. A worn nozzle produces a wider, weaker fountain that no longer matches the recipe, and the joints change without any setting being altered.

Cleaning is the other half of maintenance. Oxide and dross build on the nozzle rim and disturb the flow, and a partially blocked nozzle can produce a fountain that looks correct but delivers less heat at one side. Nozzle life should be recorded in hours or cycles rather than assumed.

Joint Defects and Their Process Causes

Incomplete fill points at insufficient heat, which usually comes from the nozzle, the preheat or the thermal mass of the joint rather than from the alloy. Bridging between adjacent pins points at excessive contact depth or a nozzle that is too large for the pitch.

Solder balls and flags of residue come from flux that was applied outside the target area or from a fountain that touched the mask. Where these defects persist, the selective soldering recipe should be reviewed alongside the assembly drawing, since pad geometry and mask openings set limits the process cannot exceed.

Points to Confirm at First Article

Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

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

FAQ

How is the right nozzle diameter chosen? Start from the pad and barrel diameter, add a margin that keeps the flowing alloy off neighbouring features, and confirm the choice with a first article. A dense connector often needs a smaller nozzle and a longer dwell rather than a wider one.

Can selective soldering replace wave soldering entirely? For boards with a small number of through-hole parts it usually can, and it avoids heating the whole assembly. Where a panel carries many through-hole components, the cycle time per joint makes a wave process more economical.

Why do joints vary across the same panel? Local thermal mass is the usual reason. Pins connected to planes draw heat away faster than isolated pins, so one recipe produces different dwell outcomes across the board. Grouping similar joints and using separate passes is the standard fix.