Selective Soldering: Practical Checks for Production

Selective soldering makes a small fountain of molten solder and brings it into contact with one joint at a time. The nozzle that forms the fountain is the part that decides whether the process works, because it sets the shape of the flow, the area that is heated and the amount of solder that reaches the joint.

This article covers what the nozzle does, how its size is matched to the joint, how flow and dwell are set, and how the thermal profile of a local process is established.

What The Nozzle Does

A pump inside the pot pushes solder up through the nozzle, and the nozzle shapes the rising flow into a small stable wave with a defined top surface. The assembly is moved so that the joint of interest passes through that wave, or the nozzle is moved to the joint. The joint is heated by the solder itself, so the thermal profile is produced by the contact time and by the temperature of the pot rather than by an oven.

The nozzle also carries the flux, in the sense that the flux applied before the process has to survive until the joint is hot. The window between applying the flux and making the joint is short in a well designed machine, and the nozzle position and the flux applicator are programmed together so that the flux has not dried before the solder arrives. Where the joint is soldered after a reflow step, the fluxes are the same family in most cases, and the alloy decision made for the rest of the assembly applies here too, as discussed under lead free versus leaded solder.

Nozzle Size And Joint Geometry

The nozzle opening is sized to the joint and to the surrounding clearance. It has to be large enough that the wave covers the joint and both the pad and the lead are wetted in a single contact, and small enough that it does not touch a neighbouring component or a connector housing. A nozzle that is too small produces a joint that is wetted on one side only and needs a second pass, which overheats both the part and the board.

The clearance around the joint usually decides the nozzle diameter rather than the joint size itself. A connector with a housing a few millimetres away leaves less room than the joint needs, and the process then uses a narrow nozzle with a longer dwell, or an angled nozzle that reaches under the housing. Where several joints are on the same pitch, a multi-hole nozzle may be used so that a row is soldered in one contact; this shortens the cycle and evens out the thermal load, but it demands that the joints be exactly on the nozzle pitch. The layout, and how it constrains the process, is treated under how layout decisions affect production.

<img src="https://www.gopcba.com/wp-content/uploads/2026/06/SMT车间一角.jpg" alt="Nozzle forming a small solder fountain under a connector” />

Flow, Wave Height And Dwell

The pump speed sets the height of the wave and the velocity of the solder at the top. A higher flow gives a more vigorous contact, which wets faster and helps to displace flux and gas, and a lower flow gives a gentler contact that transfers less heat per unit time. The wave height must be set so that the joint is contacted without the solder overflowing onto the mask around it, and so that the level is stable over the working day as the pot level falls and the alloy is replenished.

Dwell time is the other half of the pair. A longer dwell produces a hotter joint, more intermetallic growth and a thicker fillet, and it also allows more time for the flux to act. A shorter dwell risks a cold joint with poor wetting, particularly on a pad connected to a heavy copper area that drains heat away. The correct combination is found on the actual assembly with a thermocouple attached to the pad and to the lead, not from a table of standard values, because the heat drawn away depends entirely on the copper connected to the joint.

Thermal Profile In A Local Process

A selective process has no oven, so the profile is defined by the preheat stage, by the solder temperature and by the contact. The preheat is usually applied by convection or by an infrared lamp over the whole assembly, and it serves to remove moisture, to bring the flux up to its activation range and to reduce the thermal shock of the solder contact. Without a preheat the joint sees a very steep gradient, which stresses both the component and the laminate.

The profile measurement is taken at the joint, and it typically shows a rapid rise on contact, a plateau while the joint is in the wave, and a decay after the nozzle moves away. The plateau is the part that matters for the joint, and its temperature and duration are the two numbers that are recorded. A profile that varies between the joints on one assembly indicates that the preheat is uneven rather than that the nozzle is wrong, and the fix is usually in the preheat arrangement.

Flux Application And Residue

The flux is applied as a spray, a drop or a brush pass, and each method puts a different amount on the board. A spray covers a wide area and dilutes the flux where it lands; a drop places a controlled volume on the joint; a brush pass wets the whole row. The choice depends on the geometry: a recessed joint needs the flux delivered to it, while a row of pins can be brushed in one movement.

Because the process is local, the residue is local as well, and a selective operation on an otherwise no clean assembly usually specifies a no clean flux so that no further cleaning is needed. Where the residue has to be removed, the cleaning step has to reach the joint, which may be under a component body or beside a housing. The compatibility of the flux with the rest of the assembly, and the question of whether the joint will be coated afterwards, are the same issues that apply to the wider assembly process. The general rules that keep such a process manufacturable are collected under design guidelines for manufacturability.

Programming And Verification

The programme controls the position of the assembly relative to the nozzle, the approach and departure angles, the dwell time and the flow. The approach angle is chosen so that the joint enters the wave from the least obstructed direction, and the departure angle so that the solder drains away from the joint rather than towards a neighbouring part. Both angles should be recorded with the programme, because a nozzle that is changed without changing the approach can produce a bridged joint.

Verification combines a visual check of the fillet, a cross section of a sample joint per batch to confirm the penetration into the hole, and a temperature profile taken after any change to the programme or the preheat. A joint that looks correct but has not filled the hole is the characteristic defect of this process, and it appears when the dwell is too short or the heat is drawn away faster than the nozzle can supply it. The measurement that catches it is a section, not an inspection of the surface.

FAQ

Can selective soldering replace wave soldering entirely? It can where the number of through hole joints is small and the parts are heat sensitive. Where a board carries many through hole parts, a wave process with a pallet is faster, and the two are often used together on the same assembly.

Why does the same programme produce different joints on different boards? The difference is normally in the copper connected to the joint or in the preheat, not in the nozzle. A joint tied to a ground plane needs more contact time than its neighbour on the same assembly.

Is the solder in the pot the same as the solder in the paste? It should be the same alloy so that the joints do not form a mixed structure, and it should be replenished and analysed on a schedule, because the pot is a small bath whose composition drifts as it is used.