Selective Soldering: Nozzle Wear, Flux Application and Pot Control
Selective soldering replaces the wave for boards whose through-hole joints sit among tall surface mount parts, and it does so with a nozzle the size of a pencil. The process is governed by three things that wear or drift: the nozzle, the flux deposition and the chemistry of the solder pot.
How Selective Soldering Differs From the Wave
A wave soldering machine floods a whole side of the board with alloy, while a selective machine moves a small nozzle to each joint in turn and deposits a defined amount of solder there. The programme is written per joint, so every parameter that the wave applies to the whole board becomes a per-joint decision.
That freedom is also the risk. A joint with a short contact time and a long thermal path will not fill, and the machine will complete the cycle without complaining, so the process is verified by looking at joints rather than by trusting the programme.
Nozzle Types and Wear Mechanisms
Electromagnetic pumps and mechanical pumps both deliver alloy through a nozzle whose bore is typically 3 mm to 8 mm, and the bore wears from the oxides and intermetallics that circulate with the alloy. Wear opens the bore unevenly, which changes the flow pattern rather than the flow rate, so the joint fills on one side and starves on the other.
Flow is checked at the start of each shift by holding the nozzle over a witness plate for a fixed time and measuring the height of the resulting wave. A drop of more than 10 percent against the reference, or a visible change in the shape of the wave, is the criterion for replacing the nozzle rather than waiting for a defect.
Flux Application and Dwell
Flux is applied by spray, by drop jet or by a pin transfer, and the two figures that matter are the volume deposited and the time between deposition and the solder contact. A typical spray setting deposits 0.5 mg to 1.5 mg of flux per square centimetre, and the flux has to be dry at the joint before the nozzle arrives.
Dwell time on the joint is usually 2 to 5 seconds, and it has to be long enough to heat the barrel and short enough not to cook the flux. The preheat that supports it is measured on the board rather than in the air, with a top side preheat of 90 to 110 degrees Celsius being a common working band.
Pot Chemistry and Alloy Control
The solder pot is a small volume of alloy that is continuously contaminated by the copper it dissolves from the barrels, and copper is the element that moves most quickly. A lead free alloy is normally maintained between 0.7 and 1.0 percent copper, with analysis every few thousand boards and a change of the pot when the figure leaves the band.
Other elements move more slowly but they accumulate in the same way, and iron above about 0.02 percent raises the liquidus enough to change the fill. The pot level also matters, because the nozzle flow depends on the head of alloy above the pump, so the level is topped up to a mark rather than by eye.
Thermal Profile for the Joint
The thermal profile of a selective joint is measured with a thermocouple attached to the barrel, not to the board surface, and a typical curve reaches 100 to 120 degrees Celsius at the joint before the nozzle contacts it. The contact then raises the joint to 250 to 260 degrees for a lead free alloy.
The profile is written per connector rather than per board, because a 2.5 mm pitch connector with a heavy body draws heat away from the pins. The same programme applied to a light connector overheats it, so the nozzle height and the contact time are stored with the joint list.
Fixtures and Board Support
A selective machine holds the board on a fixture, and the fixture has to support the board directly under each joint or the nozzle will push the board away as it rises. Support pins are placed under connectors and away from components, and the pin height is set so that the board is flat within the machine’s own tolerance.
Clearance for tall parts is the other constraint, because the nozzle has to reach a pin that may stand behind a transformer. Where the geometry does not allow a straight approach, the joint is either soldered by hand or the component is placed after the selective step.
Nitrogen and Dross Control
Nitrogen blanketing holds the oxygen around the nozzle below about 1000 parts per million, which reduces dross and changes the wetting behaviour of the alloy. Less dross means fewer nozzle blockages and a more consistent wave, and the flow rate itself changes when the blanket is lost.
A nitrogen flow that is too high cools the nozzle tip and disturbs the wave, so the setting is found by measuring the wave rather than by turning the regulator up. The consumption is recorded with the shift so that a leak is visible as a change in gas use The same sensitivity to the atmosphere appears in reflow, and our profile verification notes describe how the measurement is taken in both cases.
Verifying the Process
The first article is sectioned to confirm the fill, and after that the check is visual for the fillet and for bridging, with periodic X-ray or section on a sampling basis. Top side fillets form only where the alloy has pushed through the barrel, so their presence is a useful indicator of fill on a joint that cannot be X-rayed.
A short fill on a selective joint is usually traceable to one of three causes: nozzle wear, flux that was not dry, or a pot chemistry that has moved out of band. Our solder joint acceptance criteria describe the figures that separate the three.
Maintenance Schedule and Records
The maintenance list is short but it is the whole process: nozzle flow check per shift, flux nozzle and filter check per week, pot analysis per interval, and a full thermal profile for each connector type at first article and after any fixture change.
The records should name the nozzle in use, because two nozzles of the same nominal bore behave differently once one has worn. A change of nozzle is a process change, and our production process flow notes treat it as one that requires a new first article.
FAQ
How do I know when a nozzle is worn? By the flow check rather than by looking at it. A bore that has opened by a fraction of a millimetre still produces a wave, but the wave is wider and the contact area changes, and the first symptom is a joint that fills on one side only.
Can a selective machine solder a joint that the wave cannot reach? Often yes, because the nozzle can approach from an angle and because the heat is applied locally. The limit is mechanical access, and a pin surrounded by tall parts may still need hand soldering or a change in the placement order.
Why does pot chemistry matter so much on a small machine? Because a small pot has less alloy to dilute the copper it dissolves, so the copper figure rises faster than it would in a wave pot. Analysis frequency follows the board mix rather than the calendar for that reason.
