Selective Soldering Process Window and Nozzles
Selective soldering places molten alloy on individual joints rather than passing the whole board through a wave. It exists because some assemblies cannot take a wave, and its process window is narrower than the wave it replaces, which is why it is usually the fussiest step on the line.
Where Selective Soldering Fits
An assembly with through hole parts on both sides, with tall components that a wave cannot pass, or with a dense area that must not be wetted is a candidate. The alternative is hand soldering, and selective soldering is chosen where the consistency of a machine is worth the tooling.
It is not a general replacement for a wave. The cycle time is longer because the nozzle has to visit each joint, so the process is used where the board cannot be waved rather than where the board could be waved faster. Our joint criteria notes define what the result has to look like.
Flux Deposition and Its Control
Flux is applied to the site rather than to the whole board, which is the main reason a selective process exists. The quantity has to be enough to clean the joint and small enough to leave a residue the assembly can tolerate.
Spray fluxing is the common method, and the deposit depends on the nozzle, the pressure and the distance. A change in any of the three moves the residue and the soldering result together, which is why the settings are recorded rather than adjusted at the machine. Our cleanliness notes describe the residue side of that.
Preheat and Thermal Mass
Preheat brings the site to a temperature where the nozzle can finish the joint without dwelling long enough to damage the board. On a mixed assembly the thermal mass varies from site to site, so the preheat is set for the heaviest site and the light ones run hotter than they need.
Top side preheat is the usual choice, with bottom side preheat added for a heavy ground plane. The measurement is taken on the top side at the site rather than at the board edge, because the difference between the two is enough to change the result. Our profile notes describe how that measurement is arranged.
Nozzle Selection and Dwell
The nozzle is a small fountain of alloy, and its size is chosen to match the joint rather than the pitch alone. Too small and the joint takes too long to fill; too large and the nozzle wets two pads at once, producing a bridge that no profile can remove.
Dwell time is the second half of the pair. A short dwell leaves an incomplete fillet and a long dwell heats the laminate and the nearby components. The window between them is set by experiment and then held by the machine, not by the operator.
Fixturing and Board Support
The board is held flat and still, because the nozzle stands within a fraction of a millimetre of the surface and a board that moves changes the gap. Support is normally provided under the soldering area, and the fixture has to allow the nozzle to reach the joint from below.
A fixture that holds the board by the edge only allows the middle to sag, and the sag changes the gap at the sites in the middle. That is why the fixture design is part of the process development rather than a fixture bought afterwards.
Defects Specific to Selective Soldering
The characteristic defects are a cold joint from insufficient dwell, a bridge from an oversized nozzle, a skip from a nozzle that has not been cleared, and flux residue that has been baked on by a preheat that was too hot for the deposit.
Each has a different correction, so the investigation should start with the site rather than with the profile. A single bad joint on a board is usually a nozzle or a clearance problem, and a whole area that is bad is usually a preheat problem.
Verification and Records
Verification is by visual inspection of the fillet, by X-ray where the joint is hidden, and by a coupon that carries the same thermal mass. The coupon is worth having because it can be sectioned without destroying a product board.
The record that makes the process repeatable is the flux setting, the top side preheat at the site, the nozzle identification, the dwell and the travel. Our paste deposition notes describe the same discipline on the printing side of the line.
Process Control and Verification
On a design of this kind, dwell is the item that decides how the rest of the board is arranged. 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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.
The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.
Process Control and Verification
On a design of this kind, dwell is the item that decides how the rest of the board is arranged. 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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.
Process Control and Verification
On a design of this kind, dwell is the item that decides how the rest of the board is arranged. 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. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.
FAQ
Can selective soldering replace hand soldering entirely? On most assemblies it can, and the exceptions are joints the nozzle cannot reach without touching a neighbouring part.
Is the flux residue different from a wave process? The chemistry is similar and the quantity is smaller, because the flux is applied to the site rather than to the whole board.
What does gopcb provide for selective soldering? We provide site-by-site program development, flux deposit and preheat measurement at the joint, nozzle selection against the pad geometry, fixtures that support the soldering area, coupon sections for verification, and records that tie the settings to the assembly.
