Selective Solder Flux Spray Control: Deposit, Coverage and Dwell
In selective soldering the flux spray is applied to a moving target: the head tracks a path across the board, and the deposit depends on the path speed, the spray pattern and the distance to the surface. Controlling the flux deposit means controlling those three together.
What the Flux Has to Do in a Selective Soldering Process
The flux has to remove the oxide from the pad and the lead, protect the surface until the solder arrives, and then leave a residue that does not have to be removed on a no clean process. In selective soldering the interval between fluxing and soldering is short for the first joint and long for the last, so the flux must remain active for the whole duration of the program rather than only at the start.
That time span is the defining constraint. A program that visits twenty positions over two minutes leaves the first joint fluxed for two minutes before the solder arrives, and the flux has to survive that dwell without losing its activity. Choosing a flux with a long activation window and verifying the deposit are therefore two halves of the same control.
Spray Parameters: Volume, Atomisation and Pattern
The spray is produced by a nozzle fed with flux and atomising air, and the deposit is set by four parameters: the flux flow rate, the atomising air pressure, the distance from the nozzle to the board, and the speed at which the head travels. Increasing the flow rate deposits more flux, but it also produces larger droplets that do not spread evenly and can leave a wet film that flows into the wrong places.
Atomising pressure controls the droplet size. A high pressure produces a fine mist that deposits evenly and dries quickly, but it also throws flux beyond the target area and increases the loss to the exhaust. A low pressure produces a coarse spray with a heavy deposit in a small area and a sharp edge at the pattern boundary. The pattern itself is a function of the nozzle design and the distance. A round pattern covers a circular area and needs overlapping passes for full coverage; a fan pattern covers a strip that matches the width of the joint area. Match the pattern to the geometry of the product rather than to the nozzle that happens to be fitted.
Flux Deposit Measurement and Its Limits
The deposit is measured as a mass per unit area, typically in milligrams per square centimetre, or as a wet film thickness. Both are destructive or indirect on a production board, so the practical approach is to measure on a test coupon that travels through the same program, weighed before and after fluxing, and to use that figure as the process control value.
Weighing is the simplest method and it is accurate if the balance resolves a milligram and the coupon is handled consistently. The coupon has to be the same material and the same size as the production board section, because the deposit depends on the surface energy and the thermal mass. A coupon that is a different alloy or that has been handled with bare fingers will not give the same result. Where a wet film measurement is used, remember that the wet thickness converts to a solids deposit only through the solids content of the flux. The number that the soldering process responds to is the solids deposit, not the wet film, and a flux change alters the conversion.
Coverage: Where the Spray Misses
Coverage is the more important variable and the harder one to measure. The spray reaches the joints that face it and does not reach the ones in the shadow of a connector body, a tall component or the board itself when the spray is applied at an angle. The uncovered joints are the ones that produce incomplete fill and excess dross, and they are invisible unless the coverage is checked deliberately.
Check coverage by spraying a test board with a dye or a UV tracer under the same parameters and inspecting the board under ultraviolet light. The pattern shows exactly which areas received flux and which did not, and it takes only a few minutes. Repeat the check after any change of program, nozzle, board or fixture, because all of them move the spray relative to the joints. Where a joint cannot be reached, the answer is a change of spray angle, a second pass from the other side, or a nozzle that produces a narrower pattern aimed at the gap. Adding flux volume to compensate for a coverage gap simply floods the reachable joints and leaves the shadowed ones dry.
The Activation Window and the Preheat Interaction
The flux has to be at the right temperature when the solder arrives. Too cold and the activator has not yet reacted; too hot and it has already been consumed. The preheat therefore sets the activation window, and the flux deposit sets how much activity is available. A heavy deposit on a hot board is wasted flux, and a light deposit on a cold board does not activate.
Measure the board temperature at the point of flux application and again at the point of soldering, and compare the difference against the flux supplier’s window. In a selective process the two are close together for the first joint and far apart for the last, so the window has to accommodate both ends of the program. Where it cannot, the program order or the head speed has to change. Following the same preheat and flux balance used for spray fluxing density control, record the measured board temperatures with the deposit so that a change in either can be identified separately.
Flux Solids and Residue After Soldering
The solids content of the flux determines how much residue is left after soldering. A high solids flux leaves a visible, sometimes tacky residue that must be removed or that will be coated over; a low solids no clean flux leaves a thin, dry film. Where the process is no clean, the solids content should be low enough that the residue passes the ionic cleanliness requirement without washing.
The residue is also where the flux chemistry shows up as a defect. An under activated flux leaves sticky residue that collects dust and creates leakage paths; an over activated flux leaves a dark, carbonised deposit that is difficult to remove. Both are corrected by adjusting the deposit and the preheat rather than by changing the flux. Where the product will be coated, verify that the residue is compatible with the coating. A coating applied over an incompatible flux residue will de laminate, and the failure appears weeks after assembly. Test the combination on a coupon before the process is released, using the same ionic testing described in our ionic contamination method.
Nozzle Condition, Clogging and Maintenance
The flux nozzle clogs with dried flux, and a partly blocked nozzle produces an asymmetric pattern with a heavy deposit on one side and none on the other. Because the pattern is judged visually as a mist, a partial blockage is easy to miss. Purge the nozzle at the start of every shift and inspect the pattern on a card before production begins.
Cleaning should use the flux manufacturer’s recommended solvent and should include the air cap and the needle seat, not only the tip. A nozzle that is cleaned externally but left with hardened flux inside the air passages will produce a coarse, spitting spray. Record the cleaning with the shift and check the pattern afterwards. Replace nozzles on a schedule rather than on failure, and keep the identification with the program. A nozzle that has been changed for a different pattern will require the program speed and distance to be re verified, which is the same discipline described in our nozzle maintenance routine for the solder nozzle itself.
Measuring and Recording the Deposit
Record the coupon mass before and after fluxing, the calculated deposit, the measured board temperature at flux application and at soldering, the parameters in use and the nozzle identification with each production run. A deposit that falls outside its window is a process change, whether or not the joints look acceptable.
Trend the deposit against the run count. A gradual fall indicates a partially blocked nozzle or a depletion of the flux supply; a sudden change indicates an adjustment, a nozzle change or a change of flux lot. Recording the lot number with the deposit makes the last of those distinguishable from a genuine process drift.
Records, Drift and Change Control
Treat a change of flux type, flux lot, nozzle, fixture or program as requiring a fresh coverage check and a deposit measurement. Each of those moves the spray relative to the joints, and the effect on the shadowed joints is larger than the effect on the exposed ones, so a check that only looks at the easy joints will pass an unacceptable process.
Review the deposit and the temperature records together at a defined interval and look for the pair moving in opposite directions. A deposit that is rising while the activation temperature stays the same is an over application problem; a deposit that is stable while the temperature falls is a preheat problem. Separating the two is what turns a recurring defect into a scheduled correction.
FAQ
How much flux should a selective soldering process apply? Measure it as a mass per unit area on a coupon that travels through the same program, and set the window from the flux supplier’s recommendation for the alloy and the dwell time. The figure that matters is the solids deposit, not the wet film thickness.
Why do some joints get no flux at all? Coverage, not volume, is usually the problem. The spray cannot reach joints in the shadow of a connector body or a tall component. Check coverage with a UV tracer on a test board and change the angle, add a second pass or fit a narrower nozzle rather than increasing the volume.
How long can flux sit on the board before soldering? Only as long as the activation window of the flux allows. In a selective program the first joint may wait two minutes while the last waits seconds, so measure the board temperature at flux application and at soldering and confirm the flux remains active across the whole program.
