Wave Solder Contact Length and Dwell Control
Contact length is the distance a board travels through the solder wave, and it decides how long every joint stays in contact with molten solder. It is set by the conveyor speed, the width of the wave at the board surface and the depth of immersion, so it is a machine result rather than a single control knob, and it has to be measured rather than assumed from a recipe.
What Contact Length Means
The board enters the wave on an inclined conveyor, so the contact patch is the width of the wave at the board surface rather than the width of the nozzle. Contact length is that width, normally expressed in millimetres, and the dwell time is the contact length divided by the conveyor speed.
<img src="https://www.gopcba.com/wp-content/uploads/2026/05/GPS-to-CAN-module-PCBA.png" alt="Board entering the solder wave on a wave soldering machine” />
A patch of 25 to 40 mm with a conveyor running at 1.2 metres per minute produces a dwell of about 1.5 to 2 seconds, which is the range in which most through hole joints fill completely. The two figures move together, so doubling the conveyor speed halves the dwell unless the contact length is increased to compensate.
How Contact Length Sets Hole Fill
Filling a plated through hole is a thermal race. The solder has to heat the barrel, the pad and the lead above the melting point of the alloy and then wet the copper before the board leaves the wave, and the time available for that sequence is the dwell.
Where the dwell is short, the fill stops part way up the barrel and leaves a joint that passes a visual check and fails a section. Fill percentage, rather than the appearance of the fillet, is therefore the figure against which a contact length window is set.
Machine Settings That Change It
Three settings move the contact length: the conveyor speed, the pump speed that sets the wave height, and the height of the board above the wave crest. Raising the wave increases the contact patch, while lowering the board into the crest has a similar effect and also raises the risk of solder reaching masked areas.
The width of the wave changes with the nozzle and with the baffles behind it, so a machine that has been rebuilt or re-nozzled has to have its contact length re-established. The measurement should be repeated after any maintenance that touches the pump, the nozzle or the conveyor.
Contact Length and Flux
Flux sets the surface condition that the solder meets, so the dwell cannot be optimised without knowing how the flux behaves. A formulation with a short activation window, which is common among low solids no-clean materials, can be spent before a long contact is finished.
A long contact also drives more flux off the board and produces more fume and condensate inside the machine. Where contact length is increased to improve fill, the flux volume should be reviewed at the same time instead of being assumed to stay correct. The relationship between the two is set out in the guidance on flux activation with preheat. A longer contact also loads the machine with fume and condensate that the cleaning routine has to remove.
Contact Length and Pot Temperature
Pot temperature and contact length attack the same problem from different directions. A higher alloy temperature reduces the time needed to heat the joint but increases oxidation and dross, while a longer contact keeps the temperature unchanged and gives the joint more time to fill.
The two are usually adjusted together, which is exactly where a process drifts. Setting the pot temperature first, verifying it with a profile, and then tuning contact length keeps the number of variables at one and makes the result reproducible on the next shift.
Short Contact and Its Defects
A contact that is too short produces incomplete fill in plated holes, especially on thick boards and on joints connected to heavy copper planes. The visible fillet can still look acceptable because the top of the hole wets early while the barrel below it stays cold and unfilled.
The same shortage produces poor wetting on joints of high thermal mass, a dull surface where the alloy solidified before it could flow, and a fill result that varies across the panel with the copper distribution. That positional pattern is the signature of a dwell problem rather than a flux problem. A joint that is open at the top of the barrel but filled at the bottom is characteristic of a board that entered the wave too hot, which is a preheat setting rather than a contact length one.
Long Contact and Its Defects
Increasing contact length improves hole fill until the board spends so long in the wave that other defects appear. Excess solder builds on the pads, bridges form between closely spaced pins, and the board leaves the wave at a temperature that keeps the alloy fluid long enough to sag.
Long contacts also lift more flux from the surface before the wave arrives and load the machine with fume, which condenses on the preheaters and on the cooler surfaces downstream. At the extreme, the board itself is heated beyond its design temperature, which is a laminate risk rather than a soldering one. The safe direction of adjustment is therefore to lengthen the contact until the fill result stops improving, then to shorten it slightly so that margin remains for the next machine rebuild.
Interaction with Conveyor Speed and Angle
Conveyor speed changes the dwell and also the heating rate in the preheat zones, so a speed change cannot be treated as a single parameter. A slower conveyor gives more preheat time and more contact time at once, which is why fill often improves disproportionately when the speed is reduced.
The conveyor angle sets how the board enters and leaves the wave. A steeper entry produces a narrower effective contact patch and a sharper exit, and a shallower one widens the patch. The angle is a machine constant, but it has to be recorded, because a repositioned conveyor changes the dwell at the same speed.
Setting and Verifying the Window
The window is described by contact length, conveyor speed, wave height, board height above the crest, pot temperature and flux volume. It is established for each assembly and confirmed by sectioning a sample of holes rather than by inspecting fillets.
In production the verification is a profile run with a thermocouple on a heavy joint and a section of a filled hole at the start of the build. A vertical fill result of at least 75 percent of the board thickness is the usual acceptance, and the machine settings that produced it belong in the record beside it.
FAQ
What is a normal contact length in wave soldering? Most lines run a contact patch of about 25 to 40 mm, which with a conveyor at 1.2 metres per minute gives a dwell of roughly 1.5 to 2 seconds.
Does a longer contact always improve fill? It improves hole fill up to the point where excess solder, bridging and laminate heating appear, so the window is bounded on both sides and is confirmed by sectioning.
How is contact length measured? By measuring the width of the wave at the board surface, or by timing the contact with a test board and multiplying the dwell by the conveyor speed.
