Machine and product aspects must fit to ensure the best joints.

Questions often arise as to recommended wave soldering settings, or if general setting data can be provided. An important aspect in wave soldering is that all joints, in principle, will get the same dwell time. This means joint designs must be such that they all can provide a sound joint; the surface and thermal solderability, and the layout of all joints to be soldered, must be in accordance with the process used. Only when these basic requirements are met can one run a sound process.

Conveyor speed, conveyor angle, contact length, fluxing, preheating, solder temperature, solder alloy, exhaust and nitrogen use are machine-related aspects. Board type, thermal aspects, solder resist, SMDs, board layout, bending of the board, and pallet use are product-related aspects. All aspects together must make the right fit, especially since wave-soldering machines use different settings and recipes.

Process boundaries. A board must be touched by the solder wave for a sufficient time to make a good joint. The board may not enter the wave at such a depth that the solder will flow over the top side of the board. To fulfill these requirements, the solder wave setting must be such that the top of the solder wave crest is half the board thickness, or about 1 mm lower than the top side of the board in the conveyor.

Contact length. With a conveyor angle of 7°, the contact length of the main wave on a glass plate at the same level as the board will be about 25 mm. The real contact of a joint in the solder wave depends also on the protruding length of the leads at the solder side, and on the board layout. This real contact length can therefore be twice as much. This is important to know if one checks the actual dwell time. Some board bending may be permitted, but be aware that wave height does not permit much bending. Boards can often be kept sufficiently flat with a board support (wire or skate) or with a pallet.

Wave height. The wave height should be kept low to avoid too much dross formation. In general, a wave height of 6-8 mm is the best setting. Lower settings may permit component movement during soldering, as the leads may touch the nozzle rim. A wave setting should be constant within a few tenths of a millimeter. For this reason also, the pot’s solder level should be monitored and corrected automatically.

The correct setting of a chip wave or smart wave depends on the SMD layout and if the board has slots or large apertures through which this wave may penetrate to the top side of the board. If only low wave settings can be used, it might be necessary to reduce the conveyor speed to avoid skipped joints.

Conveyor speed. A general conveyor speed setting is in the range of 1-1.5 m/min. (17-25 mm/s). The speed setting depends on the board type. Single-sided boards can often be soldered at high speed, since they often have a low thermal demand for the joint formation. Conversely, a multilayer PCB may have such a high thermal demand for the joint formation that 1 m/min. is even too fast. The layout of the joints on the solder side can be a decisive factor in the speed setting, in order to promote optimal separation conditions to prevent solder bridging.

Fluxer. The spray fluxer setting is related to the conveyor speed. Important aspects include application of the correct amount of flux. How much flux should be applied can be found in the flux supplier’s datasheets. Next, one must measure the flux flow with different settings, because the general data in the machine manual are just general. It is important to control the flux volume; too much may interfere with the soldered product’s electrical reliability, depending on the board application requirements.

Functionally, flux removes oxides from the joints to be soldered and from the solder wave in contact with the board. The flux must also provide a sufficient “tail” activity when the board leaves the wave to promote bridge-free soldering. The main issue here is that a correct fluxer setting is of primary importance, not only for the soldering process, but also for the reliable application of the circuit. For this reason, no real general recommendation can be given.

There are so many different fluxes with different activators, rosin or resin-based, alcohol-based solvents or VOC-free, that one should contact the flux supplier and discuss the demands for the use of the board (circuit/equipment). But also the type of solderable finish and solder resist used are parameters to consider.  These facts need to be known to receive good advice about the best flux and process settings. For consumer electronics, these demands may be quite different than those for medical equipment, for example.

Preheating. Preheating the board is necessary to evaporate solvent from the flux and to prepare the board and flux for soldering. Different systems can be used for preheating. As long as the flux layer is wet, a medium wave quartz rod IR preheating is recommended. If the flux becomes sticky, forced air cooling can be used. A quartz heater can be used to boost the preheat temperature, in case different recipes need different preheater settings. The preheater setting must be optimized with relation to conveyor speed.

Soldering temperature. The solder pot temperature setting depends on the solder type used, but may also be related to the product to be soldered. In general, low temperature settings are recommended. Low temperatures will create less dross, but will in most cases also extend the lifetime of the flux, so that it has a better tail activity. For SnPb solders, 245-250°C is a common setting; for SAC-alloys, 260-265°C is the recommended setting.

Nitrogen application. Nitrogen may be helpful to support flux activity during the separation of the board from the wave. During this separation process, solder should stay on, but not in between, the joints. Solder oxide formation at this stage increases the risk of solder bridging considerably (Figure 1); this is why the flux must have sufficient activity left to reduce these solder oxides. Nitrogen used in that area can displace the air (oxygen) at that region and thus assist in better drainage conditions.


Exhaust. Machine exhaust affects most other process settings as well, which is why the exhaust conditions, once set at the demanded values, should be kept constant.

Ursula Marquez de Tino is a process and research engineer for Vitronics Soltec (vitronics.com); umarquez@vitronics-soltec.com.

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