Introduction
Often the question rises what settings for the wave soldering process should be recommended, or if general setting data can be provided. Although these questions are very straight and clear, the answer is not as simple as one often would expect. This document will give some elucidation why some answers need more knowledge of the product that has to be soldered and on the aspects regarding the flux to be used.
General
Although basically wave soldering is a simple process, where a board is fluxed, preheated and dragged over a solderwave via a conveyor system, the ins and outs around this process must be known and understood to make this process a sound process. A sound process will deliver soldered joints on boards that will perform without failures during the expected lifetime of the equipment where that board is part of. An important aspect in wave soldering is that all joints in principle will get the same dwell time. This means that the design of the joints must be such that they all can give a sound joint. This means that the surface solderability and the thermal solderability and the layout of all joints to be soldered must be in accordance with the process that will be used. Only when these basic requirements are met one can run a sound process. Conveyor speed, conveyor angle, contact length, fluxing, preheating, solder temperature, solder alloy, exhaust, the use of nitrogen are machine related aspects. Board type, thermal aspects, solder resist, SMD's, board layout, bending of the board, use of pallets are product-related aspects. These are all aspects that together must make the right fit. That is also the reason that wave-soldering machines have the possibility to run with different settings and recipes. Next we will summarize the important aspects that should be considered regarding the different settings of the process.
Process boundaries
A board must be touched by the solderwave 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 topside of the board. To fulfill these requirements the solderwave setting must be such that the top of the solderwave crest is half the board thickness or about 1 mm lower than the topside 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 solderwave 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 allowed, but be aware that the wave height does not allow a too strong bending. Boards can often be kept sufficiently flat with a board support (wire or skate) or with a pallet.
Waveheight
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 give a movement of the components during soldering as the leads may touch the nozzle rim. A wave setting should be constant within a few tenths of a millimetre. For that reason also the solder level in the pot should be monitored and corrected automatically. The correct setting of a chipwave or smartwave depends on the SMD-layout and if the board has slots or large apertures through with this wave may penetrate to the topside of the board. If only low wave settings can be used it might be necessary to reduce the conveyor speed in order 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 is depending on the type of board. Single sided boards can often be soldered with a high speed since they often have a low thermal demand for the joint formation. On the other hand a multi-layer may have such a high thermal demand for the joint formation that 1 m/min is even a too high speed. Also the layout of the joints at the solder side can be a decisive factor for the speed setting, in order to prevent solder bridging.
Fluxer
The sprayfluxer setting is related with the conveyor speed. Important aspects in this setting are the demand that the correct amount of flux is applied on the board. How much flux should be applied can be found in the datasheets of the flux supplier. Next one must measure the flux flow with different settings with that flux, because the general data in the machine manual are just general. It is important that not too much flux will be applied, since that may interfere with the electrical reliability of the soldered product. This is of course also depending on the application requirements of the board. The flux has a function to remove the oxides from the joints to be soldered and from the solderwave that is in contact with the board. The flux must also provide a sufficient 'tail' activity when the board leaves the wave to improve bridge-free soldering. The main issue here is that one should be aware that a correct fluxer setting is of main importance. Not only for the soldering process, but also for the reliable application of the circuit. For that 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 that is used are parameters that need to be considered in this respect. These facts need to be known to get a good advice about the flux and process settings that should be used for this flux. E.g. for consumer electronics these demands may be quite different compared to those for medical equipment.
Preheating
Preheating of the board is necessary to evaporate the solvent from the flux and to prepare the board and the flux for soldering. In the data sheets of the flux the recommended settings are given. For preheating different systems can be used. A long as the flux layer is wet a medium wave calrod preheating is recommended. If the flux is becoming 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 in relation with the conveyor speed.
Soldering temperature
The solderpot temperature setting depends on the type of solder that is used, but may also be related to the product that has 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 tin-lead solders 245 - 250°C is a common setting. For SAC-alloys 260 - 265°C is the recommended setting.
Application of nitrogen
The use of nitrogen may be helpful to support the flux activity during the separation of the board from the solderwave. During this separation process the solder should stay on the joints but not in-between the joints. Solder oxide formation at this stage increases the risk of solder bridging considerably. That is why the flux must have sufficient activity left to reduce these solder oxides, so that solder bridging will be prevented. The application of nitrogen in that area can displace the air (oxygen) at that region and so assist in better drainage conditions.
Exhaust
The exhaust of the machine affects most other process settings as well. That is why the exhaust conditions once set at the demanded values, should be kept as constant as possible.