Optimizing process development: harmonizing all the parameters is no simple feat.
A wave soldering process breaks down into the following categories:
Fluxing. The correct amount of flux to be applied per board is based on the flux supplier's specifications. Excessive flux may interfere with the product's electrical reliability, and a moderate amount of flux may not provide sufficient tail activity to reduce bridging and to obtain good through-hole penetration when the board leaves the wave. It is extremely important to optimize fluxer settings, which are related to conveyor speed. Visual testing should be used to ensure proper overlap and penetration of flux. With alcohol-based flux, thermal fax paper can be used on the bottom of the assembly and processed through the fluxer only. A visual footprint then can be seen and areas missed by the flux pattern identified. The same is true if the paper is applied to the top of an unassembled board. The paper must be fixed to the assembly to avoid movement during flux application. For water-based flux, pH paper can be used. Also commercial test fixtures can be used for flux test application. The appropriate flux type (i.e., alcohol-or water-based fluxes) depends on the application, board surface finish, solder resist, board complexity and other issues.
Preheating. Board preheating is necessary to evaporate flux solvents and to prepare the board and flux for soldering. Preheating also is used to reduce thermal shock of components and to promote better through-hole penetration, especially for multi-layer boards. The flux supplier specifies preheating parameters. For the case of alcohol (i.e., isopropyl) fluxes, the board topside temperature should be above 82[degrees]C, and for water fluxes above 100[degrees]C. Complete evaporation of the solvent is important to reduce soldering defects such as openings, voiding, and solder balling. Depending on the type of flux and board thickness, higher preheat temperatures may be required to activate it.
When surface mount components (i.e., chip components) are glued to the bottom side of the board, it is important to ensure that the [DELTA]T between the temperature of the components and solder temperature is between 100[degrees] and 110[degrees]C. It is important to follow the supplier's specifications for wave soldering surface mounted components.
Conveyor speed. A typical conveyor speed setting will be in the range of 1 to 1.5 m/min. The speed setting depends on board complexity. Single-sided boards often can be soldered at high speed because they often have a low thermal demand and no plated-through barrel, and thus do not require topside fillets. A multilayer board may have a high thermal demand and 1 m/min could be too fast. To optimize the conveyor speed, it is also important to consider the board layout at the solder side, which can be a decisive factor to prevent solder bridging.
Soldering temperature. The solderpot temperature setting depends on the type of solder, but also may be related to the product to be soldered (i.e., board complexity, pallet use, or exposed bottom-side surface mount components). In general, low temperature settings are recommended to avoid board war-page and component damage. Lower temperatures create less dross, and extend the lifetime of the flux so that it has better tail activity. During soldering, the topside board temperature must be below the melting point of the surface mount component joints to avoid double reflow. For SnPb solders, 245[degrees] to 250[degrees]C is a common setting. For SAC alloys, 260[degrees] to 265[degrees]C is the recommended setting. It is important to keep the solder bath volume constant to maintain soldering temperatures.
Dwell time. A board must touch the wave for a sufficient time to make a good solder joint. The real contact of a joint depends on the protruding length of the leads and the board layout. The typical contact time for Pb-free applications is between 3 and 6 sec. The dwell time also may affect board warping. To avoid excessive warping, board supports or pallets can be used.
Wave height. The wave height should be kept low to minimize dross formation. In general, lead clearance of 6 to 8 mm relative to the bottom of the assembly to the wave formers in the solder pot is preferred. Lower settings may move components during soldering, as the leads may touch the nozzle rim. A wave height setting should be constant within a few tenths of a millimeter. For this reason, the solder level of the solderpot should be monitored and corrected automatically.
Wave type. There are three types of waves: chip, main and smart wave. Depending on the type of assembly and flux, it may be best to use one wave former. Chip wave is a turbulent wave and is used as first wave to enable wetting of chip components, which are surrounded by non-wettable component bodies. Main is the second wave (Figure 1) and is a smooth wave that prevents bridging. The smart wave is located over the main wave and produces turbulence, which may be beneficial for through-hole penetration.
[FIGURE 1 OMITTED]
Nitrogen. Nitrogen may be helpful to support flux activity during the separation of the board from the solder wave. During this separation process, the solder should stay on the joints and not in between joints. Bridging occurs because of solder oxide formation at this stage. Solder oxides are formed due to lack of flux activity and the presence of air. By applying nitrogen at that stage, it can displace the air and assist in better drainage conditions due to reduced oxides.
Cooling. As soon as the board leaves the wave, the solder joints cool rapidly at a rate of -10[degrees] to -15[degrees]C/s. Heat from the solder joints is absorbed by the component leads and the board's copper traces/layers, resulting in rapid joint solidification. Here, the cooling system will not affect the microstructure of the solder joints, but can be used to reduce the board temperature for handling purposes.
Good wave soldering machines should have proper control and produce repeatable temperatures to ensure the profile is the same for all boards. To monitor the repeatability of a machine, SPC software is used. This helps monitor several machine parameters, detect process shifts, and recommend preventive maintenance. This way, the user can be sure all components and joints reach correct soldering temperatures.
Ursula Marquez de Tino, Ph.D. is a process and research engineer at Vitronics Soltec, based in the Univis SMT Lab (vitronics-soltec.com); firstname.lastname@example.org. Her column appears monthly.
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|Title Annotation:||Wave Soldering|
|Comment:||Optimizing process development: harmonizing all the parameters is no simple feat.(Wave Soldering)|
|Author:||Tino, Ursula Marquez de|
|Date:||Aug 1, 2009|
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