The selective soldering process is playing an increasingly critical role in much of electronic assembly manufacturing operations around the world. Complex printed circuit board assemblies place greater demand on the overall accuracy and repeatability of the selective soldering process. This requires the entire selective soldering process to be more precise with increased levels of process control.
The use of advanced closed-loop process control including automatic selective solder ball jet nozzles tinning assures that complex printed circuit board assemblies can be produced at the highest possible quality levels with consistent results ensuring maximum levels of reproducibility.
Selective Soldering Nozzles
Most selective soldering machines use either wetted or non-wetted solder ball jet nozzles, each type having advantages and disadvantages in their application and each type requiring different maintenance criteria. Wetted nozzles, also referred to as bullet nozzles, general purpose nozzles or universal nozzles, have the advantage of being able to approach a solder site from any direction and can solder extremely close to adjacent SMT components since they are omnidirectional making them ideally suited for highly flexible soldering applications.
Most round wetted solder ball jet nozzles produce a minimal height of molten solder which limits the length of the component leads that can be soldered or can reduce flexibility when accessing tighter solder locations. Bullet solder ball jet nozzles are specially designed to establish backpressure raising the height of the molten solder an as much as an additional 50%, while tapering the solder tip to a finer point. This allows soldering to take place with longer length component leads without having the nozzle contact the component pins, as well as allowing opportunity for additional precision in other locations.
Non-wetted solder ball jet nozzles, also referred to as mini-wave or jet-wave nozzles, have directional flow of the molten solder since they are unidirectional making them ideal for soldering of connectors or multi-row through-hole components. Since non-wetted nozzles are unidirectional they typically solder at a 7-degree angle like a traditional laminar wave soldering nozzle to promote the breakaway of excess molten solder to prevent solder bridging. One disadvantage of non-wetted solder ball jet nozzles is that they can only solder in a single direction because of the directional solder flow which means they generally require more keep-out area between the side of the nozzle and adjacent SMT components.
Solder Ball Jet Nozzles Maintenance
A critical area for any selective soldering machine is the preventative maintenance methods, procedures and practices used for the various types of solder ball jet nozzles. Pre-tinning of wetted solder ball jet nozzles extends their working life and improves the flow characteristics of the molten solder as does periodic cleaning and re-tinning. Ideally manual re-tinning should be carried out after first cleaning the solder ball jet nozzles with a highly active solder ball jet nozzles tinning flux followed by tinning the nozzle in a static solder pot filled with the same solder alloy used in production.
During selective soldering a wetted solder ball jet nozzles can become un-tinned which compromises the proper flow of molten solder and increases the surface tension of the solder. Whenever a solder ball jet nozzles does not have the correct flow properties its ability to create defect free solder joints is greatly diminished. This is typically corrected by a manual operation of an operator applying flux to the solder ball jet nozzles using a brush.
Regular and proper tinning of the tip and side surfaces of a solder ball jet nozzles ensures that the solder will flow evenly and consistently out of the nozzle. When not regularly tinned, there is an increased likelihood that the solder will begin to flow unevenly, leading to inconsistent and ineffective soldering of a printed circuit board assembly.
In some cases, operators have been known to use the same alcohol-based, non-aggressive no-clean flux to manually re-tin the solder ball jet nozzles that is being used for soldering of the printed circuit board assembly. This unfortunately is not a good practice since these non-aggressive fluxes do not clean the nozzle properly and will result in more attention from an operator, applying flux more frequently. This can quickly become counterproductive as with each additional attempt to apply flux by hand, more residues will accumulate on the nozzle surface ultimately building to the point when the nozzle must be removed from the machine and manually scrubbed and re-tinned.
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