Improving Consistency and Quality with Automatic Solder Nozzle Tinning
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 nozzle 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 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 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 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 (Figure 1).
Non-wetted solder 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 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 Nozzle Maintenance
A critical area for any selective soldering machine is the preventative maintenance methods, procedures and practices used for the various types of solder nozzles. Pre-tinning of wetted solder 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 nozzle with a highly active solder nozzle 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 nozzle can become un-tinned which compromises the proper flow of molten solder and increases the surface tension of the solder. Whenever a solder nozzle 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 nozzle using a brush.
Regular and proper tinning of the tip and side surfaces of a solder nozzle 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 nozzle 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.
The frequency of solder nozzle re-tinning is dependent on many things, one of which can be the solder alloy used. For example, high melting point solder alloys that are used for wellhead electronics applications because of their ability to withstand extreme thermal and mechanical stress, exhibit reduced fluidity when in a liquidous state resulting in sluggish solder flow. These high melting point alloys tend to be very corrosive on the solder nozzle surfaces and using them tends to require nozzle re-tinning more often.
Liquid fluxes for selective soldering are available in many types including alcohol-based fluxes, water-soluble fluxes, rosin-based fluxes, low pH fluxes and fluxes with high solids content. The selection of a flux type for the selective soldering process is generally specified for the end application of the product and is critical for the resulting solder joint integrity.
When using no-clean fluxes with selective soldering, the preferred method for applying liquid flux is to use a drop-jet as opposed to an atomizing type spray head or an ultrasonic spray fluxer. This is because the drop-jet flux applicator applies small droplets of flux that remain within the path of the solder nozzle so that the no-clean residues are consumed by direct contact with the molten solder.
Bullet solder 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.
While drop-jet dispensing is a proven technology for no-clean processing, flux solvents can evaporate over time or impurities within the flux can cause a drop-jet orifice to partially clog and slightly dispense flux off center of its true position. The amount of flux dispensed by a drop-jet dispenser is so minuscule and is extremely critical to the quality of the selective soldering process, that automatic flux verification is essential especially in a high-volume lights-out operation.
A common occurrence with selective soldering processing is the application of an excessive amount of liquid flux attempting to resolve solderability issues, which should be addressed by resolving the root cause as opposed to applying more flux. Applying an excessive amount of flux can result in various solder defects including solder voids due to out-gassing of flux solvents, formation of solder balls, or flux entrapment. The application of excessive flux also contributes to the accumulation of flux residues on the solder nozzle requiring more frequent cleaning of the solder nozzle.
Nozzle Cleaning versus Nozzle Tinning
Several selective soldering machines incorporate various types of automatic solder nozzle cleaning for minimizing machine downtime by eliminating manual nozzle cleaning. These different methods include ultrasonic nozzle cleaning, spraying or brushing the solder nozzle with adipic acid or organic acid, or spraying a liquid or powdered flux directly onto the outer surface of the solder nozzle. While these methods clean the solder nozzle by removing residues, they do not necessarily perform a re-tinning of the solder nozzle which is critical to improving the flow characteristics of the molten solder.
Flux properties required to properly clean and stabilize the surface tension of a flowing solder nozzle can be quite aggressive. While some of these automated solder nozzle cleaning methods present the flux by spraying a liquid or powdered flux, introducing air pressure or high-volume air currents into the flux material is necessary to present it to the flowing solder. This can lead to inaccuracy of flux placement exposing aggressive material to areas that are undesired, or not being effective in stabilizing the solder flow within the nozzle. An alternative method is to have the flux encased within a solder wire until the moment it is presented to the solder nozzle which is cleaner for the machine and provides a more precise placement which is critical when cleaning and re-tinning smaller diameter solder nozzles.
Automatic Nozzle Cleaning and Tinning
To resolve the issue of ineffective nozzle cleaning and re-tinning, several end-user customers have installed a Nordson SELECT automatic solder nozzle tinning system that eliminates spraying altogether. This patent pending system uses a proprietary flux core solder wire to clean and re-tin the solder nozzle in a single integrated operation (Figure 2).
Unlike other nozzle cleaning systems, this patent pending system does not spray liquid or powdered flux. The automatic solder nozzle tinning system keeps solder nozzles meticulously clean by automatically removing oxidation and residues without any overspray or contamination of the printed circuit board assembly or the selective soldering machine. This automatic solder nozzle tinning system can be adapted to all Nordson SELECT selective soldering machines and is compatible with lead-free and tin-lead solder alloys.
This automatic solder nozzle tinning system can be adapted to all Nordson SELECT selective soldering machines and is compatible with lead-free and tin-lead solder alloys.
The advantage of the automatic solder nozzle tinning system is that it removes oxides and residues from the solder nozzle improving solder flow to ensure consistent soldering quality, duplicating the manual action of re-tinning a solder nozzle. Since the automatic solder nozzle tinning system does not produce overspray or contamination of the printed circuit board assembly or selective soldering machine, it is advantageous for clean production shop floor environments where caustic materials are not permitted.
At programmed intervals, the solder nozzle moves into position under the automatic solder nozzle tinning system, after which the nozzle tinning system indexes a predetermined amount of flux core solder wire introducing it directly into the molten solder above the solder nozzle (Figure 3).
Upon contact with the flowing solder wave, the solder wire melts immediately exposing the specific flux core to the molten solder within the nozzle. Through the natural motion and heat of the flowing, molten solder, the flux is brought to activation temperature and washed over the exterior of the nozzle, stabilizing the solders surface tension (Figure 4).
This automatic solder nozzle tinning system replicates the action of an operator manually applying tinning flux with a brush to the exterior of the solder nozzle and manually re-tinning a solder nozzle, but without costly machine downtime.
Based on the positive experience of numerous installations, the automatic solder nozzle tinning system is establishing itself as a highly effective alternative to other nozzle cleaning systems and is successfully ensuring the highest levels of reproducibility and consistent solder joint quality.