[electronics manufacturing expo Knowledge] Automatic soldering tin plating can improve solder joint quality and consistency

11 Jul 2019

Selective soldering processes play an increasingly important role in electronics assembly manufacturing. Complex printed circuit board assemblies place higher demands on the overall accuracy and repeatability of the selective soldering process. The entire selective soldering process is required to be more precise and to improve process control levels.

The use of advanced closed-loop electronics manufacturing expo process control systems, including automatic selective tip soldering, ensures that complex printed circuit board assemblies are produced at the highest possible quality levels with consistent results to ensure maximum reproducibility.

Selective tip

Most selective welding equipment uses wettable or non-wetting tips, each type has advantages and disadvantages in its electronics manufacturing expo application, and each type requires different maintenance standards. Wettability tips, also known as bullet-type tips or universal tips, have the advantage of being able to access solder joints from any direction and can solder components that are very close to adjacent SMT components because they are omnidirectional, making them very Suitable for highly flexible welding applications.

Most round wettability tips produce a very low level of molten solder, which limits their soldering of long lead components or reduces flexibility when soldering tighter solder joints. The warhead tip is a proprietary design that produces back pressure and increases the height of the molten solder by an additional 50% while gradually turning the solder tip into a electronics manufacturing expo finer tip. This allows the solder to be soldered to long lead components without the nozzles touching the component leads, and in other cases to improve accuracy. As shown in Figure 1.

Non-wetting tips are also known as mini-wave or jet-wave tips with directional flow of molten solder because they are unidirectional, making them ideal for soldering connectors or multiple rows of via components. Since non-wetting tips are unidirectional, they are typically soldered at 7° like a conventional electronics manufacturing expo laminar wave tip, which helps excessive molten solder detachment and prevents solder bridging. One disadvantage of non-wetting tips is that they can only be soldered in a single direction due to the directional solder flow, which means that they typically require a larger isolation area between the tip side and the adjacent SMT component.

Tip maintenance

A key part of any selective soldering equipment is the preventative maintenance methods, procedures, and practices for each type of tip. Pre-tinning of wettable tips prolongs their working life and improves the flow characteristics of molten solder, such as periodic cleaning and re-tinning. Ideally, the tip is first cleaned with a high-active tip tin-plated flux, then tinned in a static solder pot, filled with the same electronics manufacturing expo solder alloy used in production, and finally manually tin-plated.

During selective soldering, the wettability tip becomes non-tinned, which affects the proper flow of molten solder and increases the surface tension of the solder. When the tip loses its correct flow characteristics, its defect-free solder joint electronics manufacturing expo productivity is greatly reduced. This is usually corrected by an operator's manual operation, and the operator applies the flux to the tip using a brush.

Regular and correct tin plating on the tip and sides of the tip ensures that the solder flows out of the tip evenly and consistently. Irregular tin plating increases the likelihood of uneven solder flow, which can lead to inconsistent and ineffective soldering of printed circuit board assembly.

In some cases, the operator has known that for the soldering of the printed circuit board assembly, the same ethanol-based, non-etching, no-clean flux is used to manually re-tamp the tip. Unfortunately, this is not a good practice because these non-erosive fluxes do not properly clean the tip and will cause more electronics manufacturing expo operator attention and then apply flux more frequently. This will quickly be counterproductive, because every additional manual application of flux will leave more residue on the surface of the tip and eventually accumulate in the tip, which must be removed from the machine and manually scrubbed and Re-tinning.

The frequency at which the solder nozzle is re-tinned depends on many factors, one of which is the solder alloy. For example, high melting point solder alloys, because they are capable of withstanding extreme thermal and mechanical stresses, exhibit low flow in the liquid state, resulting in slow solder flow. These high melting electronics manufacturing expo point alloys tend to be highly corrosive on the surface of the welding nozzles, and the use of them often requires more frequent nozzle retining.

Flux application

There are many types of liquid fluxes for selective soldering, including alcohol based fluxes, water soluble fluxes, rosin based fluxes, low pH fluxes, and high solids fluxes. The type of flux used in the selective soldering process is usually specified by the final application of the product and is critical to solder joint integrity.

When no-clean flux is used for selective soldering, the preferred method of applying liquid solder is to use a droplet-jet nozzle instead of an atomizing electronics manufacturing expo nozzle or an ultrasonic spray flux applicator. This is because the droplet-jet nozzles apply small droplets of flux that stay in the path of the tip so that the no-clean residue can be consumed by direct contact with the molten solder.

Although droplet-jet spot coating is a proven no-clean technique, over time the flux solvent evaporates and impurities in the flux cause the droplet-jet exit portion to become clogged, causing the flux to deviate slightly from its true The center of the coating electronics manufacturing expo position. The droplet-jet applicator coats very small amounts of solder, which is critical to the quality of the selective soldering process, especially in high-volume, unmanned operations where automatic flux verification is essential.

A common selective soldering process is to apply an excess of liquid flux to solve the solderability problem. This is not advisable. It should be solved by solving the root cause instead of applying more flux. Applying excess flux can result in a variety of soldering electronics manufacturing expo defects, including voids, solder balls, or flux entrapment due to exhaust of the flux solvent. Excessive flux application can also cause accumulation of flux residue on the tip, which requires more frequent cleaning of the tip:

Tip tinning cleaning tip

Some selective welding equipment uses various types of automatic tip cleaning methods to minimize downtime by eliminating manual cleaning of the tip. These different methods include ultrasonic tip cleaning, or spraying or brushing the tip with adipic acid or organic acid, or spraying liquid or powder flux directly onto the outer surface of the tip. Although these methods can clean the tip by removing the electronics manufacturing expo residue, they do not necessarily re-tin the tip, which is critical to improve the flow characteristics of the molten solder.

The flux required to properly clean the tip and stabilize the surface tension of the solder flowing through the tip is quite aggressive. While these automatic tip cleaning methods provide flux by spraying liquid or powdered flux, this must introduce air pressure or a large amount of gas flow into the flux material to present it to the flowing solder. This may result in inaccurate solder placement, exposure of aggressive electronics manufacturing expo materials to unwanted areas, or ineffective stabilization of solder flow within the tip. Another method is to wrap the flux in the wire until it is presented to the tip, which can make the device cleaner and provide a more precise flux placement, which is for cleaning and re-tinning smaller diameter tips. It is important.

Automatic tip cleaning and tin plating

To solve the problem of ineffective tip cleaning and re-tinning, some users have installed Nordson SELECT's automatic tip tinning system that completely eliminates the spray electronics manufacturing expo operation. The system is patent-pending and uses proprietary flux-cored wire, tip cleaning and re-tinning integration operations (see Figure 2).

Unlike other tip cleaning systems, the patent-pending system does not spray liquid or powdered flux. The automatic tip tinning system maintains the nozzle's absolute cleanliness by automatically removing oxides and residues without any overspray or contamination of the printed electronics manufacturing expo circuit board assembly or selective soldering equipment. This automatic tip tinning system is suitable for all Nordson SELECT selective soldering equipment and is compatible with lead-free and tin-lead solder alloys.

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