In many cases, automated tinning of leads is a requirement of today’s assembly processes. This is a controlled, strictly managed, and preferably automated process, because it requires consistent tinning quality and good repeatability. Requiring tin plating on pins first began about twenty-five years ago when the military decided that a non-fusible plated surface was not suitable for a high-reliability environment. It was found that if the plated surface was not durable enough to resist oxidation attacking the pins below the plating, the solder joints would fail in the field.

Today, the most common reason for tin plating on lead surfaces in high-reliability applications is to remove gold from gold-plated surfaces on electronic devices. One reason gold is undesirable at the solder J-joint is that gold can cause the well-known phenomenon of gold embrittlement. During the tinning process, these components are dipped in the solder bath, which washes out the gold. It is then re-tinned to create a tin/lead surface. This process uses two solder pots—one in which the existing surface or intermetallic compounds and other metals are washed off, and the other in which a new surface is plated on the component.

The second most common reason for tinning the surface of the pins is that there are a large number of components left over from the past that have been in storage for a long time, either due to oxidation or simply due to time, their solderability may have decreased , — there is no guarantee that a strong connection will be formed during the assembly process.

Remove the original gold-plated surface of these components, and evenly plate a new layer of tin with good solderability (exhibiting good wetting characteristics) on the surface by dipping. When these components are assembled, at least it can be guaranteed. relatively high success rate.

Another reason for tin-plating the pin surface is to convert modern components with a lead-free surface to be compatible with tin-lead assembly, which is again for high-reliability products. Since the implementation of RoHS, the inventory of tin-lead surface finished components has become less and less, so these components must be dipped in a high-temperature tin-lead solder bath to be suitable for use in high-reliability products.

Dip these components in hot solder, which, if done properly, can “wash” the tin off the components so that the pins can be properly re-tinned with tin-lead solder. In an automated pin-on-pin system, a system with two solder pots is required for tinning: one solder pot absorbs contaminants—the purity of the solder in the solder pot must be monitored when gold or undesirable When the content of the substance reaches a certain saturation point, another solder can is used. Using a second, unused can of solder, re-tin the pair of pins with fresh and uncontaminated solder. Don’t do all of these things with one solder can! Of course, the reverse is also normal – to remove the lead from a tin-lead plated component and re-tin it for use in a RoHS-compliant component, also take turns Use two tin cans.

While all of these practices are not common, tin plating is also required in the case of traditional components with tin-lead surfaces on the board in modern products that require boards to be RoHS compliant. Therefore, we need to remove the plating or surface treatment on the pins and replace them with RoHS compliant plating or surface treatment. Reducing tin whiskers is another reason for tinning the pin surface. The importance of tin whisker reduction is that everyone seems to be concerned about tin whiskers when moving to high tin lead-free alloys. Jiali Chuang laser steel mesh

NASA has published a paper stating that, in fact, the only reliable way to mitigate tin whiskers and prevent their growth is to immerse the pins in molten alloy. Doing so will create a layer of “fused” intermetallic compound on the surface of the pins, unlike a non-fused plating finish – which is rather like a coating on sand, without (and the metal beneath the surface layer) ) fusion, easy to grow tin whiskers under certain conditions.

For a successful lead surface tin plating automation process, there are several basic but very critical requirements. As mentioned above, two solder cans must be used, one to wash off the unwanted metal and the second is the uncontaminated can to re-tin the pins with the correct alloy.

The second requirement is that the solder in the solder can is flowing, not static, because the flowing solder (especially the solder in the scrub tin can) will pollute the surface of the pin or remove the material on the surface of the pin, Such as removing gold from adjacent pins. When the component is pulled from the solder, the flowing solder keeps the contamination from being pulled back and deposited on the component’s leads. Some agitation of the solder is also very important for first scrub tin cans, as doing so will actually help remove the gold or solder from the pins. This can be easily done by mechanically holding the element in the cell.

It is best to fill the second solder pot with nitrogen or inert gas, which can improve the finish of the surface treatment, reduce solder dross, and reduce the formation of burrs and bridges. These are real concerns, especially when using lead-free solder. Controlling the flux and tinning in an inert atmosphere can only really be done in a controlled environment, so be sure to use an automated process with a controlled environment.

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