Publication date: 06 December 2011
There are a number of good reasons why one might have to re-tin components from the stockroom prior to assembly. For example, let’s say that I'm building a hi-rel assembly that's put together using traditional lead-bearing Sn63 alloy. Unfortunately, one of the components that I need to use in that assembly is only available in a RoHS-compatible finish, in this case, electroplated tin. What do I do? If the component is plated with tin, shouldn’t it be compatible with tin/lead solder?
Well not exactly. The added volume of tin in the plating, however slight, will change the percentage of tin in the solder joint, creating a new alloy that may have undesirable characteristics, such as being more brittle. The other reason for hot solder re-tinning is to prevent the formation of tin whiskers, which tend to grow out of a plated finish, but won't grow out of a fused alloy finish applied through dipping in hot solder. One cannot simply solder a component with tin plated leads to a military circuit, because any portion of that lead surface that is not wetted with solder has the potential for tin whisker growth and catastrophic failure of the circuit through shorting.
Therefore, what one has to do is remove that pure tin plating and replace it with a fused finish of the same solder alloy that is going to be in the connecting solder joint. This is the typical method of converting modern components that were born after RoHS with lead-free finishes that are compatible with tin/lead assembly, usually found only in hi-rel or military applications. Since RoHS, there have been fewer and fewer components available with tin/lead finishes as an alternate choice.
If not plated with pure tin, these components may also have a gold plating finish on the leads that also must be removed. One does not want gold to be present in the solder joint because it can lead to a phenomenon known as gold embrittlement. In the tinning process, these components, whether tin or gold plated, are dipped into a pool of solder which washes away the undesired plating. The components are then re-tinned with a tin/lead finish.
The tinning process basically involves the use of two solder pots, the first one being the ‘sacrificial’ pot that removes the unwanted plating of, gold, tin, or other alloy. The second pot is the ‘virgin’ pot that contains the alloy that is going to be applied to the component so that it can be assembled. Essentially, the component is picked up, dipped in flux, then dipped in the first pot to remove the unwanted plating. During this initial dipping, the tinning system should employ some manner of agitation in the pot to produce a ‘scrubbing’ effect for complete removal of the unwanted metallization.
The component is then fluxed again and dipped into the ‘virgin’ pot to apply the new fused finish. The component isn’t dipped flat, but is tilted, so that one side at a time is dipped. There are a number of reasons for doing this, primarily to prevent icicling, for a clean peel away of the solder. Inerting is also used to minimize icicling, particularly when using lead-free alloys.
t's very important that this entire process is automated. Reasons for this include the obvious desire for repeatability and consistency of results component to component, but also to take it out of the hands of the operator, to prevent damage to delicate fine pitch leads. It's much better to automate and thoroughly control the process precisely from start to finish. Dipping times for removal of plating, and for re-tinning average about 2 seconds for each row of leads. Also, flux is applied through a dipping process to ensure uniform application and thorough fluxing for proper wetting. One wants enough flux to achieve the objective without applying excess and creating a cleaning issue.
Thus, the process, from removal of the component from the tray, through fluxing, preheating, dipping, visual inspection, followed by post-cleaning and washing, drying, inspecting, and put back in the carrier that it came from, should be completely automated. Visual inspection mid-process is to ensure the absence of bridging, a real potential problem when tinning fine-pitch components such as QFPs. This is why the tinning process requires pre-heating, at a controlled temperature ramp to a desired set -point, especially important with ceramic components, prior to both the ‘removal’ dip and the ‘re-tinning’ dip. The preheating also minimizes any potential for thermal shock or damage at the lead/body interface.
Depending on what material is being removed, the level of contamination in the ‘sacrificial’ pot must be monitored, and it must be exchanged for a fresh pot when the level of gold or unwanted metal reaches a certain saturation point.
For Further Information On This Process Contact:Alan Cable, PresidentA.C.E. Production Technologies3010 N. First St.Spokane Valley, WA 99216Office: 509-924-4898Cell: 509-993-6571mailto:acable@ace-protech.comwww.ace-protech.com
