|Ask the Experts|
October 21, 2020
Legend Marking Discoloration
Can you recommend a process to prevent this from happening?
|Expert Panel Responses|
Ensure overheating is not taking place. Measure the temperature of the areas of interest with thermocouples. If OK, try reworking in a nitrogen (low oxygen) environment, this approach has minimized the darkening of light colored boards (G10) for both site Rework and mass Reflow.
Regional Sales Manager
Engineering and Operations Management
The darkening is of course caused by thermal decomposition of the ink. This type of degradation is typically accelerated above a threshold temperature that dependent on the ink chemistry, and possibly on the chemical exposure history of the ink. If the rework is being done with a hot air system, consider conducting an experiment.
Expose some legend ink to the air flow using the normal rework temperature set point. Control the time and distance, and note how quickly the discoloration occurs. Now start reducing the temperature in steps of say, 20C, and for each step record the same information.
You will find that there is a temperature below which it takes a very long time to discolor the ink; you already know that it's not discolored by exposure to your reflow profile, so that provides one data point.
The question is, will the maximum workable set point that avoids degradation still provide adequate heat to rework the components in a reasonable time? If not, you may need to investigate whether you can change the ink to a more robust product. Another alternative might be to pre-heat the assemblies to be reworked, which will lower the set point required during rework.
I cannot think a process to prevent this from happening, other than shielding the total area around the component where the markings exists. However discolored markings are acceptable, so discoloration should not be a problem.
Vice President, Technical Director
I will make the assumption that you are using hot air to perform the rework. If this is true you will need to make sure that the air temperature blowing on the rework site stays below 300C.
Using lower air temperatures will extend the rework cycle a bit, but should give you better results with both the solder joints and cosmetics.
If you want to get rid of the discoloration without changing the thermal process, then apply Kapton tapeover the markings prior to rework.
Process Sciences, Inc.
The symbolization epoxy is usually initially anatase-based, which provides the brilliant definition. However, this phase changes to rutile in the presence of heat and then the crystals become almost like xenotine. Both the rutile stage and xenotine have more of a pinkish tan or almost beige color. So when heated, the anatase properties become rutile and thus the color change.
Also, the anatase is optically negative and rutile is positive. This tends to accentuate the color change. This is very common, especially when rosin-based fluxes are in contact with the symbolization. It is a chemical reaction brought on by heat. I have seen it happen many times, but it was not really a concern as the symbolization was still very legible. It is still inert. It is a cosmetic issue only.
Apply kapton tape or adhesive backed heat shield blanket. (235-4010 Heat Shield Blanket, 5" x 7" ) CircuitMedic sells them and are not expensive.Sundaram, EIT LLC
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