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December 5, 2012

White Legend Ink Turning Pink

We have PCB assemblies using white legend ink. The white ink has turned pink after the assembly process. Why has the ink turn pink?

J.O.

Experts Comments

Gold salts (e.g., from ENIG surface finish) left on the surface (typically from a dirty final rinse) will react with titanium dioxide used as a pigment, yielding a tint ranging from pink to purple. This phenomenon is also possible with white solder mask (popular with LED PCBs).
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Robert "Bob" Lazzara
President
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.
Let's think about the possible ways that a pink color could be generated:
  1. The ink picked up traces of a red dye from an assembly process

  2. A pigment or component in the ink was affected chemically by the assembly process and changed color
Both are possible. If all the ink turned pink, look at processes that expose all the ink to the process; one good possibility is a wash process. If only some of the ink turned pink, look at what the pink areas have in common. Following are some questions that you may want to answer, either by research or experiment, that may shed light on your issue:
  • Is the color problem limited to a specific board supplier and/or date code? If so, look into material or process changes that may have occurred at the supplier. Keep in mind that epoxy-based inks are less susceptible to absorbing dyes, and that under-cure can raise permeability of all inks, including epoxies. So:
    * Did you change board suppliers?
    * Did a supplier change the material used?
    * Did a supplier have a process control problem that may have affected cure?

  • Is the color problem limited to boards that see a certain process, or does it show up at a certain point in the process? If so, look at processes immediately prior to the change.

  • If you put a relatively impermeable label over a portion of the ink, such as a reflow-rated Kapton label, does the ink under the label remain white? If so, it's not likely to be a heat-related change.
There are other ways you can isolate process effects, such as taking  sample bare boards of lots known to produce the problem (assuming they are available and it is cost-effective) and subjecting them to varying combinations of processes using a designed experiment approach. This should hopefully not be necessary, and should only be considered if simpler means of isolating the root cause do not succeed.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
In almost 30 years in this industry I have never seen that happen. I would suspect the flux vapors in the reflow oven environment reacting with the legend ink. Perhaps exposing a defect in the ink cure, etc. Is this a new phenomenon is it seen on the whole lot? I would start my investigation upstream of the assembly process.
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Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.
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