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PCB becomes darken (yellownish)

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#56107

PCB becomes darken (yellownish) | 22 August, 2008

Hi All,

I have question regarding the PCB. My PCB is immersion silver furnished. I found out that the board is darkening from immersion silver to yellownish colour after exposed to the environment. From my finding, i think it may be caused by the sulphur contents. Am i right? Any others ways to get rid of the yellownish from the surface?

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#56110

PCB becomes darken (yellownish) | 22 August, 2008

Sulfur is probably the culprit. Board supplier should be providing boards with anti-tarnish sheets between the individual boards. Also, cardboard is a major source of sulfur and so boards should not be shipped in cardboard.

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#56111

PCB becomes darken (yellownish) | 22 August, 2008

I have seen this problem before. If the color is yellow usually it is caused by moisture left on the boards from poor drying after immersion silver. One of the ways to get rid of this is by dipping in acid cleaner solution. If the board supplier uses an anti-tarnish after immersion silver before drying the boards then that will give a better window as far as not getting any discoloration at all on the boards.

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#56130

PCB becomes darken (yellownish) | 23 August, 2008

What part of the process is it turning yellow in? That will give more help defining the problem.

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#56131

PCB becomes darken (yellownish) | 23 August, 2008

Ugh, there are more reasons that immersion silver [IAg] to go yellowish than pigs at the county fair. That aside, sulphur is one of the reasons, as you say. The sulphur can be from a variety of sources, such as: * Pollution in the air * Ill advised packing material choices * Proximity the surphur fumes from plating baths * Lack of, or lowered level, anti-tarnish in the plating bath * Yellowishness increases with each thermal cycle the plating sees on the board

Consideration of getting rid of the yellowishness raises the questions: * Are the boards yellowish and solderable? * Are the boards yellowish and unsolderable?

If boards are: * Yellowish and solderable, then do nothing and go solder away. * Yellowish and unsolderable, then replate the IAg. It is very easy to do and one of the many charms of IAg.

Get a copy of "The Study, Measurement, and Prevention of Tarnish on Immersion Silver Board Finishes" [Lenora Tuscano & Don Cullen, IPC Works 2003 Fall meeting]. It documents that the "yellowish" tarnish found on IAg boards can be silver sulfide that is still solderable.

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#56183

PCB becomes darken (yellownish) | 26 August, 2008

Thanks you for all of the infomation. By the way, i would like to prompt a question. What can i do on those contaminated PCB as i need to remove those stains? by HCL?

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#56186

PCB becomes darken (yellownish) | 27 August, 2008

Logan, My boards have gone through the baking process. After that, we ran through reflow of bottom side and washing. Due to inadequate parts, then we have on hold the reflow of top side. We kept the boards in a room. After 1 week, we noticed there was a yellownish layer on the board. this would be the scenario of it.

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#56190

PCB becomes darken (yellownish) | 27 August, 2008

Your board fabricator can replate them for you.

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#56229

PCB becomes darken (yellownish) | 28 August, 2008

Hi robinj As you have mentioned, if the color is yellow usually it is caused by moisture left on the boards from poor drying after immersion silver. However, our boards were received from the supplier and the changes happened in our side. So, what did you do with those boards?clean them with acid cleaner solution? What kind of acid solution was used?

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#56319

PCB becomes darken (yellownish) | 3 September, 2008

If the tarnish is really dark, it is very difficult to remove. Since it is yellowish it is only in the early stages. Replating will definitely work. I have used sulfuric acid based cleaners to remove this yellowish color. But rinse well and dry quickly. You don't want any acid residue on the boards. Your board fabricator should know this. The main culprit of silver tarnish is the presence of sulfur. Make sure the paper or any packaging for these boards are sulfur-free.

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