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Part of PCB silkscreen turns yellow brownish

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#72648

Part of PCB silkscreen turns yellow brownish | 28 August, 2014

Hi all,

Our production reported that they found 3 panels out of 53 panels with strange phenomenon after reflow.

1/ Blister / Delamination of PCB panels, 2pcs blister/delamination under solder mask, 1pc under Gold plating at Gold finger

2/ Silkscreen colours changes to dim dark colour; Some are both sides full panels, some are most of the units (i.e. 6pcs out of 8pcs have changed color)

We also have other observations on those defect PCB panels:

1/ Fiber color changes dim dark 2/ Gold finger Gold color change 3/ Breakaway fiber colour is different

Detail please refer to attached file.

Remarks:

a/ We discovered this problem when we were performing inspection after reflow(2 sides). b/ We had confirmed the reflow oven temperature is followed IPC requirement (~235'C max) for lead-free soldering, with the technical data from profiler. c/ All panels were completed solder printing to reflow process within 2 hrs. d/ No repair and abnormality(i.e. under repair process) of those problem Panels.

My question: 1/ I am not sure whether blistering and discoloring are come from the same root cause, I can bake PCB before production to avoid blistering, but I did not know can that be removed both problem. Please comment. 2/ PCB supplier just tell me the base materials and silkscreen materials are correctly used after they check the production record. Any other thing I can ask to confirm there is not problem with their production and materials?

I have ~1000pcs PCB stock in our warehouse, I cannot tolerate to have this % of repair rate, to pull all BGAs and reballing IC to replace PCB. You professional advice would help a lot!

Attachments:

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#72650

Part of PCB silkscreen turns yellow brownish | 29 August, 2014

I'll try to deal with the issues as you've presented them.

1. Blistering/delamination, shown in photo 2. The most likely cause of blistering/delamination like this is moisture in the boards. The moisture can get into the boards in multiple ways...at the fabrication house, in transit, or in your own facility. Insure that your fab house is sealing the boards, and shipping with dessicant. Insure that you are storing the boards in those sealed packages until use. Try to control the environment of the assembly area when the boards are in production.

There is also a chance that there is some construction issue at the fab house. I've had this issue on two layer boards before that was eventually tracked back to insufficiently cured pre-preg on the boards. It was a task to get the board house to admit this.

Advise your board house of the issue, ask them to do a failure analysis on the boards; and consider using an independent lab to do a failure analysis, if it sounds like the board house won't help you.

2. Discolored silk-screen as shown in photo 1. I've had this issue numerous times, and never did determine a true root cause. I suspect it's something in the formulation of the silk screen, perhaps being insufficiently mixed. It does not create issues in assembly, nor does it affect operation of the boards, so, in general, I have ignored the issue.

3. Fiber color change, as shown in observation 1. I've never noted this enough to document it as you've done here; but, I have seen some color changes. In general, it hasn't affected operation, so it's been ignored. Some possible causes: 1. Different FR4 material used from one panel to another. 2. Curing issues during PCB fabrication.

Can you confirm that the color change happens in reflow, and is not existent before you process the boards?

If it happens in reflow, there's a chance that an improper TG material is being used for the different boards, and the reflow temps that you're getting to are affecting it. I've had board houses change material due to availability without advising me of the issue (until I question it), which is mildly aggravating. In general, they should change to a same spec or better material...in practice, that doesn't always happen.

In photo 3, I'm not sure what we're looking at for discoloration on the panel, so, I'm not able to comment.

In observation 2, dicoloration of the gold fingers...while I see what you're talking about, I'm not sure that there's an issue there. Again, you'd need to determine if the change in color is happening in reflow, or if it exists before processing of the boards. It's possible that the board houses plating tanks had become contaminated in some way that's causing the gold to change colors. If it exists before processing, it's likely a variation in the plating chemistry/process. If it's looking more like a tarnish after processing, there's likely some contaminate in the finish. Again, an independent lab analysis may be your best bet, as I've found board houses are slow to admit variations or problems in their process.

Finally, you didn't mention it, but, in photo 1, beyond the silk screen discoloration, there is a variation in the green color between the two panels. This is caused by a difference in the solder mask that the board house is using...either the first color wasn't available when they were needed it, or they mixed the color up differently. Either way, it's an indication to you that the boards were not all manufactured at the same time, and there is a high probability that there are material and chemistry differences between the lots. Any of those material or chemical differences could be responsible for the coloration issues that you're seeing, and, even could be responsible for the delamination (one lot ran with one material with no issues, the other lot had a material issue which is contributing to the delamination). Again, independent lab analysis would be able to confirm this situation, if the board house does not want to help with true root cause analysis. Getting the board house to admit there's a difference in materials and/or chemistry can also be difficult if all of the boards have the same date-code. From what I can see, there's very little chance that they were all fabricated at the same time, regardless of what the board house tells you.

Good luck!

cheers, ..rob

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#72672

Part of PCB silkscreen turns yellow brownish | 2 September, 2014

Thanks Rob for your detail comment. It is really helpful!!

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#72673

Part of PCB silkscreen turns yellow brownish | 3 September, 2014

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#72682

Part of PCB silkscreen turns yellow brownish | 4 September, 2014

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