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Black pad on BGA after removal

Jimmie

#34079

Black pad on BGA after removal | 4 May, 2005

We have seen the black dot on the BGA pad after the component has been removed from a PCB. When most likely could this corrosion have occurred, while the component was in storage, during manufacturing of the unit? We have a number of these BGA's that are several years old, we are not sure how they have been stored prior to our receipt. Is there anything that can be done prevent the growth of this corrision on the remaining BGA components? Best regards jimmie

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#34088

Black pad on BGA after removal | 4 May, 2005

First, you cannot determine if this is "black pad" by looking at it. You must do laboratory analysis, either microsectioning or EDS. [But it sure sounds like "black pad."]

Second, if it is black pad, it was created by the company that fabricated the board with the pads that are failing ... the BGA interposer. There is very little that anyone [except the interposer fabricator and the BGA assembler, obviously] can do to decrease the potential of these failures occuring.

Finally, probably the best thing you can do, at this point, is start damage control and attempt to recover your costs.

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#34090

Black pad on BGA after removal | 5 May, 2005

I tried soldering to several of the black pads and was successful, the solder stuck with no hesitation. so i guess this is not truly "black pad"? would that be a correct statement?

What causes this blackening that looks like oxidation under the microsope? (maybe mild oxidation?)

These components are not in our possesion (long story), i have a feeling they are not vacuum packed. Will this promote any oxidation?

I have seen pads in the past that looked the same and solder would not stick AT ALL, i had to scrape the pad before it would stick.

Do you have any links for studies or documents that i could pass on? Thank you! jm

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RDR

#34094

Black pad on BGA after removal | 5 May, 2005

Just went through this with a customer. Removed BGAs and noticed discolered pads at some locations. It was determined that these were open connections that had some type of flux migration between the ball. They also took solder readily. This may be what you are seeing also.

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#34096

Black pad on BGA after removal | 5 May, 2005

Was it determined what caused the flux migration?

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RDR

#34097

Black pad on BGA after removal | 5 May, 2005

Bad cleaning practice is suspect. It is believed that W.S. flux was used to solder adjacent components and then hand cleaned. This is not definite but it is pretty certain.

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#34098

Black pad on BGA after removal | 5 May, 2005

could there have been a solderablity issue with that pad to begin with, allowing the flux in (from improper cleaning of adjacent work) to expadite an error may have happened later down the road?

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RDR

#34100

Black pad on BGA after removal | 5 May, 2005

Not in this case, these BGAs were lead free and the profile used was for a lead part. I guess I will give you the whole story.

These boards were sent to us from a customer to have the BGAs replaced for an unknown to us reason. We removed the parts which were lead at that time and replaced them with lead free parts that they did not tell us they had changed. since we perfrom this operation for them on a continual basis we reflowed using a noclean paste flux with the same profile we have always used which obviously was too cold. This left these components with a number of open connects (the majority had "reflowed"). This customer then had their assembler add additional components in the surrounding area. They utilize hand cleaning after they perform this operation. They do not know for certain that no-clean flux was used but we believe it is water soluble due to the dark grey pads/solder that we observed. I have seen this dis-coloration before on un-washed assemblies that utilized the water soluble processes. This is how we determined that this was the most likely cause.

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#34105

Black pad on BGA after removal | 5 May, 2005

Would a lesser degree of black pad still allow solderability as i seen in my case?

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RDR

#34106

Black pad on BGA after removal | 5 May, 2005

I don't believe that you are seeing black pad. I think you are seeing an oxidized pad from your statement that these pads take solder. I would guess that these will be fine. But I would re-tin all pads prior to attaching the BGA to make sure.

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#34107

Black pad on BGA after removal | 5 May, 2005

The black pads that we see are on the BGA component. We removed the component from its site and on 2-3 pads 100% of the ball stuck to the pad on the PCB and the corresponding BGA pad was black.

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RDR

#34108

Black pad on BGA after removal | 5 May, 2005

You got what I think someone has called a "cold melt". If you were to run your removal profile awhile longer you would not see this. Anyway, you don't have any issues as this is very common (at least in my experience). FYI I was thinking your oxidized pad was on the board and all of the stuff I was saying would be irrelevant to this situation.

Russ

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#34109

Black pad on BGA after removal | 5 May, 2005

Thanks Russ and Dave for time and patience.

Best regards jm

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