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SMT electronics assembly manufacturing forum.


About the PCB gold finger contamination.

bugsjoe

#21630

About the PCB gold finger contamination. | 18 September, 2002

I'm working China ,in my working factory is a electronic OEM factory and got COB and SMT. But recently, COB got a big problem because the PCB gold finger got a contamination, that make us got a most defect product. So anyone can answer me how to clean out the contamination? I've been use the rubber or cleaning solvent to clean but not successful. In now I still got a lot contaminated PCB at warehouse. Please anyone can give me a suggestion.

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#21631

About the PCB gold finger contamination. | 18 September, 2002

Please give us background on your situation, so that we can focus on the specifics, for instance: * This is a bare board problem, correct? Then, this contamination causes what the problems in your process? * What is the type [ie, FR4, CEM1, etc] of board, including solder mask [ie, LPI, wet film, etc] and solderability protection [ie, ENIG, HASL, etc]? How thick is your gold? Is there nickel beneath the gold? If so, what is the thickness of the nickel? * What is your COB process? What is you die attach? Is this flip chip or wire bond? If wire bond, what wire type and size and what attach process? * Where in your process flow is your COB attach? * When you contamination of the gold finger, are you talking about the fingers at the edge of the board that makes contact with a connector or the pads that are connected by solder or wire?

... that kind of stuff.

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bugsjoe

#21632

About the PCB gold finger contamination. | 19 September, 2002

Thank you ur reply Dave. Let me explain our COB workflow: 1. unpack PCB (FR4,LPI,HASL)(the thickness of gold and nickel we can't be find)(No SMT before) 2. clean the pcb by rubber 3. blow the pcb by di-ionic gun 4. attach die by adhesive gel 5. cure the adhesive gel at oven (100 C ,30 min) 6. bonding (wire bonding,bonder ASM AB509 ,Al wire 1 um) At step 6 we discover the pads and the wire cannot be connect.

At this moment we suspect the PCB vendor give us a bad quality PCB, but can i solve the problem ASAP? For your experience, what kind of contamination on the PCB? If need any other info i'm willing to give you. Thank you for your helping!

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#21643

About the PCB gold finger contamination. | 19 September, 2002

Comments are: * Use an eraser if the surface of Au is not clean, but make sure the eraser has no/low ionics (such as sulfur that is a common additive for rubber). * If the plating surface required eraser to remove the top surface, the plasma normally would not do the trick. Plasma is great to remove MINOR organic residue. * If your gold is too hard (impurity or voids concentration are high), you will have problem in bonding and have a reliability problem. (Eraser, brush, plasma, etc. are not going to help you)

Specific steps to resolve this problem will depend on the composition of the contaminant.

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bugsjoe

#21644

About the PCB gold finger contamination. | 19 September, 2002

* What kind of the eraser can clean clearly?I've been use the eraser but the defect still present.

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Sam

#21645

About the PCB gold finger contamination. | 19 September, 2002

We had encountered similar problem before. But the cause was found out to be the poor good plating, instead of contamination. The way we dealt with that was sending them to the supplier for rework. It may be the fastest and most relaibility way.

Good Luck Sam

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#21646

About the PCB gold finger contamination. | 19 September, 2002

I'm not saying that an eraser will necessarily work. What I'm saying is that some erasers may cause more trouble than the eliminate. Look here http://aic.stanford.edu/jaic/articles/jaic28-02-003_2.html

We prefer a brand called Ruby Red. The "ole pink pearl eraser" repair method does not work nearly as slick as electronics mythology would lead you to believe! The composition of most erasers includes several things that act as very good abrasives thus scrubbing off a fair amount of many kinds of surface contamination, like the sulfur that I mentioned earlier. But - you also leave a great deal of embedded eraser particles in/on the "treated" surface which can lead to latent field failures depending on your use environment.

If you put to much thought into the eraser bit, you'd drive yourself crazy. Here's what could happen using an eraser. Most erasers are silicone that is loaded-up with some grit. So, you're taking a silicone and rubbing 99.99% soft gold bond leads. Rubbing soft gold with an abrasive will remove stains, but as well remove the 30-50 micro inches of gold and exposing the nickel. Silicone is an organic and in essence, you are spreading organics all across the gold surface. I have wire bonded to erased gold, with exposed nickel, and had good bond pull XBAR (high teens) and low sigma (<0.75).

While I'm talking about brands, I've never tried this because we don't have the stuff here, but you may, being in the Far East, but I heard that "Darlie" worked wonders! Use the 'Original' formulation. Are you're better off with your eraser or your toothpaste? Tough choice, eh?

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#21647

About the PCB gold finger contamination. | 19 September, 2002

Sam,

You're correct. And that's what Joe would prefer to do with his contaminated boards, but he's trying to keeping line running, I believe.

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#21683

About the PCB gold finger contamination. | 22 September, 2002

Stop the lines...! Stop building defects and rework into your product. Contact Fab Supplier and have pipeline filled for small amount of replacement fabs to keep lines running and limiting production scheduling and impact. Request to have the board supplier send in quality representative to assess defect condition. Remember to generate and deliver a Supplier CAR requesting investigation of root cause and corrective actions short and long term. Often times a supplier will have noted a process issue by lot code, and implemented corrective action prior to delivering your fab's. I have been involved with one such issue recently involving an Aluminum Scrub that caused many of the issues you've identified. Scott

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dragonslayr

#21694

About the PCB gold finger contamination. | 23 September, 2002

yea - yippie- Scott's the man!!!!

good answer - I support the notion that in the long run, stopping the line, getting the root cause determination, is not only effective but efficient.

It is hard to do because we all are so used to fire fighting on a daily basis. In fact, I feel it is in our inherent nature to do fire fighting. Nothing like an adrenaline rush as the engine gets running into high RPM's, the sirens a wailing and we get to pull our hoses out to play with all day.

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