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Solderability on Immersion Gold

#4774

Solderability on Immersion Gold | 29 February, 2000

We have what appears to be a recurring problem. On immersion gold boards only, we are having solderability problems. After reflow, selected parts can be popped right off the board. There is then a dark haze left on the pad. The paste on the component is reflowable, but the pad will not reflow unless we scrape the "black stuff" off the pads. We initially indicted the boards and the board house admitted that the soldermask was put on after immersion gold instead of before and it was not properly cured and had gotten on the pads. However, we have now seen this problem on 3 different assemblies from 2 different board houses on 5 different parts pointing to a process problem. I would suspect profile, except for the unsolderable black haze left on the pads. Board storage cannot be an issue because the last 2 patches were build within 1 week of receiving the boards. Anybody dealt with this before?

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Glenn Robertson

#4775

Re: Solderability on Immersion Gold | 29 February, 2000

Steve - I believe you should ask who has NOT dealt with this before. Search the archives for "dark pad" or "black pad". It's a common problem for immersion Gold. Does anyone Know any other descriptive names that I missed (and can be printed here)?

Glenn Robertson

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#4776

Re: Solderability on Immersion Gold | 1 March, 2000

Steve: Someone is trying to trick you. He/she is trying to be very smart, but he/she didn�t anticipate this "dark haze" coating problem. I suspect that this person is someone you speak with every day in your plant. They may be the one that came-up with a really "well thought-out" method for your suppliers to package your boards or a similar in-house process at receiving.

Consider running: � Some bare boards through reflow to help determine when the haze shows on your boards in sorting through the haze-source. � Chemical analysis of the coating down at the chemistry or material science lab of a local university to ID it�s content.

My2�

Dave F

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#4777

Re: Solderability on Immersion Gold | 1 March, 2000

Glenn: I hear you, but when Steve talks about a "dark haze" and "scrap(ing) the �black stuff� off the pads" to make them solderable, but I don�t think of "black pad." I think of "black pad" as a surface coating that typically can not be removed by scraping. Also, he�s got the stuff on a bunch of different boards, from different suppliers. I think Steve�s boards have the crud. My2� Dave F

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#4778

Re: Solderability on Immersion Gold | 2 March, 2000

Hello,

I had experienced this kind of problem; Because the Au film is very thin (flash gold..) it is sometimes porositive and the Ni barrier below get Oxidation. After reflow there seems to be good solder fillets (visualy perfect!) but the leads with the "nice" solder fillets comes appart AT THE INTERFACE Au/Ni.

This in one of the disadvantages of an electroless process.

Roni

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#4779

Re: Solderability on Immersion Gold | 2 March, 2000

Allmost forgot:

The "black pad" is simply the oxidized Nickel.

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Dave Pearcy

#4780

Re: Solderability on Immersion Gold | 2 March, 2000

We are experiencing the same problem. We just spent $1,200 to figure out that the "black stuff" is a Nickel Oxide. I am going to look into the profile end of this problem. Any suggestions would be very helpful.

Dave

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Glenn Robertson

#4781

Re: Solderability on Immersion Gold | 2 March, 2000

Dave -

You certainly have a good point regarding the multiple part numbers and board suppliers, but a rework process for black pad has been reported. See the SMTAI 1999 Proceedings article by Zequn Mei (page 407). It involves flux and solder wick - not scraping. Being a relatively short timer in the business, There are a lot of things I have not seen. What type of "crud" do you have in mind?

Glenn Robertson

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eLDON sANDERS

#4782

Re: Solderability on Immersion Gold | 2 March, 2000

Gentlemen,

I too have been atacked by that "soldering to gold" and will add my 2 cents.

I also had parts that would pop off the board when the fillet looked perfect. After lots of pain, experimentation, and sleepless nights, I solved it through a better understanding of the soldering process of gold boards. The fix was in the profile.

The boards were small and thin and my first mistake was using a fixture to reflow them in mass. To get a reasonable thermal profile I needed more time in the oven. I was also afraid of getting the boards too hot, and kept the peak temp at 210. With this I created two problems. One was the preheat time. It was too long. After the flux burned off there was time enough to oxidize the nickle anywhere oxygen could get to it. I proved this by applying some flux to the bare board and running it through the oven, getting dark pads at the other end. Without the flux the pads were not nearly as dark when just the bare board was sent through the oven. The other problem was the liquidous time and temp. With gold, the solder joint forms to the nickle, not the gold. The gold is removed by the tin in the solder. The tin breaks the tin-lead bond and forms a bond with the gold,leaving the lead behind. (As a sidenote, when too much gold is on a board, the joint will be very weak as the tin-gold alloy is brittle and the lead seems to concentrate at the pad surface, which is nickle. This lead nickle interface is extreemly weak.) What you are really soldering to is the nickle, not copper and not gold. To solder to nickle the temperature needs to be hotter. The time must also allow the tin-gold alloy to mix in with the rest of the solder, otherwise you have all of the tin-gold at the interface of the pad, along with the lead the tin left behind. You want this interface to be solder and nickle, and form a proper intermetallic.

In my situation, I removed the boards from the fixture to minimize the mass, I cut to the minimum time for the preheat period based on the paste manufacture recommendations, I increased the peak temp to 225, and I kept the solder liquidous for about 50 seconds. The time and temp were decided on by destructive testing of many boards in an experimental basis to determine the greatest resistance to solder joint breakage. This profile for me resulted solder joints breaking at the component end, or the component itself breaking.

I put about 6 months of effort into this problem, and it was not fun. But I learned a thing or two, and I hope I have passed some of that on. Good luck!

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#4783

Re: Solderability on Immersion Gold | 2 March, 2000

Dave: It's good to know what that stuff is. With your NiO, you have the typical ENIG problem. I can imagine some causes for your disaster: * too thin gold plating * oxidized nickel under gold plating * porous nickel plating * wrong soldering profile.

I hope Eldron's profile suggestions are helpful.

Good luck

Dave F

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#4784

Re: Solderability on Immersion Gold | 2 March, 2000

Glen: As I mention in an earlier response in this thread, I think someone at Steve's plant (or may be in his packaging spec for the fabs) is doing something to leave a residue, but I'm also thinking solder mask ...

ENIG can be applied before or after solder mask. There are advantages/disadvantages to both.

MASK FIRST: Applying solder mask then gold is typically cheaper since less gold plating is involved. The biggest potential drawback is the solder mask can be attacked by the nickel plating bath. The extent of the attack is dependent on the solder mask used, some masks are more susceptible than others.

MASK AFTER: Applying gold then solder mask can cause solder mask adhesion problems to the gold surface. Solder mask residues can also be left on the gold, resulting in solderability issues during assembly.

Good luck

Dave F

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