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Gold Immersion - Soft Joints

William

#8831

Gold Immersion - Soft Joints | 25 October, 1999

All, I am having a problem with joints being soft after SMT. Especially 25 mil and smaller parts. It seems as though you can slide the leads right off of the solder. It takes only about 30-40 oz. of compression at 45 deg. to move leads off of joints. With HASL boards, however, I almost rip the pads off before the lead seperates from the solder. I have increased the time of reflow to almost 80s, but the joints have had no substantial increase in strength. We have experimented with more agressive 63/37 solder pastes, and that has helped some, but the joints still appear to be weak. We have also monitored the board house's gold process, and they have ensured us that the gold is w/in 5 micro inches. They also check their nickel bath every 4 hours for phosphrous content. Their nickel bath, however, is electroless. I have heard this may cause problems. We have also requested they decrease the time of gold bath by half, and this has not helped. In any case, we are not experiencing this problem with another PCB supplier. We currently use 5 mil stencils with 50-55% aperture reduction. I have never seen this problem before, but I don't have much experience with gold. Some help here would be greatly appreciated.

Does anyone have any suggestions on what we can do to increase the strength of these joints?

Does anyone have a spec for compression tests on joints?

Is another type of solder paste (other than 63/37) preferable w/gold?

Regards, William

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#8832

Re: Gold Immersion - Soft Joints | 25 October, 1999

William: Three things:

1 If the gold is porous, it will not protect the nickel and the nichel wiil oxidize and become unsolderable. 2 If the nickel is not solderable, the gold overcoat will not improve that. 3 Ag or Au over electroless Ni creates brittle joints because of phosphorous in the Ni plating bath. The P migrates into the over-plating ... Electrolytic (Ni & Ag/Au) plating should not be a problem.

Good luck

Dave F

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William

#8833

Re: Gold Immersion - Soft Joints | 25 October, 1999

I am pretty sure I am not having a problem w/porous gold or nickel that does not solder. How about electroless versus electrolytic? I thought electrolytic was for goldfingers and such applications. Can you use electrolytic plating for fine pitch and discrete surface mount? Is that an option in trying to solve this problem?

Regards, William

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#8834

Re: Gold Immersion - Soft Joints | 25 October, 1999

William: That your solderability improves with more aggressive fluxes tells me that you do have a gold porosity problem. So what makes you so sure about the thickness of the gold plating? How did you measure it? I think your supplier should do some lab work on good and bad boards and tell you the answer.

The issue is not electroless versus electrolytic. The issue is thickness of the gold plating, because "high" (>3%) concentrations of gold form brittle intermetallic compounds with the tin in your solder.

Electroless gold is applied with an autocatalytic process that self-limits around 5 mils thickness. You're correct electrolytic gold is used for goldfingers and similar wear applications. Electrolytic gold plating thickness does not self limit.

Ta

Dave F

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William

#8835

Re: Gold Immersion - Soft Joints | 26 October, 1999

Dave, Last question(s). What do I do about a gold porosity problem? Dave: How would I approach the board house to check for this? They use a very well known chemistry, but is that where the pblm lies? Or would it be in their process, cycle times, temperatures, etc.? The more I think about, you might be very correct about that. That is why I would see perfect gold thicknesses, no phosphorous contamination in nickel bath or after completion of process, and noticeably good joints to naked eye. But some of the nickel is getting oxidized due to porous gold, therefore I solder but not well. Appreciate your input on this....William

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#8836

Re: Gold Immersion - Soft Joints | 26 October, 1999

William: In answer to your questions:

1 "What do I do about a gold porosity problem?"

Get it fixed, before you go out of business!!! You�ve been yapping about it on the internet for three days now!!! Get a more on!!! I don�t understand your company�s problem resolution process, but I would expect some focal point for solving problems, like Quality Assurance, and would have the following to get the process started:

� A trouble reporting method, like a corrective action request. AND � A team oriented troubleshooting process, like a Material Review Board.

If you don�t have something like that, contact the buyer responsible for purchasing boards and work with him/her on developing an approach.

2 "How would I approach the board house to check for this?"

Again, I�m not sure how your company goes about stuff like this, but I expect it would go something like this:

� I wouldn�t contact the fab directly without release from Purchasing to do so, it�s a nice courtesy. They should be aware of the possibility that you�re going to trash their supplier and/or increase their costs. Odds are, Purchasing will make the first contact with the supplier and get back to you. � Next, Purchasing tells you the supplier will meet with you just after lunch, that�s 2 PM in purchasing agent time, to discuss the problem. � Purchasing and the supplier return from lunch, waddling noticeably. � Purchasing says "I�ll leave you two to get on with this." Which translates from purchasing agent speak to: "Phew, I could use a nap!!!! Bye now." � You take the supplier to the shop and do your A, B comparison thing with the boards. You know, "we took special care to use identical processes on these boards, one from supplier A and the other from supplier B. Yes, same storage, same board age, same paste, same reflow profile, same oven �bla, bla, bla. Now look, the components on board A pop right-off the board, yet the components on board B are firmly attached." And on and on. � Stand back and let the supplier to his/her thing. � Don�t let the slimy little jerk get his/her hands on the good board, because you�ll be killed. First, your other supplier will kill you and drop your company as a customer, and then you�ll have no suppliers, and then the buyer will kill you. � Tell your supplier you need a written root cause analysis and corrective action. And that�s it up to him/her to agree with Purchasing on the plan for that and how the defective product will be handled. � Return the supplier to Purchasing. Of course, knocking first.

3 Does the problem lie in the board house�s chemistry or process?

I don�t know (and what�s more I don�t care, although as you are, I am curious). The fab is responsible for providing boards that are built to spec, according to schedule. How they do that is their business and that�s why they get paid BIG BUCKS. But then again, their children do seem to have a strange far-away look in their eyes and they�re all bald. (Right EM?)

Answer me this:

1 You say, you "see perfect gold thickness." How do you know that? 2 You say, that you see "no phosphorous contamination in nickel bath or after completion of process." How do you know that?

One final note: As someone (I wish I could remember who it was, so I could give him proper credit) said (I�ve taken liberties with the paraphrase) recently: It�s not clear that the benefits of the planar surface of ENIG is worth the hassles of trying to solder to that surface.

Good luck

Dave F

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William

#8837

Re: Gold Immersion - Soft Joints | 1 November, 1999

Thank you so much Dave for all of your wonderful help. You know, all I really wanted to know was a little more about the gold porosity problem. Since I am not an expert on gold immersion, nor do I claim to be, I just wanted some information so that I could approach the board house with some kind of knowledge. Everything else you wrote was a complete waste of mine and your time. I had already been in contact w/the board house, visited their facilities, examined their process controls, measuring techniques, etc. They have been to my place also and examined our process as well. We have both been running numerous controlled experiments trying different proposed solutions. I just thought you might know something that could possibly help w/this problem. And for your information and "curosity," it wasn't a gold porosity problem at all. The solution lies in simple physics. And Dave, why drill someone when they were only asking a question?? To say you missed the point would be to kind.

Best Regards, Will

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#8838

Re: Gold Immersion - Soft Joints | 2 November, 1999

William: I'm sorry to seem to drill you that was not my intent. I'm glad you have a solution to your problem in-hand. Dave F

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jason

#21373

Re: Gold Immersion - Soft Joints | 30 August, 2002

Hi William,

Can u help me out here. I am new at handling Au imm. PCBs, and I hope you can answer me these questions. 1) If Au has soft joints issues, why use it at all ? Not to mention it is extremely expensive. 2) What are the precautions that I have to take dealing with this Au imm. boards. Thanks in advance.

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