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Unusual solderability issue

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Unusual solderability issue | 15 April, 2010

Hi all,

Long story ahead.... We are having issues with board vendors on some of our higher layer count (thick) boards. Specifically under BGAs with high aspect ratio vias (14:1). We are using ENIG finish and a lead free process.

Basically we get two or three (sometimes many more) random pads under the BGAs where the gold is still perfectly in tact after reflow and removal of the BGA. It also occurs on decoupling caps under the BGA. We've verified our profiles. I've also done simulations without a component by applying solder paste to the pad and using hot air to reflow the solder. The pad does not wet, the gold does not flash off. However if I use and iron and lightly move the iron over the bad while soldering I can get the pad appear to wet. I cannot however confirm an intermetalic. (no SEM in house!) Failure rates on affected boards range from 10% to 40% failures.

These defects only occur in areas of high density, high aspect ratio vias. It can occur on either side of the board (BGA side or decoupling caps). We know there's something over the gold but we can't be sure what.

We've had the issue with three different board shops, each has come to their own "solution".

1. The first shop has changed it's rinse/dry cycle on these cards. The theory is that there is chemistry trapped in the vias that is affecting solderability. We've only ran limited boards from this shop since the issue. It took them so long to figure it out that we had to move the boards to other shops.

2. The second shop moved to a SMOIG process. Keeping the vias clear of SM to help the cleaning process after ENIG. So far we are seeing better results but recently we've hit a small patch of issues but these appear to be more like black pad not the exposed gold issue.

3. The third shop is struggling. What's odd is they were our best shop and have been building these difficult boards for us for almost 2 years with no issues. Now we're seeing this creep in on several PNs. Their initial root cause was incorrect mesh size used during via tenting. They implemented changes but now we've seen it again. We've tried to hint at the solutions from the other board shops but they have been avoiding looking in that direction.

In the end I'm not sure any of the shops have really found the root cause.

I'm sure I've missed some key info along the way but that's the basics. If you have any ideas on what the real root cause of these issue could be please comment.


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Unusual solderability issue | 17 April, 2010

Sounds like you and your board suppliers have done alot of work with this issue trying to solve the problem. When I got into this situation previously, I relied on an outside lab to help me. We didn't have SEM or any testing capability at my shop either. Contact Larry Bross or Ron Sweezo at Specialty Labs ( Larry would give me alot of information over the phone and help me to understand potential causes. Their test results were impeccable and affordable. It would help me to save time if an issue was dragging out too long and the problems were still occuring. Good Luck!

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Unusual solderability issue | 19 April, 2010

Hi, if you want to verify the gold contamination, is mandatory to use the XPS and FTIR analysis, but is difficult think about a gold contamination from 3 different manufacturers. Do you have cleaned the PCBs after an erroneous screen process ? If not, I suggest you to verify better the reflow profile and the chemical of solder paste. Many lead-free solder pastes lost most of chemical activity before reaching the peak zone. So the solder pastes show too much activity in the first part of the soak zones, resulting in poor performance in the higher temperature areas of the solder process. In any case, the answer can be found only after a correct tests program. There are a lot of thing to verify, but in order to understand better all the phenomenons that occur during the production, is important to study a good handbook before, to avoid to loose a lot of money in erroneous failure analysis... Good Luck,

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Unusual solderability issue | 19 April, 2010

We have had some unusual reflow conditions with BGA's as well. We did not come to the conclusion that something was covering the lands though. It typically occured with only the BGA lands (not the decoupling caps on the back side). We found that the mass from the lands,PCB substrate, and part, took more heat through the reflow process than expected. We literally drilled a hole through a board underneath the BGA and afixed our thermocoupler from our heat profiler to the PCB between the BGA and the PCB. The actual temperature between the component and the PCB was much to low to get a good relfow/wetting. Since we increased our oven profile temp we have not had any of these issues.

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Unusual solderability issue | 19 April, 2010

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* Trace Laboratories - East; 5 North Park Dr, Hunt Valley, MD 21030; 410-584-9099 F9117

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Unusual solderability issue | 19 April, 2010

Thanks to all for your input.

We did have a third party do an analysis on boards from the first shop. They were shipped directly from the board shop to eliminate the possibility that we could contaminate them. The third party (Forsite) did find Silica along with traces of chloride, sodium and sulfate. They stated that the level of silica was high enough to cause solderability issues and recommended a cleaning process. The cleaned boards were perfect. Unfortunately by the time we received these results we had moved all our high end boards away from this shop so we still can't confirm that the shop was able to produce good boards ongoing.

We've gone through our profiles many times. What makes us relatively sure it's not profile related (I'm not naive enough to say it's imposible!) is that the pads won't solder in our hot air test. We simply place solder paste on the pad and use hot air to bring it to reflow right in front of our eyes. On any other pads, even fiducials the solder wets across the pad as soon as the solder reaches liquidous but on the suspect pads it balls up and will actually blow off the pad if you're not careful. Also note that the defect shows up on the bottom of the board as well but only on parts directly under the BGA (decoupling caps etc). This rules out the mass of the BGA as a cause, at least for the bottom side. I've attached pics of bottom and top side defects. The bottom side clearly shows the solder wetted to the parts but not to the PCB. The BGA pad is totally devoid of solder and I was unable to solder to the pads unless I mechanically scraped the pad with the soldering iron.

I'm expecting an update from the third shop in the next two days. I'll keep everyone updated. Thanks again for your input.


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Unusual solderability issue | 23 April, 2010

Hi all,

The board shop finally got back to me with some results. They did find residue present on some of the pads. They have sent a board out for evaluation to learn what it is. Hopefully from that we can determine the root cause.

The good news is that they are able to clean the residue. We confirmed by running two boards through our print and reflow process without components. The first board was left as is and the second had been cleaned at the board shop.

As you can see from the attached photo the contaminated board many pads did not solder. Again this only shows up on BGA pads.

It will be interesting to see what the results of the analysis are and to finally figure out the root cause. I’ll post when we have further results.



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Unusual solderability issue | 6 May, 2010

Unfortunately nothing ground breaking to report. The board shops analysis (SEM/EDS)showed high levels of carbon and oxygen, likely indicating an organic based contaminate. They are still stumped on where it is coming from.

We recieved some boards yesterday with the same issue right out of the packaging.

My guess is chemicals (plating or uncured SM) leaching out of the vias after the final rinse. they are using only DI water which I suspect would have a hard time flushing out .008" diameter holes @ .140" thick with one end plugged. We've suggested they try a saponifier to reduce the surface tension and possibly allow for a more thorough cleaning. Just my thoughts though. We are considering removing the capping process to give the board shop a better chance of cleaning the vias properly as well.

The saga continues. I'll update again as more events occur.

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Unusual solderability issue | 7 May, 2010

Hi Derek,

just curious of one thing - are the vias within the BGA's the only vias covered or are all vias on the board covered. Just trying to figure out what is making this area different from the rest of the board.

Regarding leaching, I do not believe this would affect you on the capped side of the board, typical if there were trapped fluids or residues they would leach out of the uncapped side or you would see blistering were the caps have lifted or would show staining.

Contaminates with Enig, silver & tin finishes can happen in quite a few placed do to they are put on just after Solder mask. which means boards still need to go through routing or scoring, electrical testing & packaging, this leave a few areas were contaminates can be introduced. the boards would go through two rinse dry cycles were my guess it is getting the contaminates - first with this product would be in the enig line and second would be Rinse drying after boards were routed or scored to remove dust. If I was to guess typically it would be due to a poor rinse cycle after the Enig was applied.

FYI: The only two finishes I know of that are applied just before vacuum sealing and see less handling and less chance of contamination would be OSP & Omnicron Silver line. Not many shops use Omnicron silver process.

Regards, BoardHouse

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Unusual solderability issue | 7 May, 2010

Yes, the vias under the BGAs are the only ones capped.

I tend to agree with you that the leaching should affect the bottom more than the top and that was actually the case with the two previous shops that had this problem. We suspected with those shops that the only reason the top was affected was because the boards were stacked leaving the BGA pads exposed to the bottom of the board above.

This is what makes this one harder to figure out. Symptoms are exactly the same except that all the defects from this most recent shop are all on the top (BGA) side. Oddly this shop is the only one that puts a sheet of paper between each board before packaging in the MBB.

We're also trying to focus the shop on any process/chemistry changes they've made. I still find it strange that they've literally made hundreds of good boards for us and it's suddenly changed? The other shops had trouble with these boards right from the start.

The picture attached is a SEM image of the contamination. Just for fun!

Keep the ideas/questions coming! Thanks.


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Unusual solderability issue | 28 October, 2010

Ok, just to put closure on things I figured I should post the conclusion of this whole ordeal.

We've acquired a new BGA rework station that allows us to get away without tenting our vias under the BGAs. This allowed us to change our PCB spec to remove the tenting. Since the change we have seen almost no solderability issues. Hopefully this is the end!

Thanks to all who gave insight.


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Unusual solderability issue | 29 October, 2010

For my opinion, this will not the end for you yet if supplier unable to provide you the rootcauses and corrections. I faced this more than 3 times within 2 years. All type of lab tests (SEM, FTIR,cross-sectioning,solderability tests, and etc.)

They are all Plating related issues.

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