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ENIG poor wetting

Rick Lathrop

#20938

ENIG poor wetting | 29 July, 2002

I have a customer with up to 8% scrap from one board supplier and 1% from another supplier for solder dewett and no wet over ENIG finish with mask defined pads. Initially I suspected mask residue on pads because of the no wets but I have come to find out that the electroless Ni and Immersion Au are applied after mask. These are 0402 pads and almost always end up in a tombstone. I have a local university doing some XPS work at the moment. Any ideas wouls be helpfull. I have seen the customes process and profiles and all loks normal. Our conpetiions paste yeilds similar failure rates.

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#20946

ENIG poor wetting | 29 July, 2002

Stand-ups are a common problem with 0402. Search the fine SMTnet Archives to get started.

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#20955

ENIG poor wetting | 30 July, 2002

When you talk about 'poor wetting', 'dewett', 'no wets', etc.; are you seeing solder pulling back or away from the pad? Please describe the condition.

Dewetting. The condition in the solder joint in which the liquid solder has not adhered intimately with one or more the components. Characterized by an abrupt boundary between the solder and the component lead or conductor. Can be distinguished by a "rolling back" of the solder from the lead or conductor.

We have seen dewetting, but it was in lots of non-wettable areas. It is most commonly caused by abrasive particles are implanted in the metal. But that's probably not what is causing your problem, because your have isolated it to 0402.

In your case, if these 0402 pads appear to be dewetted: * How does your customer assure that they are actually putting paste on the pads? * And where does the paste that they actually put on the pads go after reflow? * Are the terminations of the stand-up 0402 carrying the extra paste? * Does the volume of dewets per board, change over the course of a production lot? * Finally, when soldered by hand, do the problem pads take solder?

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Dason C

#20958

ENIG poor wetting | 30 July, 2002

Dave, I am very interesting to know about the dewetting or pull back, we are currently use the uP78 paste. Will nitrogen, profile setting help to reduce the dewetting.

Thank.

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Rick Lathrop

#20965

ENIG poor wetting | 30 July, 2002

Hi Dave, The paste applied to the board ends up on the tombstone. The pad has a thin covering of solder most of the time, this is why I think they call it a dewet. Occasionally the pad shows some areas of gold and very rarely are some pads not wet at all.They use the 2D vision on the printer to verify solder presence. As for isolating to 0402's, this is the only device size the customer places on these types of packages. The solder alloy is 96.5/3.5 Sn/Ag, peak reflow arround 245 for 40 sec.This is a water soluable application so activation is robust.

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#20973

ENIG poor wetting | 30 July, 2002

Dewetting: Solder does not adhere to lead or land, caused by: * Poor solderability of lands. * Poor solderability of leads. * Solder paste integrity. * Lead plating integrity.

Need more information, please: * What is dewetting [ie, component, pad, erwat]? Talk to us about the metalization on that surface. * How wide spread is the wetting, across the assembly, assemblies, components, component types, etc. * Tell us about the temperature profile that you measure where the dewetting is occurring. [If you are dewetting by force of your profile the dwell is probably too long.] * �UP78� is Alpha UltraPrint 78, a no-clean solder, right? Improper amounts of Cu, Au, Al, Zn, and S can cause decreased wetting and spread. So, have you been adjusting your paste?

Nitrogen will improve wetting in some NC fluxes.

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#20974

ENIG poor wetting | 30 July, 2002

Comments are: * Peak at 245�C for 40 sec seems like a very long time. I�d expect the time at 245�C to be 10 to 15 sec. This long time at peak could dewett the pads. * Really, their 2D vision on the printer can verify solder presence? * A thin coat of solder across the pad is NOT dewet. Islands or mounds of solder is more like dewett. Like this http://www.bobwillis.co.uk/defect_browser/pages/04.htm

* �This is a water soluable application so activation is robust.� See, that�s another thing that bothers me about this dewetting thing. It all just makes me want to trouble shoot 'tomb stoning', rather than 'dewetting'. It just doesn't feel correct.

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Rick Lathrop

#20986

ENIG poor wetting | 31 July, 2002

Perhaps I was too brief in my profile description the 40 seconds was time liquidous (T>221C). 2D vision can very effectively detect % pad coverage, I use it on my DEK and this customer has an MPM which I also believe to be capable. Here is another fact, when the customer uses Brand X solder paste the defects due to what they call pad dewet drops from 8% to 3% scrap, he showed me lot to lot yield data and this scrap varies quite a bit but the average is 8% with our material. I still keep gravitating to a board source in that board supplier S from Korea yields a 1% scrap level and board supplier U from Tawain yields an 8% scrap level for pad dewet. Also I have witnessed partial and complete pad no wet with good robust fillets surounding these defects. Riddle me this. I will be interesting to see what the PHD's in one of our local universities come up with using XPS.

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#21001

ENIG poor wetting | 31 July, 2002

Reflecting back on earlier comments, the majority of dewetting that we see was �in lots of non-wettable areas. It is most commonly caused by abrasive particles are implanted in the metal. But that's probably not what is causing your problem, because your have isolated it to 0402.� In dismissing that alternative, I made the incorrect assumption that there were components in addition to the 0402 on your board. Actually all you have is 0402. So, abrasive particles imbedded in the metal could be the source of your problem on just 0402.

This gets back to an unanswered question. => * Finally, when soldered by hand, do the problem pads take solder?

Yes, the XRF analysis will be interesting.

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Rick Lathrop

#21019

ENIG poor wetting | 1 August, 2002

Good morning Dave F. I am not going to try to touchup the solder pad until I get surface analysis work done, don't want to ruin the evidence. I am getting XPS work done not XRF, XPS is X-ray Photoelectron Spectroscopy and can see very thin films of organic. After reading up on the Black Pad syndrone my customer may have too thin Au, permiting the underlying nickel to be oxidized.

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