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SAC305 preforms not wetting to ENIG pad in fluxless process

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JerryM

#86501

SAC305 preforms not wetting to ENIG pad in fluxless process | 13 April, 2021

I have a SAC305 preform with which I am attempting to bond a gold coated component to an ENIG coated PCB pad. I've specified 2 microinches of gold for the ENIG. It worked previously but now with a new batch of boards, I am having an issue where the preform is just not wetting at all to the pad. The preform wets totally fine to the component, but not at all to the PCB pad. The component practically falls off after the reflow. Ultrasonically cleaning the boards in Acetone/IPA hasn't helped. Wetting to the boards with normal flux cored solder and a soldering iron works fine, so flux would probably help if I was allowed to use it (I'm not).

A few more details about the process. It is done under a cover gas to prevent further oxidation of the preform, and our peak temperature is sufficient so the surface oxides on the preform should dissolve into the bulk of the preform, allowing for soldering to commense.

I have noticed that from my PCB supplier, the visual appearance of the PCB pads has changed across batches. Some are very golden and shiny, others rather dull looking. Some even have occasional black spods on them(!!!). But I don't know if the issue is black pad as when the component falls off, the gold layer almost looks untouched. Could I be looking at a contamination or purity issue on the ENIG surface? The shiny surfaces seemed to bond better. Could specifying a thicker gold layer be helpful? Maybe 5 microinches.

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#86509

SAC305 preforms not wetting to ENIG pad in fluxless process | 14 April, 2021

Sounds like you may have already found the issue: inconsistency from your supplier.

Seeing as Gold really doesnt oxidize and you're using an inert environment (N2 I suspect), I would turn my attention to the supplier. You said yourself, the "Shiney" ones appear to solve any issue so I would start by suspecting there is some contamination in your supplier's process or your internal preparation. Gold gets dull over time as biofilms and organic contaminents coalesce on the surface. (eg. a fingerprint) - Have you tried cleaning the pads before reflow to see if that helps? (Bring out the shine*, as you say) Additionally, flux is designed to prep parental substrates for the bond to take place (eg. the flux is the cleaner). Without it, contamination can be a big influencer in the quality of your bonding.

I might also suggest that solder "migrates" towards the heat source. I would ensure your gold land target is getting equal heat in your process and isn't heat sinking on some aluminum fixture. Just a few items to check. Hope this helps!

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#86512

SAC305 preforms not wetting to ENIG pad in fluxless process | 15 April, 2021

In addition to confirming that the pad temperatures reach above the melting point of the preforms, consider checking the O2 PPM concentration.

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#86538

SAC305 preforms not wetting to ENIG pad in fluxless process | 26 April, 2021

Sounds like you are getting oxidation of the nickel under the gold layer. If the gold is porous it will allow the nickel underneath to oxidize, which is the dark dull layer you are seeing, and why when you apply flux it solders just fine - the flux is removing the oxides from the nickel layer. Check some boards from next batch where you see the dull appearance for porosity in the gold layer and some from a batch with the shiny appearance. Suspect porosity in one but not the other. Need to review gold plating process with the supplier.

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JerryM

#86542

SAC305 preforms not wetting to ENIG pad in fluxless process | 27 April, 2021

I have tried cleaning the pads. That was our first step. Ultrasonic cleaning in acetone and IPA, and also tried etching in a 10% HCL solution. Our board is definitely achieving the correct temperature.

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JerryM

#86543

SAC305 preforms not wetting to ENIG pad in fluxless process | 27 April, 2021

@KWalker I think that could be the case. Though I read somewhere a while back that nickel oxidation can be quite tough to remove, even with flux. Am I wrong about that?

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#86553

SAC305 preforms not wetting to ENIG pad in fluxless process | 27 April, 2021

Jerry,

just out of curiosity, why can't you use flux? Flux is essential part of soldering and it is there for multiple reasons. Flux we can also clean afterwords - even No Clean.

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#86555

SAC305 preforms not wetting to ENIG pad in fluxless process | 27 April, 2021

The fact that your cleaning didn't really help reinforces that it probably is oxidation. Cleaning won't remove that.

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#86556

SAC305 preforms not wetting to ENIG pad in fluxless process | 27 April, 2021

No, you're correct. Nickel oxidation can be difficult to remove

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JerryM

#86560

SAC305 preforms not wetting to ENIG pad in fluxless process | 28 April, 2021

@Evtimov I can't use flux as the components being soldered can't cope with any contamination whatsoever. Even an ultra low solids no-clean flux will be too much. I also can't clean the boards after soldering due to any residual contamination from the cleaning process.

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#86566

SAC305 preforms not wetting to ENIG pad in fluxless process | 28 April, 2021

Could you share some pictures? Without any flux or paste, how could you place preforms on pads? SMT Pick and Place ?

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