Does HASL or ENIG Reduce Creep Corrosion in Corrosive Environments?



Does HASL or ENIG Reduce Creep Corrosion in Corrosive Environments?
We make electronics equipment used in corrosive environments. Do you recommend the use of HASL versus immersion gold for our application? The Assembly Brothers, Phil Zarrow and Jim Hall, answer this question.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
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Phil Zarrow
Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.
Jim Hall
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript


Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers, pick and place. We are here to discuss your process, applications, assembly problems and anything else that happens to even remotely fall into that category.

Today, Jim, we have a question that is a surface finish question. This is from M.G. We make electronics equipment used in corrosive environments. Experts suggest we use hot air solder leveling, HASL, as a finish on circuit boards instead of immersion gold. Supposedly this resists corrosion better. Do you recommend use of HASL versus immersion gold for our application? Would you recommend another type of finish?

Jim
Let's back up. HASL vs. immersion gold, I am assuming they mean ENIG, immersion gold over electroless nickel. Corrosive environment, I am guessing we are talking about creep corrosion and it happens in corrosive environments specifically where there is sulfur in the environment around the circuit board. The sulfur reacts with principally copper and in some cases silver to form a corrosive layer that spreads out and can cause shorts on your board.

Key difference, for those of you who are familiar with dendrites and electro-migration, which is copper plated out between conductors and causing shorts. Electro-migration dendrites require an electrical potential to drive the reaction. In other words, you have to have a voltage difference between two copper connectors that cause this reaction.

That is not true with creep corrosion. Creep corrosion is a reaction of the materials on the board with the sulfur in the atmosphere. It does not need any electrical potential to make it happen. It can happen anywhere on the board. It can spread out and it can be really ugly. It can cause short circuits. Experts suggest HASL vs. ENIG. With creep corrosion, my knowledge of it is that it is a phenomenon that is not well quantified.

Still a lot of testing, trying to figure out how to measure this stuff on a repeatable basis is still going on. The test reports that I have seen shows that there isn't some difference between different surface finishes but the results that I have seen are not consistent. I don't know that anybody is saying that hot air solder leveling is better against creep corrosion than ENIG.

I am guessing that the idea is that with hot air solder leveling because it is done as a soldering process it coats the edges of the pad, perhaps better, than the plating processes do in your ENIG. The exposure, the ability of that sulfur to get at some copper on the edges of the pad is maybe a little more likely on an ENIG board rather than a HASL board where the solder finish is wrapped around that corner because of the solder immersion during the process. That would be my guess.

My experience, if you are talking about creep corrosion it is not well-defined. I could not recommend one surface finish over the other with the exception of putting immersion silver as the most susceptible because of the potential silver reaction with the sulfur in the air. It is generally assumed that immersion silver has the highest tendency for creep corrosion. But between the other finishes, I have seen creep corrosion show on all of them. HASL vs immersion silver, I wouldn't comment.

But stepping back to the bigger picture of using HASL on your printed circuit board. Would you want to comment on that?

Phil
I think our participants know that I am not a big fan of HASL because of the variation of the topography of the finish it leaves on the board. Now if you are not dealing with fine pitch components, you could probably pull it off. One recommendation we would make is that if you are going to resort to HASL, if at all possible go find a shop that can HASL using the horizontal methodology. It is more even than the vertical. It is not new. People say you guys are dinosaurs, HASL has been around. No, horizontal HASL has been around for a long time.

It does give a slightly more even finish than vertical does. However, it is a much more expensive machine. Most shops do not have it. Most shops use vertical. If you have to resort to HASL, see if you can find somebody that can do a horizontal HASL. Beyond that, I would say that because you are dealing with a corrosive environment you have to do some of your whole testing it would probably make sense to experiment with different finishes.

Going beyond expert advice, antidotal or otherwise. Beyond that, show me the data. See what the data is that supposedly supports HASL over these other finishes or what the negativity of using an ENIG finish or something along those lines might be.

Jim
The only other thing is we have made the blanket assumption that we are talking about creep corrosion here. There are other potential corrosive things that can go on. All bets are off, if you are experiencing one of the those as opposed to creep corrosion.

Phil
Beyond that, I would say if you have a creep doing your soldering, just make sure he doesn't solder like my brother.

Jim
And don't solder like my brother.

Comments

Creep corrosion is a mean thing and you have to avoid it. The best bet would be to use a conformal coating. Use the IPC-HDBK-830 as a guidance for the best type for your application. If that is not possible or wanted then actually exposed copper at the side of lands and traces would be possible with all finishers. With the etching process the sides are exposed and can start to oxidize. The shorter the intervals between the steps in the manufacturing process between etching and applying the finisher the better, since then even the sides might get covered and there will be less copper exposed. If you would want HASL and a relatively flat surface then horizontal HASL would be best but only if the manufacturer can do it inhouse.
Frank Huijsmans, PIEK Int. Education Centre
IPC subcommittee 3-11G investigated creep corrosion with ENIG and found it could be a serious problem with the right mix of sulfur, humidity, and a catalyst like chlorine in the environment. Anyone using ENIG and concerned about sulfur should review their publications. It might sound non-intuitive, but immersion silver can be resilient vs creep corrosion when coupled with a durable anti-tarnish agent. A leading chemistry provider offers a coating that binds the silver, survives reflow thermal cycles, and leaves a finish that not only resists sulfur but also eliminates the unsightly tarnishing otherwise common to silver finishes.
Naji Norder, Emerson Test & Measurement
ENG is the way to go. In every case, be sure the boards are washed after soldering, to eliminate the chance the flux residue might still have some active chemicals that would lead to even more corrosion. The previous comments assume sulfur is key to corrosion. Well, another corrosive element is chlorine, as found around swimming pools. This can be an aggressive corrosive which most conformal coatings cannot provide protection from. Most coatings use chlorine as an activator in the chemistry and continued exposure to chlorine causes the coating to break down. I have found only one conformal coating that does not using chlorine in it's chemistry and is specifically rated for chlorine environments. If cost permits, after washing the boards, conformal coat them.
Jaye Waas, Renkus-Heinz
If you have any fine-pitch SMT components on the PCB (e.g. BGAs/CSPs) then keep away from HASL because you will run into trouble with reliability issues caused by the uneven thickness. If you PCBAs are going to be used in corrosive environments, you for sure should consider conformal coating. That will not only reduce risk for creep corrosion but also hinder Electrochemical Migration. Be aware that not all coating types will stop Sulfor so select material with care. Regarding creep corrosion risk, the Type of PCB surface finish is not relevant if you protect with a good coating.
Claus W├╝rtz Nielsen, E-Consult International ApS
I have direct experience with this exact situation and my experience has been that if you have a trace extending to a pad for a via or PTH which exits from under solder mask, there is likely going to be a small (micron scale) non-wet by the HASL at the interface between the soldermask and the copper trace.

We operate in a liquid environment that can have varying amounts of sulfur and sulfur compounds present and have experienced both creep corrosion across fairly significant distances as well as really a crevice corrosion extending under the solder mask. The ENIG finished versions of the same hardware do not exhibit the same exposed copper potential that the HASL boards do. And yes - a good thorough conformal coating will improve resistance to creep corrosion.
Mark Hampson, Collins Aerospace
Wouldn't conformal coating make a bigger a difference and also make the difference between various finishes less significant?
Stephen Olan, Stim Canada

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