Blow Holes After Switching from Lead-Free HASL to ENIG

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting. Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow With over 50 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 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 from ITM Consulting, The Assembly Brothers. We are here to discuss techniques, methodologies, problems with your electronic assembly. Jim, our first question is from CS.

After switching our PCB finish from lead-free HASL to ENIG blow holes started occurring in the solder joints of the through-hole LEDs. The PCBs and LEDs are from the same suppliers as before, only the finish changed. Can the epoxy bump on the bottom of the LED create an air-tight seal on the top of the through-hole pad causing solder to blow out the bottom during wave soldering when using ENIG versus HASL finish. Any other thoughts?

For this application, ENIG would be the better choice of finish. The reason being it is a very flat finish. It is very uniform in terms of thickness. Versus HASL, even the newer version of HASL still create a topographically uneven surface. I know we are going to hear from the HASL fan boys, but just stating the truth. Assuming the ENIG finish would be the better choice. Jim, what do you want to weigh in on this?

Jim
If as you say, nothing else has changed but the finish and if the topography, I agree with you Phil. If we think about creating a HASL plated through-hole with lead-free alloy where the process for HASL is to completely fill that hole with molten solder, molten SAC assuming that is what they are using. Then blow it out with an offset air knife and then level it with a balance of air knifes on both sides of the board. Surface is not uniform.

First off it is bowed because it is put on molten and is controlled by the surface tension. But also, just that blowing out and leveling process, you can see the knee of the hole as not being very uniform. Therefore in this case, allowing gaps if the LED is pressed down and the surface of the epoxy bump on the bottom of the LED is not totally uniform knee of the hole geometry you might have holes that can allow gases to expose. Whereas that would not happen with ENIG because it is uniform, you get a nice uniform knee on your hole.

So, if they stay with the same printed circuit board house it would have to come down to the actual ENIG process. It would have to be either the gold or what else can be causing blow holes? Well, if there is any contamination or other chemicals that get involved and get into that solder joint when you are trying to wave solder it. They could be boiling and expanding and creating blow holes also.

So where do they come from? Well, they could come from improper application of the ENIG finish. You probably think more of this Phil. I am thinking core cleaning. All of the plating process final gold is put on the PCB, including the barrel of the hole. That has to get gold on it too. Now you have to put on an electroless plating process.

There is a whole bunch of other chemicals involved with the gold to make that plating reaction take place. All that individual chemistry has to be removed. Was it cleaned properly? Is your PC fab although it is at the same place, it is two different processes they are doing HASL vs. ENIG. So maybe their HASL process is okay, but their ENIG either the gold isn’t being applied properly, there is some contamination in the bath, or you are not cleaning all of the residues out of that hole at the end.

Either of those could cause there to be other materials in the hole when you try to fill it with solder and they could boil, expand, and cause blow holes. The solution for this kind of issue is typically to stand off the components. That is very traditional with connectors. Where you put the whole body of the connector down and you get blow holes and things because as CS indicates to create a tight seal at the top of the hole you can attract bubbles which cause blow holes. So, the solution is to stand off the component.

I do not know at all if this is practical for these LEDs, which tend to be really small. Whether there is a practical way to ensure that there is a stand off so that you have a gap regardless of what surface tension that you have a small gap between the bottom of the LED and the top of the hole.

Phil
Jim, the only thing I’ll add to that is just in the name of thoroughness and to make sure we are dotting all our i’s and crossing our t’s it probably wouldn’t hurt to send out some samples of the LEDs to make sure the solderability of the leads have integrity. Send them out to like a lab or test them yourself. It may not change your suppliers, but we have seen cases where the quality, solderability of components has varied from the same manufacturer. Just in the name of thoroughness, that would cover it.

It is interesting that you are only seeing it on the LEDs unless those are the only through-hole components you have there. Yeah, it wouldn’t hurt to take a look at the solderability of those leads.

Jim
I can understand the problem because as indicated here the epoxy bump on the bottom of the LED forms like a partial sphere surrounding the lead on the bottom. Yeah, that could drop into a whole and potentially cause a perfect seal and prevent the gas from escaping. I acknowledge that I think they have thought this through pretty reasonably and that could be a potential problem throughout and again the standing off would be the traditional solution.

Phil
Well, we hope that we answered CS’ question. On behalf of Jim and myself, we thank you for listening to Board Talk. As you go forward whether you are using wave soldering, or whatever finish you are using, please don’t solder like my brother.

Jim
And don’t solder like my brother.

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