Ask the Experts
January 4, 2024 - Updated
September 30, 2021 - Originally Posted

Solder Joint Blow Holes

After switching our PCB finish from Pb 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 the solder to blow out the bottom during wave soldering when using the ENIG vs HASL finish? Any other thoughts?


Expert Panel Responses

Without clearance between the component and the destination side land of the PTH, this phenomenon is possible and even more so with ENIG.

You are obstructing the hole, likely not achieving acceptable vertical fill for class 2 or 3 product. The thicker HASL finish reflows during wave soldering, allowing solder to flow to the destination side, whereas the ENIG does not.

John De Leeuw
Manufacturing Process Engineer
TE SubCom
John has more than thirty years of precision metal fabrication, contract manufacturing, fiber optics and electronic manufacturing experience in quality and production management and process engineering capacities. He has been a Certified IPC Trainer for more than ten years. He is currently a Manufacturing Process Engineer for High Reliability Optoelectronic assemblies used in undersea fiber optic cable systems.

You have a good hypothesis for a root cause here. It is certainly possible that if a component makes an air-tight seal on the top edge of the hole, expanding gas in the hole could create a blowhole. The only other source of outgassing would be from the PWB, and that would seem rather unlikely.

One way to test this is to build boards with LEDs that are intentionally prevented from making a seal, either by standing them off a bit or by indenting the epoxy bump to prevent a seal. If this stops the blowholes, then you can conclude that this was in fact the source of the gas. If it does not stop the blowholes, then the PWB is the main suspect.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Interesting question. It initially appears that there is trapped moisture in the PTH that since the ENIG is an ambient plating process and the HASL but both go through final rinse and dry. Are the new ENIG boards just a few days old from fabrication?

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Hi, the characteristics of HASL and ENIG finish are quite different and need modify the current process for HASL to ENIG, also your PTH component generally need to have some kind of separators to avoid this failure modes.

As you mention the planarity and precision of the ENIG finish can be one variable that generate a air-tight seal combined with the LED shape and avoid out gassing. If this happens you can try reducing the flux amount and work with you profile on the preheat area to assure the right evaporation of the solvents before wave to reduce to the minimum the generation of gas on the wave. In a few words try that you PCB arrives all most dry to the wave.

There are other variables than can generate blow holes like humidity, contamination, PCB oxidation etc...

If never work with ENIG in the past, i recommend to you read the IPC 1601 Printed Board Handling and Storage Guidelines to understand better the differences between board finishes and modify the handling process.

Andres Rojas
Engineering Director / Master IPC Trainer (MIT)
AMMSA Solutions
More than 20 years of technical experience in the electronics industry in roles ranging from Process & Project Engineer to engineering manager and Technical Applications Engineer for Latin Americas. IPC Master Trainer, International speaker and consultant.

It may be possible that the ENIG plating process trapped moisture or process chemistries in the through holes. During wave soldering, trapped moisture or chemistry evaporates explosively creating a blow hole in the solder. I recommend baking the circuit boards to release any trapped moisture prior to soldering.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

I do not believe that the epoxy "bump" you refer to would be the cause. Creating an airtight seal would be virtually impossible. In actuality through hole components tend to rise, float, when they enter the wave. This would eliminate the epoxy bump as a potential cause.

When searching for the solution to a new problem that has arisen in a process always look for "what has changed". You've indicated that the supplier has changed the finish. You did not indicate if the finish was on the LED leads or the barrels on the thru holes.. IMO... The fickle finger of fate points to the change in the finish, either one, causing solder voids/blow holes.

Jerry Karp
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.

The epoxy bump on the bottom of the LED shouldn't cause a new blow hole problem just by changing from HASL to ENIG. I would expect that it would have shown itself to some level even with HASL.

Many times blow hole problems have to do with the hole drill quality. I think the way to get to answers quickly is to cross section the soldered holes with the LEDs. This should give you good information about whether it is the board quality or something to do with the LEDs.

Even though you are getting PCBs from the same supplier, the ENIG process is different than the HASL process. It may be something different in that process for them—it might be a different drilling process, or the ENIG PCBs may even be coming from a different facility, and even possibly a subcontractor.

Eddie Groves
Director of the Selective Soldering Academy (SSA)
Selective Soldering Academy
Eddie Groves has over 32 years' experience in the electronics manufacturing industry, specifically in the soldering of circuit boards. His first 11 years was with Lucent Technologies (AT&T), then with Kester, Qualitek and then Pillarhouse. He has over two decades of Selective Soldering experience as a user and a provider of Selective Soldering equipment. His particular focus is Process Development, Process Troubleshooting, flux, and component & PCB solderability.

It is possible and been seen before, due to the unevenness of the HASL it was enough to let the vapours from the flux escape where as now they are trapped in a bubble in the hole and the solder wave rides over them.

I would screen two ident thin lines with solder mask legend on top of the PCB around the LED body so the LED isn't flush with the surface and will allow hot gases to escape.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Potentially it could, one of the results of using a HASL finish is that the surface of pads are never flat, in extreme cases you can see a lump on one end of a pad compared to the other and this can indicate the direction the hot air flows over the pad to remove excess solder. ENIG on the other hand creates a very even coating over the whole of the pad which is great for surface mount devices. The negative effect you may be seeing is due to the LED making a good seal or gasket on the top pad of the through hole component.

During the wave soldering process when the component lead comes in contact with the wave the flux trapped in the through hole super heats and turn to vapour, unfortunately as the component is gasketing the top of the hole these vapours can only go back down the hole and erupt out through the molten solder causing the blow hole to appear. You could try to get the leg of the LED component preformed with a kink in it to hold it slightly above the pad level, this may reduce the risk of the blow holes.

Richard Boyle
Global Product Champion
Henkel Electronics
Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.

It is possible to create blow holes when making an air tight seal. I think the most likely problem would be moisture in the board. You can try baking the boards before soldering to see if this eliminates the issue.

Brien Bush
Manufacturing Applications Specialist
Cirtronics Corp.
Mr. Bush has 20 years experience in electronics contract manufacturing. Major areas of expertise include through hole, SMT, wave and selective soldering.

I don't think the root cause is with air tight sealing on the top although this can aggravate the problem since one side is shut (or partially shut) for gas release. I would ask the assembly contractor to rinse and then bake the boards prior to wave soldering - to make sure the PTHs are free from contamination and fully dry.

In addition, you can ask the assembly contractor whether they can change process parameters on the wave soldering machine and also if they can offer other flux formulations.

Guy Shemesh
General Manager
ePiccolo Engineering
Mr. Shemesh has Bsc. in E.E engineering and hands-on experience with electronics (schematics & layout) since 2004. He has designed dozens of multi-layer PCBs, HDI, RF, rigid-flex, etc., and had the honor for design reviewing veteran layout engineers several times as a consultant.

The chemistries of HASL and ENIG are completely different, so my focus would be on the boards themselves. Since the soldering process is the same, I would not think the LED would act any different.

Have the plated through holes been evaluated to make sure the plating in the hole is sound and meets the requirements of IPC-6012?

Changes in the plating thickness or voids in the pth walls could be a cause for the blow holes so this should be checked out.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

I have to agree with John DeLeeuw and Fritz Byle. The fact that the blowholes affect only the LEDs with a plastic bump on the bottom, and the fact that these parts are through hole and likely clinched on the bottom side, thus sealing the hole would indicate that as being the issue. The fact that no other part on the board is seeing blowholes pretty much rules out the change in finish from HASL to ENIG. So why did the blowholes become an issue only after the plating change? Because, as John pointed out as well, the HASL melted during soldering, and this would allow solder to flow up and around any obstruction in the hole on the topside. Remember the old adage: "Nothing solders to solder like (fill in the blank) does."

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Based on the provided information, I have several questions - how do you know that the previous finish did not cause solder blow holes and this one does? if you are questioning a component that sits down on the board, should it be elevated a little? How are you inspecting the solder fill? how do you know that the HASL finish was ok and this one is not? I would say that answering these questions will solve the problem and point you in the right direction for what the cause of the defect is.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at

Based on the provided information, I have several questions – how do you know that the previous finish did not cause solder blow holes and this one does? if you are questioning a component that sits down on the board, should it be elevated a little? How are you inspecting the solder fill? how do you know that the HASL finish was ok and this one is not? I would say that answering these questions will solve the problem and point you in the right direction for what the cause of the defect is.

Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at
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