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April 23, 2014
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HASL vs. Immersion Gold
We make electronic 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 vs. immersion gold for our application? Would you recommend another type of finish?
M.G.

Experts Comments
Pros: Less expensive than gold, Gold embrittles solder.
Cons: Gold is a LESS corrosive surface than any solder. If wire bondability is important, solder is not wire bondable. Why don't you look at electroless Ni, immersion Pd as an alternative. The IP is very thin so cost is low and it's both solderable and wire bondable.
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Lee Levine
President, Consultant
Process Solutions Consulting Inc.
Lee Levine has been a Process Engineer and Metallurgist in the semiconductor industry for 30 years. He now operates his own company Process Solutions Consulting Inc where he consults on process issues and provides SEM/EDS and metallography services.
The Gold is a better finish for the corrosive environment if it is clean going into the field. HASL will typically use a very corrosive flux to prepare the surface and if this is a lead free HASL or leaded it does not matter, both are susceptible to corrosion. These must be very clean going into the fielded environment or just the residue from the HASL process can set up corrosion cells. The plating process for the gold finish is not as difficult to clean and the residues are typically very low even with RO water rinsing.
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Terry Munson
President/Senior Technical Consultant
Foresite
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.
On nature ENIG is better than HASL with corrosive environment. Be careful upon migration to HASL if you are using BGA, Fine pitch IC [pitch<20mil] because of co-planarity is not the best as in ENIG. If really corrosion is concern in present Board ENIG preference to be given for conformal coating Acrylic type - MIL grade approved to minimize corrosion as well coating should meet minimum thickness requirement per MIL std without changing finish type.
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Subrat Prajapati
Supplier Quality Leader
Ge Healthcare
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
If electronics are being used in a corrosive chemical environment it would be advised to use a polyurethane conformal coating which would protect both HASL and ENIG finishes equally. Generally a material that meets G3 noxious gas testing would be advised.
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Chris Palin
European Manager
HumiSeal
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
There is really no one-size-fits-all answer to this question. The answer for your application really depends on the following factors and probably other factors as well:
  1. The types and concentrations of corrosive species in the atmosphere in your application.
  2. The level of venting present in your product. This could be anything from well-sealed to forced-air fan cooled.
  3. The electrical nature of the product, including voltage levels, spacings, circuit impedances, etc.
  4. Whether or not the assemblies are conformal coated or potted, and if so with what material and process.
In general, ENIG finishes are relatively robust to corrosive atmospheres. HASL finish is also quite robust, though exposed solder surfaces can corrode and produce ionic species. The advantage of HASL is that the PWB finish has the same corrosion behavior as the solder joints, so it doesn't have to be evaluated separately. There is risk of exposed copper with both HASL and ENIG finishes, mainly on the edges of traces and lands. In my experience, it happens more with HASL than it does with ENIG. Exposed copper can corrode easily and produce highly ionic salts, some of which are water soluble (copper sulfate, for instance). These salts represent a high reliability risk. Other popular finishes such as immersion silver or OSP are much less resistant to corrosion than HASL or ENIG. I would not recommend either of them in a corrosive environment.
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Fritz Byle
Process Engineer
Astronautics
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.
If you are leaving exposed pads or test points that will be susceptible to atmospheric attack then there have been a number of papers published that indicate HASL's superiority over the majority of surface finishes. However HASL comes at it's own price re placement issues and it is one of the dirtier (ionic and bi-polar organic contaminant) finishes. If all test points/pads are soldered then there is no advantage to struggling with HASL as the surface finish. Depending on the atmosphere in which the end product operates may make ENIG a suitable finish - you would have to provide more information re S, SO2, Cl levels etc as well as temperature/humidity levels expected to be seen and if there is the possibility of a condensing atmosphere being in contact with the board surface.
Gerard O'Brien
President
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
This is an interesting question from the perspective of utilizing the finish solderability coating on raw boards for a protective coating in its functional life.  

The coating applied to raw boards is to guarantee and preserve some degree of solderability during its storage and assembly life cycle. The major difference between the two coating be it either HASL or ENIG is related to the types of components being assembled on the boards and their requirements for flatness of the pads. The topography of the HASL boards can at times impact the reliability of BGA component as the pads are different thicknesses and the solderability can be problematic, whereas the ENIG boards are flatter, which would results in more consistent solder joints across the components. Although there are other benefits to each type, these are the two of the biggest concerns, while the next would be the change from HASL to ENIG to create a lead-free product.

ENIG boards can be problematic from the perspective of Gold thickness and Gold porosity, where in fact the solder joint is made through an interaction between the tin in the solder and the nickel barrier on the board. One problem that has been experienced through the years has been the black pad issue which is related to gold plated boards. This issue is a result of the plating process and can be a major solderability issue with components whose solder joints are beneath the component such as BGA components. Proper qualification of the board fabrication shops and constant monitoring of those shops is a must to make sure this problem does not exists or rear its ugly head.

As for using these solderable coatings for protective measures, these coatings must be fully evaluated from a product reliability perspective which is different than their solderability requirements. To prevent interaction problems with various environments in the field many users apply a conformal coating to the assembly prior to final test and assembly into their operational product. This conformal coat does provide some measure of protective coating to the various environmental excursions the product may be subjected to during its operational life.

As to which coating I would recommend, this is most difficult as there are the failure mechanisms of the two coatings to consider which would occur based upon your requirements. Corrosive environments would impact the tin from solder coating and if the gold was porous the corrosive vapors would impact the nickel barrier coating. In both cases it would also impact the laminate material which is where the biggest issue is from my perspective and that change would be in the dielectric constant of the laminate material which will impact the functional operation of the product. This is why any product which is going to be subjected to corrosive environment should be conformal coated and sealed to protect it from the operational environment. Hope this is useful.
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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.
ENIG is a good candidate for High Phosphoric Corrosion resistance. This is the plating of choice for Printers, TeleCom and others in the industry. Care should be taken, to specify the Gold and Anvil structure, but this is industry preferred Solder Finish. HASL could offer an inert option, as a Pb Solder type, but I wouldn't believe this to be preferred method.
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Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.
Immersion gold is not intended as a final finish. Rather, it's purpose is to preserve the solderability of the metal it covers, and it's application is limited to the assembly process. Volumes are written about the shelf life of immersion coatings -- including gold -- but there is no established performance measure for immersion gold as a final finish. As such, most PCB designs have via pads covered in solder mask, leaving only the solderable lands exposed. And because these lands variously receive assembly solder, my answer favors HASL.
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Bob Lazzara
VP, Marketing & Tech Support
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.
HASL will produce a much thicker deposit of solder to wet to BUT it can leave traces of Hydrobromic Acid and Hydrochloric acid plus lots of High Boiling Hygroscopic solvents to cause Corrosion and SIR issues anyway, so be very wary. I would use a good Chemical resistant conformal coating then stick with Nickel gold finish. Most fluxes can be coated over now a days so don't clean just leave in place as cleaning itself can cause issue's with undercured solder resist absorbing conductive cleaners then not rinsing off correctly.
Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
If all the areas are soldered, then it should not matter. Both immersion gold and HASL should perform equally well. If there are locations that are not soldered, then the immersion gold layer is likely too thin to prevent the underlying metals from corroding. So if areas are not soldered, HASL would be better.
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Renee Michalkiewicz
General Manager
Trace Laboratories
Renee has been with Trace and an IPC member for 16 years. She has managed all military and commercial PB qualification and conformance testing and training, as well as product qualification and testing in the areas of solder pastes, fluxes, solder masks, and conformal coat. She is the chairman of the IPC Testing and the IPC-J-STD-004 Flux Specification Committees and the Vice Chairman of the Assembly and Joining Committee. She has published more than a dozen papers and presented at numerous electronics conferences.
Do you recommend use of HASL vs. immersion gold for our application?  Yes.  Would you recommend another type of finish? Sn/Pb plate and fuse.
Mahendra Gandhi
SME - PWB Technologies
Northrop Grumman
Mahendra Gandhi has been working in interconnect industry since 1972.
I usually recommend ENIG as a plating over HASL for those customers who have applications for printed circuit board assemblies that are or can be exposed to corrosive environments not so much for corrosion resistance but to help advance the cleanliness of the finished product specifically to prevent flux reactivation and possible dendritic growth which can thrive in such situations.

By the nature of the process, ENIG as a plating does not have residual flux residues but HASL does (in some cases excessive) and therefore the cleanliness of the board fabrication as received will depend upon the effectiveness of the cleaning process of the board fabrication supplier. As such a HASL plated board will automatically start out with elevated levels of anionic and cationic contamination (which will only be added to by the assembly process) so I would definitely recommend an ENIG plated board fabrication to be used in corrosive environments for this reason.

We do ionic chromatography testing internally on blank board fabrications and the finished product where cleanliness is concerned for high reliability applications to ensure that they meet minimal requirements prior to acceptance and repeatedly find that assemblies starting off with ENIG plating are in general cleaner than those assemblies that use HASL plated board fabrications.
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David Bonito
Sales & Marketing Manager
Technical Manufacturing Corp.
David has been active in all areas of the contract electronics manufacturing industry for over fifteen years. He is currently in charge of all Sales and Marketing related activities for Technical Manufacturing Corporation.
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