Ask the Experts
January 2, 2019
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?
Expert Panel Responses
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 analternative. The IP is very thin so cost is low and it's both solderable andwire bondable.
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 itis clean going into the field. HASL will typically use a very corrosiveflux to prepare the surface and if this is a lead free HASL or leaded it doesnot matter, both are susceptible to corrosion. These must be very cleangoing into the fielded environment or just the residue from the HASL processcan set up corrosion cells. The plating process for the gold finish is not as difficultto clean and the residues are typically very low even with RO waterrinsing.
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.
Onnature ENIG is better than HASL with corrosive environment. Be careful uponmigration to HASL if you are using BGA, Fine pitch IC [pitch<20mil] becauseof co-planarity is not the best as in ENIG. If really corrosion is concern inpresent Board ENIG preference to be given for conformal coating Acrylic type -MIL grade approved to minimize corrosion as well coating should meet minimumthickness requirement per MIL std without changing finish type.
Supplier Quality Leader
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 beingused in a corrosive chemical environment it would be advised to use apolyurethane conformal coating which would protect both HASL and ENIG finishesequally.Generally a material that meets G3 noxious gastesting would be advised.
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:
In general, ENIG finishes are relatively robust to corrosive atmospheres. HASL finish is also quite robust, though exposed solder surface scan 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.
- The types and concentrations of corrosive species in the atmosphere in your application.
- The level of venting present in your product. This could beanything from well-sealed to forced-air fan cooled.
- The electrical nature of the product, including voltage levels, spacings, circuit impedances, etc.
- Whether or not the assemblies are conformal coated or potted,and if so with what material and process.
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.
Ifyou are leaving exposed pads or test points that will be susceptible toatmospheric attack then there have been a number of papers published thatindicate HASL's superiority over the majority of surface finishes. However HASLcomes 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 aresoldered then there is no advantage to struggling with HASL as the surfacefinish. Depending on the atmosphere in which the end product operates may makeENIG 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 ifthere is the possibility of a condensing atmosphere being in contact with theboard surface.
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 interestingquestion from the perspective of utilizing the finish solderability coating onraw boards for a protective coating in its functional life.
The coating applied to rawboards is to guarantee and preserve some degree of solderability during itsstorage and assembly life cycle. The major difference between the two coatingbe it either HASL or ENIG is related to the types of components being assembledon the boards and their requirements for flatness of the pads. The topographyof the HASL boards can at times impact the reliability of BGA component as thepads are different thicknesses and the solderability can be problematic,whereas the ENIG boards are flatter, which would results in more consistentsolder joints across the components. Although there are other benefits to eachtype, these are the two of the biggest concerns, while the next would be thechange from HASL to ENIG to create a lead-free product.
ENIG boards can beproblematic from the perspective of Gold thickness and Gold porosity, where infact the solder joint is made through an interaction between the tin in thesolder and the nickel barrier on the board. One problem that has beenexperienced through the years has been the black pad issue which is related togold plated boards. This issue is a result of the plating process and can be amajor solderability issue with components whose solder joints are beneath thecomponent such as BGA components. Proper qualification of the board fabricationshops and constant monitoring of those shops is a must to make sure thisproblem does not exists or rear its ugly head.
As for using thesesolderable coatings for protective measures, these coatings must be fullyevaluated from a product reliability perspective which is different than theirsolderability requirements. To prevent interaction problems with variousenvironments in the field many users apply a conformal coating to the assemblyprior to final test and assembly into their operational product. This conformalcoat does provide some measure of protective coating to the variousenvironmental excursions the product may be subjected to during its operationallife.
As to which coating I wouldrecommend, this is most difficult as there are the failure mechanisms of thetwo coatings to consider which would occur based upon your requirements.Corrosive environments would impact the tin from solder coating and if the goldwas porous the corrosive vapors would impact the nickel barrier coating. Inboth cases it would also impact the laminate material which is where thebiggest issue is from my perspective and that change would be in the dielectricconstant of the laminate material which will impact the functional operation ofthe product. This is why any product which is going to be subjected tocorrosive environment should be conformal coated and sealed to protect it fromthe operational environment. Hopethis is useful.
Vice President, Technical Director
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 goodcandidate for High Phosphoric Corrosion resistance. This is the platingof choice for Printers, TeleCom and others in the industry. Care shouldbe taken, to specify the Gold and Anvil structure, but this is industrypreferred Solder Finish. HASL could offer aninert option, as a Pb Solder type, but I wouldn't believe this to be preferredmethod.
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 notintended as a final finish. Rather, it's purpose is to preserve thesolderability of the metal it covers, and it's application is limited to theassembly process. Volumes are written about the shelf life of immersioncoatings -- including gold -- but there is no establishedperformance measure for immersion gold as a final finish. As such, most PCBdesigns have via pads covered in solder mask, leaving only the solderable landsexposed. And because these lands variously receive assembly solder, my answerfavors HASL.
Robert "Bob" Lazzara
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 towet to BUT it can leave traces of Hydrobromic Acid and Hydrochloric acid pluslots of High Boiling Hygroscopic solvents to cause Corrosion and SIR issuesanyway, so be very wary. I would use a good Chemical resistant conformalcoating then stick with Nickel gold finish. Most fluxes can be coated over nowa days so don't clean just leave in place as cleaning itself can cause issue'swith undercured solder resist absorbing conductive cleaners then not rinsingoff correctly.
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.
If all the areas are soldered, then it shouldnot matter. Both immersion gold and HASL should perform equallywell. If there are locations that are not soldered, then the immersiongold layer is likely too thin to prevent the underlying metals fromcorroding. So if areas are not soldered, HASL would be better.
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. immersiongold for our application? Yes. Would you recommend another type of finish? Sn/Pb plate and fuse.
SME - PWB Technologies
Mahendra Gandhi has been working in interconnect industry since 1972.
Iusually recommend ENIG as a plating over HASL for those customers who haveapplications for printed circuit board assemblies that are or can be exposed tocorrosive environments not so much for corrosion resistance but to help advancethe cleanliness of the finished product specifically to prevent fluxreactivation and possible dendritic growth which can thrive in such situations.
By the nature of the process, ENIG as a plating does not have residual fluxresidues but HASL does (in some cases excessive) and therefore the cleanlinessof the board fabrication as received will depend upon the effectiveness of thecleaning process of the board fabrication supplier. As such a HASL plated boardwill automatically start out with elevated levels of anionic and cationiccontamination (which will only be added to by the assembly process) so I woulddefinitely recommend an ENIG plated board fabrication to be used in corrosiveenvironments for this reason.
We do ionic chromatography testing internally onblank board fabrications and the finished product where cleanliness isconcerned for high reliability applications to ensure that they meet minimalrequirements prior to acceptance and repeatedly find that assemblies startingoff with ENIG plating are in general cleaner than those assemblies that useHASL plated board fabrications.
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.
An ENIG finished circuit board would have no exposed copper whatsoever. HASL has, however, at the undercut. From this perspective, ENIG is a better choice; a little bit more expensive, though.
Alan Lee, Finenet Electronic Circuit Ltd., Hong Kong, PRC.
We evaluated bare board cleanliness by creating the necessary conditions in a lab and observing dendritic growth on the surface layers of PC boards. With HASL boards, it was not if they would grow dendrites, but how long it would take. If it was seconds, the boards were considered contaminated with ionic substances, likely traces of flux from the HASL process. If it took minutes to grow a dendrite, the boards were considered clean.
Jack Lucas, Ametek PPD
With the ENIG finish, it was typical that we could not grow any dendrites at all.
The reasons for this are mentioned above. The HASL process likely leaves trace amounts of flux on the boards. It also leaves exposed copper at sharp edges. The ENIG process leaves no exposed copper and likely no exposed nickel, although I'm not sure of that.
Since our company runs some high voltages on the surfaces of our boards and our products are sometimes used in humid environments, I have favored the use of the ENIG finish. The typical ENIG finish is not thick enough to cause embrittled solder joints in my opinion, so you just need to use PCB suppliers that have the controls to avoid "black pad", which is most of them.
Regarding Levine's comment that "Gold embrittles solder", that does NOT apply to ENIG. The gold thickness of ENIG, as defined by IPC standards, is typically 3-5 microinches. That small of an amount will not contribute at all to embrittlement. Please do not use that as a factor in determining which finish to use.
Richard D. Stadem
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.
Generally speaking, hot air solder leveling (HASL) as a finish on circuit boards provides better corrosion resistivity than immersion gold. However, the HASL finish is typically non-flat (with a dome shape), which may cause solder paste printing issues and difficulty in component placement on fine pitch (<25mil) areas.
Director New Product Development
Metallic Resources, Inc
David Bao has more than fifteen years of experience in developing new solder paste, wave soldering fluxes and other SMT consumables. He currently serves as the Director of New Product Development at Metallic Resources Inc. He received a Ph.D. in Chemistry at Oklahoma State University.