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December 8, 2017

Gold Plating and Embrittlement

What is your opinion regarding gold plating removal to improve soldering? I believe in the past gold plating was thick enough (>50 micro inches) to cause gold embrittlement in solder joints. However today the way gold is applied using vapor deposition (<50 micro inches) there should not be a problem with embrittlement, do you agree?

M.B.

Expert Panel Responses

Goldembrittlement occurs at 5 wt.% in the solder joint although the industryusually defaults to 3 wt.%. You can calculate easily the expected wt.% in yourjoints and see if it exceeds these values. For ENIG, we do not know ofany device features currently in use that approx. these threshold values.

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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.

Goldembrittlement can be a significant reliability issue. Most PCB designers areaware of this and careful consideration is made in the design of solder joints.Goldcontent of solder joints are fixed by design. There is a threshold level forgold which can be calculated.Thereis a great study on this by Ed Hare PH.D that you can read at www.semlab.com.

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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.

Goldembrittlement in solder joints has always been caused by a percentage ratio theweight of the gold versus the weight of the solder and the going number for theamount of gold through the years was approximately 3% of the total weight ofthe solder alloy. Theproblem and the reasoning behind changing the criterion in the J-STD-001 RevF., standard was to address a problem which was experienced by the industry.That problem as defined was the lack of turbulence in the plated though holeduring the wave solder operation. At (5) fpm and a board contact width over thewave of (1) inch the board is in the solder for (1) sec, which is quite fastrespective of the solder getting flushed out of the plated through holes duringthe soldering process. This leaves the gold on the lead to be dissolved in onlythe solder within the plated through hole which can result in a higherpercentage of gold than is necessary. Secondlyif the components are manually soldered, the amount of solder being added tothe solder joint or plated through hole, is minimal due to the physical volumeof the hole minus the volume of the lead could create a gold rich environmentwhich would be prone to crack propagation within the plated through hole. Thirdly,if the components are surface mount component, the amount of solder paste addedto the pads is smaller in volume than any of the plated through holes and theability of the gold to dissolve in that molten solder paste does increase theamount of gold per given volume of solder paste and again the gold dendriteswithin the solder joint would be the catalyst for the propagation of crackswithin the solder joint. Thereforeto be safe, remove the gold and eliminate the worry about the exact amount ofgold on the leads and terminals

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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.

Whetherthere is a risk of embrittlement depends on several variables:
  • Amount of gold expected to be leached (soldered area*goldthickness)
  • Solder volume in the resulting joint
  • Whether or not the solder is from an "infinite source" such as awave, or from solder paste
  • Whether there is any gold contribution from the PWB finish (ENIGor ENIPIG finish)
Worstcase, we can assume that all the gold will be leached (usually an accurate assumption)and that the solder source is finite and equal to the volume of the joint (truefor reflow, conservative for wave). If we make these assumptions, I have infact seen cases where component gold thicknesses below 30 microinches stillresult in marginal or unacceptable gold levels in the final joint. Where goldthickness is truly minimized, e.g. below 10 microinches, I have not personallyseen specific cases where the gold concentration becomes unacceptable. In theend, each joint configuration is unique and should be analyzed as such.

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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.

To thebest of my knowledge, gold plating in electronics at the package or assemblylevel is still not applied via vapor deposition.However,if the process uses immersion technology, then yes gold embrittlement isunlikely.

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Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.

FirstI want to clarify that we are only talking about gold-plated component lead andterminations here, and not the PWB surface finish gold plating (such as ENIGand ENEPIG and soft gold for wirebonding) applied to PWB pads. Surface platingssuch as ENIG, with only about 4 uinches of gold, do not contribute towardsembrittlement. Second,I want to point out that there are many different types of gold finish oncomponent leads, and many different geometrical configurations of gold-finishedterminals, leads, solder cups, etc., and many methods of soldering tothose leads, all of which are factors in gold embrittlement. In addition to thedifferent types of gold finish (electroless, electroplated, fused, etc) thereare many different types of base metals and barrier platings, including copperbase metal with a barrier plate of phosphate bronze, or nickel (alloy 42), orkovar with direct gold electroplate, etc. Goldembrittlement is caused by a spectacle called nucleation. All forces in naturewant to reach equilibrium, and engineering is nothing more than devisingmethods of controlling those forces. In fluid dynamics pressurized hot and coldwater wants to reach room temperature or the controlling temperature(refrigeration and steam), in electric and electronic circuits electrons andholes want to recombine (electrical power and current), etc. Magnetic fieldsare another example of forces trying to reach equilibrium. In metallurgy thereare similar reactions between different alloys that lead to galvanization,corrosion, oxidation, etc., and nucleation is nothing more than the gold in asolder joint attempting to achieve equilibrium with the other alloys within thatsame solder termination. Nucleation of gold molecules through the solder jointhappens even at room temperature, and is a time-variable phenomenon where thegold molecules will travel right through the solder joint to the intermetallicformation of the solder and the copper pad, as well as the solder and the leador termination. During soldering the gold molecules are readily and immediatelydissolved and suspended and are homogenous throughout the solder joint. Afterthe solder joint is formed, nucleation of the gold begins, and the rate and theconcentration of the agglomeration of gold molecules at or near theintermetallic boundary is determined by the amount of gold as a percentage, thegeometric formation that can cause the agglomeration to be more concentrated atcertain points of the solder joint, and the rate of nucleation as controlled bythe basis metals and their barrier plating. When we solder to gold, we don'treally solder to the gold itself, but the intermetallic formation is made betweenthe solder alloy and the barrier metal under the gold, or if there is nobarrier metal, then directly to the basis metal. Most typically component leadsthat are gold plated are actually a basis metal of copper with a barriermetallization of nickel or other barrier plating underneath the gold. The goldis plated over the nickel as a means of providing a coating over the nickelbecause bare nickel oxidizes almost overnight and becomes unsolderable (moltensolder cannot form an intermetallic bond to any oxide layer, hence the use offlux to remove the oxides). Myown experiences of gold nucleation leading to embrittlement include instanceswhere pre-tinned stranded wires were soldered into gold-plated solder cupterminals using resistance soldering back in the 1960 time frame. Yes,I already know that was a long time ago. Anyway,these wires were used to make point-to-point terminations. Resistance solderingis done by directing electrical current through the gold cup to melt thesolder. The gold thickness of the gold cup terminals was not controlled verywell, and some had much more gold than others because at that time there wereno good methods of ensuring just the right thickness of gold; we did not haveXrF or any of the other more advanced methods of determining gold thickness (orany other plating thickness for that matter). As a result, it was required (andstill is per J-STD-001F) that the gold be removed prior to soldering in orderto reduce the gold percentage to less than 3%. However, even with as little as1% of gold in the solder joint, gold embrittlement can occur. In the wire cupsthat I mentioned, the actual gold plating was chemically etched and found to beless than 2% by volume, but due to the cylindrical shape, the gold tendedto nucleate between the stranded wire and the inside surface of the cup closestto the wire. Because the operators tended to insert the wires and route themout and back, nearly all of the gold nucleated to a point near the top 25% ofthe cup between the stranded wire and the edge of the cup nearest where thewires curved back, and that is where the fractures occurred approximately 95%of the time. The fractures did not start showing up until more than a yearafter the wires were soldered to the cups. I personally from my own experiencefeel that the 3% limit is too liberal, that the maximum allowable goldpercentage should be closer to 1% or less. But that is also why the J-STD-001Falso has requirements that if you do not want to remove the gold by tinningfirst, then you are required to show objective evidence that no embrittlementleading to fractures will occur, and by objective evidence that means showingthat no embrittlement occurs over a long period of time. Samplepull-testing or flexing solder terminations immediately after soldering is NOTobjective evidence; the time variable and temperature variation over time mustalso be a part of the evaluation. There are gold embrittlement test proceduresin both IPC-TM-650 (a free download from www.ipc.org)as well as ANSI standards. Soto summarize; gold embrittlement in Class 3 (high reliability) solderconnections is still a major concern, because even vapor deposition can lead tomore than 1 or 2% of gold if not done properly. Better to tin it away than toexperience a costly bunch of field failures, or at least perform the requiredqualification and produce the objective evidence as a minimum. InGod we trust, all others bring data. Goodluck with the qualification.

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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.

Solderinggold-plated parts can be problematic as gold is soluble in solder. Solder whichcontains more than 2-3% gold can become brittle. The joint surface isdull-looking. Goldreacts with both tin and lead in their liquid state, forming brittleintermetallic. When eutectic 63% tin - 37% lead solder is used, no lead-goldcompounds are formed, because gold preferentially reacts with tin, forming theAuSn4 compound. Particles of AuSn4 disperse in the solder matrix, formingpreferential cleavage planes, significantly lowering the mechanical strengthand therefore reliability of the resulting solder joints. If the gold layer does not completely dissolve intothe solder, then slow intermetallic reactions can proceed in the solid state asthe tin and gold atoms cross-migrate. Intermetallic has poor electricalconductivity and low strength. The on-going intermetallic reactions also causeKirkendall effect, leading to mechanical failure of the joint, similar to thedegradation of gold-aluminium bonds known as purple plague. (Kirkendall effectis the motion of the boundary layer between two metals that occurs as aconsequence of the difference in diffusion rates of the metal atoms). Itis misconception that Sn62 (with 2%) can be used on Gold Surfaces, Golddissolving and forming brittle joints is same as SN63, maximum limit of 3% Auwill be applicable to Sn62 also to prevent brittle joints. Silverbearing solder like Sn62 is recommended for soldering where leads are silverplated or end termination of components are Palladium Silver to prevent silverleaching.

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KN Murli
Head-Quality
Astra Microwave Products, Hyderabad, AP India
Holds Degree in Engineering, started off as Scientist/Engineer in ISRO (Indian Space Research Organization) in Quality Assurance of Space hardware Electronics Production. Worked in the area of Parts, Material and Process; DPA, FA and Process Qualification for space and ground hardware. Later moved into Private sector and worked in the area of Quality Management Systems & ISO 9001 certification. Currently hold a position as Head-Quality in RF/Microwave Product manufacturing for Defense and Aerospace segment.

It depends on what the solder alloy is and how much solder isapplied. But in sufficient amounts gold is known to embrittle tincontaining solder joints. Depending on the reference source, 3-4% by weight of gold in aSnPb solder joint and ~10% by weight of gold in a SAC joint is typically knownas enough to cause embrittlement to occur.

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Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.

The embrittlement in solder connections occurs when theweight percentage of gold to solder increase. The alloy of the solder has aneffect on this as well. Tin lead is more prone to a sharp increase inbrittleness than SAC305. So yes in a general term your thoughts are correct.However solder joint design and pad to solder ratios have to be considered.Aging also plays a role in this embrittlement. Typically the more solder there is to disperse the goldthough the safer the solder connection. When soldering bottom terminatedcomponents (BTCs) the gold ration increase not only from some devices havinggold pads but also the reduction of the solder volume and the increased voidingthat may inhibit the gold distribution.

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Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

Reader Comment
It is mentioned that there are gold embrittlement test procedures in both IPC-TM-650 (a free download from www.ipc.org) as well as ANSI standards. Can you provide more details as to what the actual specs are?
Mary Lou Sachenik, Moog, USA


Reader Comment
While it is noted that there is no threshold in strengthversus gold content with SAC305 as there is in SnPb, the study by Dr. Hillmanclearly shows that the SAC305 consistently shears at higher forces than SnPb,both below and above the knee function for any given Au content. The best solution is still to avoid or remove goldplating, however.
David Mullins, Gavial Engineering and Manufacturing