|Ask the Experts|
January 19, 2021
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?
|Expert Panel Responses|
Gold embrittlement occurs at 5 wt.% in the solder joint although the industry usually defaults to 3 wt.%. You can calculate easily the expected wt.% in your joints and see if it exceeds these values. For ENIG, we do not know of any device features currently in use that approx. these threshold values.
S T and S Testing and Analysis
Gold embrittlement can be a significant reliability issue. Most PCB designers area ware of this and careful consideration is made in the design of solder joints. Gold content of solder joints are fixed by design. There is a threshold level for gold which can be calculated. There is a great study on this by Ed Hare PH.D that you can read at www.semlab.com.
Manufacturing Applications Specialist
Gold embrittlement in solder joints has always been caused by a percentage ratio the weight of the gold versus the weight of the solder and the going number for the amount of gold through the years was approximately 3% of the total weight of the solder alloy.
The problem 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 hole during the wave solder operation. At (5) fpm and a board contact width over the wave of (1) inch the board is in the solder for (1) sec, which is quite fast respective of the solder getting flushed out of the plated through holes during the soldering process. This leaves the gold on the lead to be dissolved in only the solder within the plated through hole which can result in a higher percentage of gold than is necessary.
Secondly if the components are manually soldered, the amount of solder being added to the solder joint or plated through hole, is minimal due to the physical volume of the hole minus the volume of the lead could create a gold rich environment which would be prone to crack propagation within the plated through hole.
Thirdly, if the components are surface mount component, the amount of solder paste added to the pads is smaller in volume than any of the plated through holes and the ability of the gold to dissolve in that molten solder paste does increase the amount of gold per given volume of solder paste and again the gold dendrites within the solder joint would be the catalyst for the propagation of cracks within the solder joint.
Therefore to be safe, remove the gold and eliminate the worry about the exact amount of gold on the leads and terminals.
Vice President, Technical Director
Whether there is a risk of embrittlement depends on several variables:
Where gold thickness is truly minimized, e.g. below 10 microinches, I have not personally seen specific cases where the gold concentration becomes unacceptable. In the end, each joint configuration is unique and should be analyzed as such.
To the best of my knowledge, gold plating in electronics at the package or assembly level is still not applied via vapor deposition. However, if the process uses immersion technology, then yes gold embrittlement is unlikely.
CEO & Managing Partner
First I want to clarify that we are only talking about gold-plated component lead and terminations here, and not the PWB surface finish gold plating (such as ENIG and ENEPIG and soft gold for wire bonding) applied to PWB pads. Surface platings such as ENIG, with only about 4 u inches of gold, do not contribute towards embrittlement.
Second,I want to point out that there are many different types of gold finish on component leads, and many different geometrical configurations of gold-finished terminals, leads, solder cups, etc., and many methods of soldering to those leads, all of which are factors in gold embrittlement.
In addition to the different types of gold finish (electroless, electroplated, fused, etc) there are many different types of base metals and barrier platings, including copper base metal with a barrier plate of phosphate bronze, or nickel (alloy 42), or kovar with direct gold electroplate, etc.
Gold embrittlement is caused by a spectacle called nucleation. All forces in nature want to reach equilibrium, and engineering is nothing more than devising methods of controlling those forces. In fluid dynamics pressurized hot and cold water wants to reach room temperature or the controlling temperature(refrigeration and steam), in electric and electronic circuits electrons and holes want to recombine (electrical power and current), etc. Magnetic fields are another example of forces trying to reach equilibrium.
In metallurgy there are similar reactions between different alloys that lead to galvanization,corrosion, oxidation, etc., and nucleation is nothing more than the gold in a solder joint attempting to achieve equilibrium with the other alloys within that same solder termination.
Nucleation of gold molecules through the solder joint happens even at room temperature, and is a time-variable phenomenon where the gold molecules will travel right through the solder joint to the intermetallic formation of the solder and the copper pad, as well as the solder and the lead or termination. During soldering the gold molecules are readily and immediately dissolved and suspended and are homogeneous throughout the solder joint.
After the solder joint is formed, nucleation of the gold begins, and the rate and the concentration of the agglomeration of gold molecules at or near the intermetallic boundary is determined by the amount of gold as a percentage, the geometric formation that can cause the agglomeration to be more concentrated at certain points of the solder joint, and the rate of nucleation as controlled by the basis metals and their barrier plating. When we solder to gold, we don't really solder to the gold itself, but the intermetallic formation is made between the solder alloy and the barrier metal under the gold, or if there is no barrier metal, then directly to the basis metal.
Most typically component leads that are gold plated are actually a basis metal of copper with a barrier metallization of nickel or other barrier plating underneath the gold. The gold is plated over the nickel as a means of providing a coating over the nickel because bare nickel oxidizes almost overnight and becomes unsolderable (molten solder cannot form an intermetallic bond to any oxide layer, hence the use of flux to remove the oxides).
My own experiences of gold nucleation leading to embrittlement include instances where pre-tinned stranded wires were soldered into gold-plated solder cup terminals 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 soldering is done by directing electrical current through the gold cup to melt the solder. The gold thickness of the gold cup terminals was not controlled very well, and some had much more gold than others because at that time there were no good methods of ensuring just the right thickness of gold; we did not have XrF or any of the other more advanced methods of determining gold thickness (or any other plating thickness for that matter).
As a result, it was required (and still is per J-STD-001F) that the gold be removed prior to soldering in order to reduce the gold percentage to less than 3%. However, even with as little as 1% of gold in the solder joint, gold embrittlement can occur. In the wire cups that I mentioned, the actual gold plating was chemically etched and found to be less than 2% by volume, but due to the cylindrical shape, the gold tended to nucleate between the stranded wire and the inside surface of the cup closest to the wire.
Because the operators tended to insert the wires and route them out and back, nearly all of the gold nucleated to a point near the top 25% of the cup between the stranded wire and the edge of the cup nearest where the wires 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 year after the wires were soldered to the cups. I personally from my own experience feel that the 3% limit is too liberal, that the maximum allowable gold percentage should be closer to 1% or less.
But that is also why the J-STD-001F also has requirements that if you do not want to remove the gold by tinning first, then you are required to show objective evidence that no embrittlement leading to fractures will occur, and by objective evidence that means showing that no embrittlement occurs over a long period of time.
Sample pull-testing or flexing solder terminations immediately after soldering is NOT objective evidence; the time variable and temperature variation over time must also be a part of the evaluation. There are gold embrittlement test procedures in 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) solder connections is still a major concern, because even vapor deposition can lead to more than 1 or 2% of gold if not done properly. Better to tin it away than to experience a costly bunch of field failures, or at least perform the required qualification and produce the objective evidence as a minimum.
In God we trust, all others bring data.
Good luck with the qualification.
Soldering gold-plated parts can be problematic as gold is soluble in solder. Solder which contains more than 2-3% gold can become brittle. The joint surface is dull-looking.
Gold reacts with both tin and lead in their liquid state, forming brittle intermetallic. When eutectic 63% tin - 37% lead solder is used, no lead-gold compounds are formed, because gold preferentially reacts with tin, forming the AuSn4 compound. Particles of AuSn4 disperse in the solder matrix, forming preferential cleavage planes, significantly lowering the mechanical strength and therefore reliability of the resulting solder joints.
If the gold layer does not completely dissolve into the solder, then slow intermetallic reactions can proceed in the solid state as the tin and gold atoms cross-migrate. Intermetallic has poor electrical conductivity and low strength. The on-going intermetallic reactions also cause Kirkendall effect, leading to mechanical failure of the joint, similar to the degradation of gold-aluminium bonds known as purple plague. (Kirkendall effect is the motion of the boundary layer between two metals that occurs as a consequence of the difference in diffusion rates of the metal atoms).
It is misconception that Sn62 (with 2%) can be used on Gold Surfaces, Gold dissolving and forming brittle joints is same as SN63, maximum limit of 3% Au will be applicable to Sn62 also to prevent brittle joints.
Silver bearing solder like Sn62 is recommended for soldering where leads are silver plated or end termination of components are Palladium Silver to prevent silver leaching.
Astra Microwave Products, Hyderabad, AP India
It depends on what the solder alloy is and how much solder is applied.
But in sufficient amounts gold is known to embrittle tin containing 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 known as enough to cause embrittlement to occur.
Technical Support Engineer
The embrittlement in solder connections occurs when the weight percentage of gold to solder increase. The alloy of the solder has an effect on this as well. Tin lead is more prone to a sharp increase in brittleness 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 gold though the safer the solder connection. When soldering bottom terminated components (BTCs) the gold ration increase not only from some devices having gold pads but also the reduction of the solder volume and the increased voiding that may inhibit the gold distribution.
Deck Street Consultants
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
While it is noted that there is no threshold in strength versus gold content with SAC305 as there is in SnPb, the study by Dr. Hillman clearly shows that the SAC305 consistently shears at higher forces than SnPb, both below and above the knee function for any given Au content.David Mullins, Gavial Engineering and Manufacturing
The best solution is still to avoid or remove gold plating, however.
In the early 1990's when the FCC opened up the 900MHz spectrum to land mobile we found that all gold had to be removed from RF power devices (the flanged lead of a RF Power MOSFET) to prevent thermal runaway and burnout of the solder joint. P-N semiconductor junctions formed in the intermetallic region and reacted with the carrier frequency. We removed the protective gold plate and tinned the flange before assembly. Its well know practice to do this now for hi power RF.Tim Swanson, NEMO Manufacturing
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