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?
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.
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.
embrittlement can be a significant reliability issue. Most PCB designers are
aware of this and careful consideration is made in the design of solder joints.
content of solder joints are fixed by design. There is a threshold level for
gold which can be calculated.
is a great study on this by Ed Hare PH.D that you can read at www.semlab.com.
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.
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.
problem and the reasoning behind changing the criterion in the J-STD-001 Rev
F., 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.
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.
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.
to be safe, remove the gold and eliminate the worry about the exact amount of
gold on the leads and terminals
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.
there is a risk of embrittlement depends on several variables:
Amount of gold expected to be leached (soldered area*gold
Solder volume in the resulting joint
Whether or not the solder is from an "infinite source" such as a
wave, or from solder paste
Whether there is any gold contribution from the PWB finish (ENIG
or ENIPIG finish)
case, 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 (true
for reflow, conservative for wave). If we make these assumptions, I have in
fact seen cases where component gold thicknesses below 30 microinches still
result in marginal or unacceptable gold levels in the final joint.
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.
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.
best of my knowledge, gold plating in electronics at the package or assembly
level is still not applied via vapor deposition.
if the process uses immersion technology, then yes gold embrittlement is
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.
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 wirebonding) applied to PWB pads. Surface platings
such as ENIG, with only about 4 uinches of gold, do not contribute towards
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.
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 homogenous throughout the solder joint.
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).
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.
I already know that was a long time ago.
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.
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.
to 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.
God we trust, all others bring data.
luck with the qualification.
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.
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
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).
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.
bearing solder like Sn62 is recommended for soldering where leads are silver
plated or end termination of components are Palladium Silver to prevent silver
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 is
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 a
SnPb solder joint and ~10% by weight of gold in a SAC joint is typically known
as enough to cause embrittlement to occur.
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 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 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.
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.
The best solution is still to avoid or remove gold
David Mullins, Gavial Engineering and Manufacturing