Gold plating the entire PCB


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MooseFET wrote:

Lately, I've been looking again at gold plating the entire PCB. It
makes a nice chemically stable layer on top of the copper. With the
price of copper these days, changes are the copper on the PCB will
cost more than the small amount of gold involved.



"Multi-layer surface finish, from the bottom to top, of electroless
Ni, electroless Pd, and immersion Au (Ni/Pd/Au) have been introduced
in the printed circuit board (PCB) industry recently. This paper
reports an evaluation of this surface finish from the perspective of
solder joint attachment reliability, especially to see if the
Ni/Pd/Au could be immune from the brittle interfacial fracture of
PBGA on electroless Ni/immersion Au, recently observed and reported
by us. PCBs with Ni/Pd/Au finishes, made from two vendors with varied
Pd layer thickness were attached with PBGA packages, and tested in
four-point bending. When joint strength is strong, bending tests
resulted in peeling off the PCB pads; otherwise, brittle fractures
occurred at the interface between solder balls and PCB pads. After
aging, solder joints on all Ni/Pd/Au and reference metal finishes
failed by the same brittle fracture at the interface between Ni-Sn
and Au-Sn intermetallic compounds. It is concluded that the
interfacial fracture was controlled by something other than the Pd,
and the existence of the Pd did not prevent the interfacial fracture.
Also, the presence of Pd could not prevent the Au migration and
subsequent fracture."

http://www.emeraldinsight.com/Insight/viewContentItem.do;?contentId=1455552



"ENIG (electroless nickel and immersion gold) plating applies
electroless nickel over copper surfaces. Nickel provides a barrier
to copper migration and protects the copper surface from oxidation.
A thin layer of gold plated over the nickel prevents nickel
oxidation until the board is processed. During soldering, if the
nickel-tin intermetallic layer becomes brittle and fractures, a
phenomenon called black pad may occur. Black pad may appear in
various shades of gray or black. Pad/land repair by wicking and
retinning may or may not be successful. There is no practical method
of prescreening PCBs for black pad. It occurs as non-wetting or a
premature joint failure due to a weakened interface at the board
surface."

http://smt.pennnet.com/articles/article_display.cfm?article_id=247127



"With direct gold-on-copper plating, the copper atoms tend to
diffuse through the gold layer, causing tarnishing of its surface
and formation of an oxide and/or sulfide layer. A layer of a
suitable barrier metal, usually nickel, is usually deposited on the
copper substrate before the gold plating.

"Gold reacts with both tin and lead in their liquid state, forming
brittle intermetallics. When eutectic 63% Sn - 37% Pb 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. Intermetallics have poor
electrical conductivity and low strength. The ongoing intermetallic
reactions also cause Kirkendall voiding, leading to mechanical
failure of the joint, similar to the degradation of gold-aluminum
bonds known as purple plague.

"A 2-3 µm layer of gold dissolves completely within one second
during typical wave soldering conditions. Layers of gold thinner
than 0.5 µm (20 microinches) also dissolve completely into the
solder, exposing the underlying metal (usually nickel) to the
solder. Impurities in the nickel layer can prevent the solder from
bonding to it. Electroless nickel plating contains phosphorus.
Nickel with more than 8% phosphorus is not solderable.
Electrodeposited nickel may contain nickel hydroxide. An acid bath
is required to remove the passivation layer before applying the gold
layer; improper cleaning leads to a nickel surface difficult to
solder. A stronger flux can help, as it aids dissolving the oxide
deposits. Carbon is another nickel contaminant that hinders
solderability."

http://en.wikipedia.org/wiki/Gold_plating


Note: with an entire trace plated in gold and solder resist
stopping most of the trace being wetted with solder, there
exists a region at the edge of the solder with a high
concentration of gold, followed by a region with solder
over undissolved gold.


Also see:
http://www.smtinfo.net/docs/Electronic%20Production/9.htm
http://www.polarinstruments.com/support/cits/AP171.html


--
Guy Macon
<http://www.guymacon.com/>

.

Relevant Pages

  • Re: Gold plating the entire PCB
    ... makes a nice chemically stable layer on top of the copper. ... cost more than the small amount of gold involved. ... I was not considering nickel under the gold. ... When eutectic 63% Sn - 37% Pb solder is ...
    (sci.electronics.design)
  • Re: American PCB fabs defaulting to lead-free
    ... they work fine using normal leaded solder. ... The downside is you have a possibility of tin whiskers ... Gold plating is bad, as it is a flash of electroless gold deposits a ... over nickel which is plated over the copper traces. ...
    (sci.electronics.design)
  • Re: Gold-PLated PCBs?
    ... Gold over nickel over copper? ... of exotic alloy for the solder? ... The only gold plating I've ever seen on a PCB has been edge ...
    (sci.electronics.design)
  • Re: American PCB fabs defaulting to lead-free
    ... they work fine using normal leaded solder. ... The downside is you have a possibility of tin whiskers ... Gold plating is bad, as it is a flash of electroless gold deposits a ... over nickel which is plated over the copper traces. ...
    (sci.electronics.design)
  • Re: Gold plating the entire PCB
    ... makes a nice chemically stable layer on top of the copper. ... cost more than the small amount of gold involved. ... This puts a very thin layer of gold over the pads. ... gold that gets absorbed into the solder joint. ...
    (sci.electronics.design)
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