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MooseFET wrote:
"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."
"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."
"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."
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.
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