The need to use immersion gold plating for soldering is due to a number of reasons.
First of all, it is an alternative to metallurgical solder coatings.
Although hot-tin (HASL) or tin-lead reflow has better solderability, they leave sagging on the board that interferes with paste and small components.
In addition, the powerful thermal shock experienced by the boards during tinning and reflow injures them and reduces the resource for ensuring the reliability of interconnections.
But the increase of printed density layout nodes through the use of BGA components with low-pitch and chip components require planar mounting surfaces.
This is what led to the use of topcoats. Providing a combination of good solderability and a flat surface for mounting and soldering highly integrated components.
Among flat topcoats, immersion gilding is not the only one.
But so far it occupies a leading position in terms of prevalence in products for responsible use.
Table. Prevalence of topcoats in the global PCB market 2020
|Organic coating (OSP household products)||10%||11%||12%||12%||12%|
|Nickel immersion gold||14%||16%||19%||26%||20%|
|Immersion tin with barrier underlay||1%||3%||8%||11%||28%|
The coating itself is a composition of copper of the contact pad, a sublayer of chemically deposited nickel, and immersion deposited gold.
A thin layer of gold 0.05-0.1 microns thick has the only function – to protect nickel from oxidation for subsequent soldering.
When soldering, it quickly dissolves in the solder, while a fresh nickel surface is exposed for wetting the solder m.
Any immersion process consists of the reaction of replacing one metal with another from a solution.
Therefore, the thickness of immersion gold in this case, in principle, cannot be large: as soon as the nickel surface is covered with gold, its interaction with the solution for the substitution reaction will cease.
This means that all areas of the nickel surface will be necessarily covered with gold, as long as they are free for the substitution reaction, and also that, despite the extremely small thickness of the immersion-deposited gold, its continuity is guaranteed by the very mechanism of the process.
Immersion gold could be deposited directly onto the copper of the contact pad, but their mutual diffusion would lead to a rapid loss of solderability due to the transformation of a thin layer of gold into the inter metalloid, which does not dissolve in the solder.
A 3–6 µm barrier nickel sublayer prevents this diffusion process and loss of solderability.
Immersion gold with a nickel sublayer (Electroless Nickel / Immersion Gold, ENIG) allows several re-soldering cycles and guarantees solderability for 6 months.
This coating has a flat contact surface and is well wetted by solder with the correct choice of flux.
Immersion gold can also be used as a wrap-around coating, a coating for push-type contacts, for connectors with zero mating force (contact without friction), for detachable connectors, provided that they are mated/disengaged no more than five times.
The Sequence of the Process of Deposition of Immersion Gold with a Sublayer of Chemical Nickel:
- Acidic cleaning;
- Chemical deposition of nickel sublayer;
- Application of immersion gold.
The acidic cleaner removes oils, oxides, and fingerprints from copper surfaces.
It has no effect on solder masks, paints, epoxy phenol substrates. The micro-etch agent evenly etches the copper surface, which gives excellent adhesion to nickel during its subsequent deposition.
The activator is colloidal palladium. Such an activator completely catalyzes the copper surface without affecting dielectrics.
The use of an activator guarantees a dense nickel deposit during the subsequent processing of the board in a chemical nickel plating bath.
The chemical nickel plating solution gives a high-quality semi-shiny nickel-phosphorus alloy coating with good ductility and excellent adhesion to the copper surface of the contact pad (Fig. 1).
A dense, fine-crystalline, shiny gold deposit of 24-carat gold should be obtained from a solution of immersion gilding (Fig. 2).
Fig. 1. Plating nickel-phosphorus, obtained from a solution of chemical nickel plating KEM NI 6000 (increase 10,000)
Fig. 2. Dense 24-carat gold plating, obtained from a solution of immersion gilding KEM A 3000
The studies carried out in  and confirmed by the practice of the PTC PP of the GRPZ showed that the phenomenon of “black contact area” is associated with excessive corrosion of nickel in the process of immersion gold deposition.
If the crystal structure of the deposited nickel has a form other than normal (Fig. 1), with large intercrystalline interlayers, as shown in Fig. 3, this means that not the entire nickel surface is involved in exchange reactions with the gilding solution, and the foreign interlayers themselves, not covered with gold, are the cause of the nucleation of corrosion centers (Fig. 4).
Fig. 3. Crystal structure of chemically reduced nickel with large intercrystalline interlayers
Fig. 4. Black nickel surface (enlarged image)
What Provokes the Formation of Excessively Large Intercrystalline Interlayers?
It is known that during the formation of a crystal structure, all components foreign to a crystal are displaced into the space between crystals – into intercrystalline interlayers.
In this case, the phosphorus accompanying the reaction of the chemical reduction of nickel can form a solid solution with nickel, or it can be displaced into the intercrystalline space.
The fine-crystalline structure of nickel with intercrystalline interlayers is formed with a phosphorus content of up to 7%.
With higher phosphorus content – from 7 to 12% – the structure of the nickel layer acquires an amorphous form, which means that it does not have a crystalline structure and intercrystalline interlayers.
In this case, the reaction of replacing nickel with gold occurs uniformly over the entire surface with good hiding power, which prevents nickel oxidation.
This leads to the first recommendation: contact pad “for chemical nickel plating, you should ensure the maximum concentration of phosphorus.
Surface processes of redox reactions are, in one way or another, associated with gradients of electrochemical potentials.
Therefore, any surface heterogeneity, including edge effects, is unacceptable for uniform gold coverage. And any local non-coverage entails the danger of a “black contact area “. Hence the second recommendation: surfaces subject to immersion gilding should be smoothed as much as possible, this can be achieved by performing a micro-etching operation.
What is the Immersion Gold Recovery Process?
The gold recovery process is accompanied by the dissolution of nickel, that is, it is the process of nickel corrosion.
At a high reaction rate, the replacement process may turn out to be unbalanced, nickel corrosion may become predominant, and a black nickel surface is already formed under the gold, which is not yet visible to the eye.
The ready-made processes and solutions for immersion gilding offered by the market  include components that inhibit the redox process. The third recommendation is to use reliable proven suppliers of chemical processes and materials.
General recommendations for ensuring the stability of the gold plating process:
- Most manufacturers use a combined positive method using a tin metal resist to selectively etch copper. For the subsequent application of the mask and gold plating, it is removed. It is important that its removal and subsequent rinsing are complete; otherwise, the remnants of the metal resist can cause local corrosion of copper with its spread to subsequent layers of nickel. For the tenting method, careful development and washing of the photoresist must be foreseen, avoiding the presence of a veil.
- Surface preparation for immersion gilding, as already mentioned above, is a fundamental operation to ensure the necessary morphology of nickel and gold coatings applied afterward. Micro-etching solutions give guaranteed results [2, 5]. During micro-etching, the copper surface is activated by removing the top “poisoned” layer and receives a micro-roughness that ensures good nickel adhesion. The uniform activation of the copper surface promotes uniform deposition of palladium through the displacement reaction, which in turn ensures uniform deposition of nickel. It is important that this should be followed by thorough cleaning of the surface to be metalized to prevent palladium from entering the nickel-plating solution, which would lead to the destruction of the solution.
- When depositing nickel, it is important to prevent the high deposition rate that creates thick and deep intergranular formations – the causes of corrosion. To do this, it is necessary to accurately maintain the pH of the solution within ± 0.1. The same applies to the accuracy of maintaining the temperature of the solution – ± 1 ° C at the total temperature of the solution in the range of 85 … 90 ° C. During operation, the nickel plating solution requires constant replenishment with nickel and a reducing agent. Therefore, it is advisable to equip nickel plating baths with a system of automatic dosing and control of pH and temperature.
- To ensure the stability of the nickel plating process, a stabilizer is provided in the solution. Stabilizer control should be part of the daily maintenance of the chemical nickel plating bath. Active stirring of the solution facilitates the delivery of the stabilizer to the metalized surface, which compensates for its deficiency.
- The uniformity and rate of gold deposition are ensured by maintaining its concentration in the solution and the temperature of the solution. Too high a temperature leads to uneven gold deposition and an undesirable acceleration of redox reactions. Too low a temperature slows down the deposition process. The low concentration of gold causes non-uniformity of the coating with non-coatings, under which nickel does not receive protection from oxidation and corrosion. The immersion time of the boards in the gilding bath should be sufficient to obtain a continuous film, but no more. Excessive stay of boards in the gilding bath insignificantly increases the thickness of the gold, but due to inevitable diffusion processes leads to nickel corrosion.
- Immersion gilding is performed on the mounting surfaces – in the solder mask windows. It is necessary to achieve full manifestation, washing, and hardening of the mask so that there are no residues in the windows, which will then violate the morphology of sediments. An incompletely cured mask will degrade in aggressive hot chemical nickel plating solutions and deposit on the surface of the contact pads. The adhesion of the coatings will be weakened.
- Some solder mask suppliers guarantee their resistance to hot nickel plating solutions, so they recommend using immersion gold plating prior to applying the solder mask. This is absolutely unacceptable! The open surface of the dielectric base of the boards, which has significant porosity, absorbs ionic products of solutions, which cannot be completely removed even with thorough rinsing. Their residues in the form of metal ions and halogens lead to a significant decrease in the quality of the electrical insulation of the boards and the loss of their reliability. Not completely washed out ionic residues of solutions reduce the insulation resistance in conditions of high humidity. Residues of chemical contaminants in high humidity conditions provoke osmotic phenomena, leading to the detachment of the solder mask and the moisture-proof coating. Ionic contamination creates an electrolyte on the board surface under the mask, in which electrochemical processes develop, ending in the formation of electrically conductive bridges – “dendrites” and, accordingly, short circuits. The increased dielectric loss of the board, due to the presence of chemical contaminants, reduces the levels of microwave signals (IPC 4252 standard). Besides:
- Treatment of the open surface of the boards in aggressive solutions will lead to the destruction of the adhesive layer of the foil and, as a consequence, to the exfoliation of thin conductors and the formation of sinuses under them, in which contamination accumulates.
- Solder mask adhesion to immersion gold plated conductors is much lower than to copper conductors. The mask may peel off during use. Therefore, the solder mask should only be applied to the developed surface of copper conductors (IPCSM-839), obtained by mechanical processing or micro-etching, and not to a thin shiny layer of gold.
- In the process of applying a solder mask on immersion gold and its thermal tanning, the gold surface is “poisoned” by the vapors of organic compounds that make up the solder mask, which impairs the wettability of the mounting surface of the contact pads with solder m and reduces the reliability of the soldered joint.
- At the final stage of processing, the board must be thoroughly washed with quality control of washing and dried and then placed in a vacuum package.
Gold plating is a process that requires a high technological culture. These recommendations are only a part of the features of the gold plating technology used by the authors.