Electroless Nickel applications Involving Heat Excursions - Part 1

[color="Navy"]This article was published in the July 2005 issue of Southern Metal Finishing. If you would like register to receive our free newsletter and review our online archives please visit www.southernmetalfinishing.com[/color]

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By Don Baudrand; Consultant to the Metal Finishing Industry

Reliable joining to electroless nickel deposits is critically important to the electronic industry as well as in hardware fabrication.  Be cause electronic devices are often structure to extreme temperature excursions during fabrication, electroless nickel deposits may change physical characteristics in bulk and on the surface where joining takes place.  High temperatures are common for multi-chip modules (MCM-C) and hybrid circuits on ceramic surfaces.

Electroless Nickel is a good electrical conductor compared with thick film materials used to metallize ceramic substrates.  Therefore, it is used to enhance conductivity as well as to improve corrosion resistance and provide a superior diffusion barrier.  The ability of electroless nickel deposits to be soldered (RMA fluxes or organic acid fluxes), easily brazed, wire and die-bonded make it a valuable and useful tool for electronic devices.  The conditions for each joining method must be correct for the type of electroless nickel deposit.  Adjustments may be necessary to compensate for the environment to which the deposit is exposed in the fabrication process.

Electroless nickel deposits have excellent uniformity, good corrosion resistance, strong adhesion to substrates and superior solderability under the right conditions.

Electroless nickel deposits have been recognized as a satisfactory barrier material.  It has a slow transport rate in the substrate and the adjoining layer, laterally uniform in thickness and structure.  The deposits are thermodynamically stable against substrate and adjoining material and have low resistivity.  (1&2)

Numerous papers have been published and presented at conferences dealing with the virtues of electroless nickel for many engineering and hardware applications.  Hardness, wear resistance and corrosion protection are characteristics which make electroless nickel popular.  The ability to plate uniform thickness over all surfaces enhances these characteristics.  One place where advanced applications are found is in the electronics industry.

Electroless nickel has many forms.  Examples are:  Nickel phosphorus deposits having different phosphorus (P) content.  The physical, electrical and surface characteristics are different for each level of phosphorus.  In addition, the reaction to “heat-treat” conditions differ widely.  It is well known that heating nickel phosphorus deposits to temperatures of 300 to 400 degrees C causes the formation of nickel phosphide which gardens the deposit.

The degree of hardening depends on the amount of P and the heat treat temperature and time.  For example, low P deposits (3-4% P) heat treat to a harder value and at a lower temperature than high P (10.5-12% P) deposits.  Nickel phosphorus plating solutions can also host alloying metals which further change the deposit characteristics.  Sometimes as little as ½ part per million (ppm) can cause profound change.

Nickel boron deposits have different characteristics from those of nickel phosphorus deposits.  Similarly, nickel boron deposit characteristics change with boron content.  Low boron deposits are best suited for most applications. A nickel boron deposit containing -0.25% boron has a melting point 1455 degrees C.  Nickel Phosphorus with 11% phosphorus melts at 880 degrees C. (a eutectic alloy)  Nickel boron deposits have higher conductivity than electroless nickel phosphorus deposits, and high as plated hardness (700-770Kn100).  Nickel boron with higher boron content (1.5-4% B) heat treat to higher values than for nickel phosphorus, and at lower temperatures. (2) Electronic microcircuits and packaging requires a suitable barrier layer which can sustain a long period of service.  Nickel has the slowest dissolution rate in solder and the slowest intermetallic compound formation rate compared with gold, silver, copper, palladium or cobalt.

Sintering Electroless Nickel Deposits
Let us examine what happens to electroless nickel phosphorus when it is heated under certain conditions.  Heating to 250-400 C, nickel phosphorus deposits harden considerably reaching a maximum at about 385 C, depending on the phosphorus content.  Oxidation takes place and the deposit changes in volume, composition and structure.  Nickel-phosphorus intermetallic compound forms.  Oxidation of both nickel and phosphorus occurs.  Above 600 C, migration of phosphorus to the surface and formation of phosphorus oxide has been observed.  Above 800 C decomposition and evaporation of phosphorus from the coating occurs.  “A kinetic study at 800 – 1000 C showed that a Ni-P deposit oxidizes about 100 times faster than pure nickel.  The addition of even a small amount of boron to the deposit decreased the amount of oxidation.  Phosphorus in the Ni-P coating is the cause of the poor oxidation resistance of the alloy and the much purer Ni-B coatings may be expected to have a much improved resistance to oxidation.” (2)

Heating nickel phosphorus in air or moist hydrogen to a temperature of 400-850 C for 10-15 minutes results in removal of phosphorus from the surface of the deposit, making it much easier to solder, braze, wire bond or die-bond.  Ohmic contacts are made to thick film layers on ceramic semiconductors by plating electroless nickel and heat treating gold, silver or platinum.  Thick-film conductors are much more electrically resistant than expected from calculated values.  Plating and then heat treating electroless nickel on these films enables the fabrication of stable, low-contact-resistance metal layers. (3)  Nickel-boron deposits do not need the thermal excursion for die or wire bonding.  However, if for certain brazing applications or exposure in laser devices, boron must be removed.  This is accomplished by heating the nickel boron deposit in a moist hydrogen atmosphere to 950 C for 30+ minutes and evacuating.  Boron hydride gas is formed and is removed by this process.  It is interesting to note that moist hydrogen atmosphere is reducing for nickel but oxidizing for phosphorus.  Phosphorus is oxidized by the oxygen supplied by some dissociation of water, and vaporized from the surface, leaving a few angstroms of pure active nickel. (3)

Soldering to Electroless Nickel Deposits Soldering is usually defined as joining metals at low temperatures (usually below 800 degrees F, 426 C) by fusing a low melting alloy.  The interaction between nickel and solder is important in determining reliability.  Nickel-tin compounds form when using tin-lead solder.  Ni3Sn, Ni3Sn2 and Ni3Sn4 have been detected at the interface. (4)  Films on the surface of the electroless nickel deposits such as soils, phosphorus compounds, and oxides of nickel can interfere with soldering.  Non wetting or dewetting results from foreign substances on the surface to be soldered.  In the absence of these films good wetting of the surfaces by the solder results in a reliable joint.

Contact angle measurements and miniscograph instruments are commonly used to measure the wetting ability of solder to the substrate materials.  It is reported that solder contact angle varies with phosphorus content.  The higher the phosphorus content, the higher the contact angle, meaning poor solderability, for non annealed deposits.  Tests were done in vacuum to avoid atmospheric interference.  Higher contact angle means less wetting.  However, when deposits were heated to temperatures in excess of 340-410 degrees C, prior to soldering, solder contact angles improved.  This was true for both 95Pb/3Sn and 63Sn/37Pb solders.  Annealing time to low contact angle was from 120 seconds minimum to reach equilibrium to about 600 seconds.

Solder Fluxes or special surface cleaning and activation processes are used to prepare electroless nickel for soldering.  Solder fluxes are many.  They are classified by the degree of activity.  Inorganic fluxes being the strongest and rosin is the weakest.  Inorganic acids such as zinc chloride, are not used by the electronics industry because the residues which may be left are corrosive.  Water soluble mildly active organic acids fluxes along with rosin mildly activated are commonly used in electronic applications.  Rosin flux is the most common for electronics because of its non corrosive nature.  However, it must be removed by alcohol or other non aqueous solvents.  Fluxes are used to remove surface oxides and sulfides, reduce surface tension of the molten solder, and prevent oxidation during the heating cycle.  R fluxes are rosin, RMA are resin-mildly activated and RA fluxes are rosin-activated.  Soluble organic acids and inorganic acid fluxes complete the list.  It is possible to use certain organic acids to prepare electroless nickel for soldering.  Non halide organic acids provide the equivalent of mildly activated flux without danger of leaving undesirable corrosive materials after soldering.  Since they are water soluble, residual flux is usually easily removed by water or mild cleaners.

See following thread for part 2 of this article..........