Soldering, Brazing, Die and Wire Bonding to Electroless Nickel;
Conditions that Influence Reliability
By Don Baudrand
Introduction
Reliable joining to electroless nickel deposits is critically important to the electronic industry as well as in hardware fabrication. Because electronic devices often are subjected 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 substrates.
Nickel is a good electrical conductor compared with thick films used to metallize ceramic substrates. Therefore, it is used to enhance conductivity as well as corrosion resistance, diffusion barrier and the ability to be soldered, brazed, wire and die bonded. 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 he 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, they 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 popular. The ability to plate uniform thickness over all surfaces enhance these characteristics. One place where advanced applications are found is in the electronics industry.

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.(3) Films on the surface of 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,
Electroless nickel has many forms. Examples are: Nickel phosphorus deposits having differing 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 degrees C to 400 degrees C cause the formation of nickel phosphide which hardens 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%) heat treat to a harder value and at a lower temperature than high P (10.5-12%) deposits. Nickel phosphorus plating solutions can also host alloying metals which further change the deposit characteristics.
Sometimes as little as 1/2 ppm can cause a profound change.
Nickel (B) boron deposits have different characteristics from those of nickel P deposits.
Similarly, nickel boron deposit characteristics change with boron content. Low B deposits are best suited for most applications. A nickel boron deposit containing 0.25% B has a melting point of 1455 degrees C. Nickel P with llwt.% P melts at 880 degrees C. ( a eutectic alloy).
Nickel boron deposits have high as plated hardness (700-770Kn100). They 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. "Nickel P starts to oxidize at about 400 degrees C and above 800 degrees C, decomposition of phosphorus and evaporation of phosphorus from the coating occurs. Above 600 degrees C, migration of phosphorus to the surface and formation of phosphorus oxide has been observed.. "A kinetic study at 800 to 1000 Degrees C showed that a Ni-P deposit oxidized about 100 times faster than pure nickel. The addition of even a small amount of boron to the deposit decreased the amount of oxidation." (2)
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-P deposits in air or in moist hydrogen results in removal of phosphorus from the surface of the deposit making it much more solderable, brazable and wire bondable. It is interesting that moist hydrogen atmosphere is reducing for nickel but oxidizing for phosphorus.(2) 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.
Ohmic contacts are made to thick film deposits on ceramic semiconductors by the use of electroless nickel and heat treat processes. Thick film conductors of gold, silver platinum etc. have DC resistivity which is several orders of magnitude higher than calcu lated values. Stable low resistance ohmic contacts having ease of solderability and reliable lead attachments using heat treated nickel-P alloys. This technique can be used to make ohmic contacts to intermetallic compounds such as gallium arsenide, thermoelectric semiconductors, bismuth-telluride, silicon and ceramic materials. Heat treating to 350 -
400 degrees C for 10-15 minutes provides the improved contact. If air oxidation increased the resistance, re-heating will restore the good ohmic contact characteristics. (4)
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 (poorer solderability) for non annealed deposits. (tests were done in vacuum to avoid atmospheric interference.) Higher contact angles mean 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/5Sn and
63Sn/37Pb solders. annealing time to low contact angle was from120 seconds minimum to reach equilibrium to about 600 seconds.
Solder Fluxes
Fluxes or special surface cleaning and activation processes are used to prepare electroless for soldering.
Solder fluxes are many. They are classified by the degree of activity. Inorganic flux es being the strongest and rosin 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 organic acids are used as mildly active fluxes and rosins are the least active.
Fluxes are used to remove surface oxides and sulfides, reduce surface tension of molten solder, and prevent oxidation during the heating cycle. R fluxes are rosin, RMA are rosin-mildly activated and RA fluxes are rosin- active. Soluble organic acids and inorganic acid fluxes complete the list.

Tests were made using dilute sulfamic acid. The results were favorable. However, although it appears that no harmful residues are left from using sulfamic acid, tests for long term corrosion have not yet been completed. One successful method to use sulfamic acid is to rinse thoroughly then dip in a dilute sulfamic acid solution and dry, leaving a slight film of sulfamic acid on the surface. Since sulfamic acid is a solid the film remains in tact until ready to solder. The rosin flux contains alcohol and some water which dissolves the sulfamic acid which in turn activates the electroless nickel deposit for good soldering.
In some cases users of this method have immersed the part to be soldered in warm water, activating the sulfamic acid film, alcohol dip to remove water and solder of use pure rosin then solder. This method is not in accordance with Military specifications, but seems to be quite effective.
It is common practice to overplate electroless nickel with gold or palladium to preserve solderability. However, the overplate must be pore free to prevent oxidation
(passivation)of the nickel at the pore sites. Porous precious metal over plating can accelerate passivation of the nickel due to the galvanic cell created by the dissimilar metals. for example, Steam-aged porous gold over electroless nickel will not solder easily due to the passivation of the nickel.
Die Bonding
Electroless nickel Phosphorus for die bonding requires a heat excursion above 700 degrees
C, usually about 800 degrees C in a Nitrogen/hydrogen for atmosphere for 8-10 minutes, leaves a nearly pure nickel surface that when gold plated, will alloy for the formation of a nickel silicide thereby giving rise to an excellent die bond. The best results were achieved when the temperature was at 500 degrees C for 3 minutes then raised to 800 degrees C for
8 minutes then die bonded using 400 degrees C for 0.75 minutes in the nitrogen/hydrogen atmosphere. Phosphorus can inhibit formation of nickel silicide during die bonding. Gold phosphorus silicide is very brittle and can cause die delamination. Nickel boron can be die bonded without this thermal excursion.(3)
Die bonding using conductive epoxy materials is usually successful on various electroless nickel deposits. Success depends somewhat on the epoxy formulation, as well as the condition of the deposit
.
Wire bonding
Aluminum wire ultrasonic wire bonding testing was performed using nickel boron plated printed wiring boards. Long term reliability of the wire bond joint was measured by the destructive pull test at time zero and after 2000 hours of temperature humidity (85 degrees
C and 81% RH)." The MIL-STD specifies a minimum time zero destructive pull strength of 3 grams. The aluminum wire was approximately 0.00125 inches in diameter, 99% Al,
1% Si, 19-21 grams tensile strength. The samples tested ranged from 13.5-14.0 grams at time zero. At the end of the 2000 hour test, all samples recorded a pull strength of 12-13

grams.(5 ) Thus electroless nickel boron deposits wire bond well. Higher ultrasonic energy is required for wire bonding to electroless nickel than for bonding to gold.
Bonding gold wire to electroless nickel boron requires thermosonic techniques. Similarly, higher ultrasonic energy is usually required. (not higher pressure).
Wire bonding to nickel phosphorus is not usually done unless specific thermal excursions are done to prepare the nickel surface as noted above. summa ry
Soldering, brazing wire bonding and die bonding can be successful using electroless nickel plating processes provided that the conditions for reliability are observed. Proper sintering condition, proper selection of flux, joining time, temperatures and the right furnace atmosphere make it possible to solder to any electroless nickel regardless of phosphorus content. Wire bonding energy level, for ultrasonic bonding to aluminum wire, preparation by sintering, if necessary, and the use of thermosonic methods for bonding gold wire make reliable joints.
Proper preparation of electroless nickel deposits make electroless nickel suitable for die bonding.
References
1.
Chwan-Ying Lee and Kwang-Lung Lin " Materials Interaction in Pb-Sn-P/Al and
Pb-Sn/Ni-B/Al Solder Bumps on Chips". Thin Solid Films, 229, 1993 63075
2.
W. Tomlinson and G Wilson. "Oxidation of electroless Ni-B and Ni-P Coatings in Air at
800-1000 degrees C., Chapman and Hall Ltd.
3, Kwang-Lung Lin and Jien-Ming Jang. "Wetting behavior Between Solder and
Electroless
Nickel". Materials Chemistry and Physics 38 (1994) 33-41.
4.
N. Gajbhite and T. Kutty. "Omic Contacts By Electroless Nickel Plating". Bulletin of
Electrochemistry May-June, 1986, pp,231
-
236
5.
D. Crotty and T Jackson. "Electroless Nickel Plating of Printed Wiring Boards For
Solderability and Wire Bonding." Sur-FIN 92