Southern Metal Finishing

Re: Electroless Nickel applications Involving Heat Excursions - Part 2

[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[/color]

Electroless Nickel applications Involving Heat Excursions - Part 2
By Don Baudrand; Consultant to the Metal Finishing Industry

Continued from previous thread........

Experiments were done using sulfamic acid, which is a good activator for nickel alloys including electroless nickel.  Soldering was successful and it appears that no harmful residues are left after rinsing, using sulfamic acid (60 g/L).  However, long term corrosion tests have not been completed.  Another successful method to use sulfamic acid is to rinse the plated device in a dilute solution of sulfamic acid (15 g/L) and allow it to dry on the surface (sulfamic acid is powder).  When the device is to be soldered, a rinse in warm water dissolves the sulfamic acid reactivating the surface.  An alcohol dip will remove water and prepare the part for soldering.  This way parts can be stored for long periods of time and remain solderable.  Storage should be in a fairly dry atmosphere.

It is common practice to over-plate electroless nickel with gold or palladium to preserve solderability.  However, the over plate 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.  Moreover, gold is soluble in solder and will eventually contaminate it to the extent that weak, dull solder joints result.
No Flux Soldering The use of electroless nickel followed by immersion gold, or electroless nickel followed by electroless palladium, then immersion or electroless gold can provide a surface which will solder without the use of a solder flux.  This process is used for high reliability devices and printed wiring boards.  For printed wiring boards, the process consists of acid cleaning, micro etching the copper clad board.  Then using a non precious metal catalyst, electroless nickel boron is deposited as a thin strike.  Electroless nickel phosphorus (7-8%P) is deposited through the holes, followed by electroless palladium, then immersion gold.  Sometimes the electroless palladium is omitted, however the highest reliability is realized when palladium is used.  The same process is used for via hole filling, wire bonding and to produce bumps for connections on flip chip assemblies.

Die Bonding  Electroless nickel phosphorus for die bonding requires a heat excursion above 700 degrees C, usually 800 C in a nitrogen/hydrogen atmosphere for 8-10 minutes on silicon wafers.  This leaves a nearly pure nickel surface that is easily gold, or other precious metal, plated.  Nickel will alloy to form 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, raised to 800 C for 8 minutes, then die bonded using 400 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 deposits can be die bonded without this thermal excursion. (4)  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 Ultrasonic wire bonding tests using aluminum wire were performed using electroless nickel boron plated printed wiring boards.  Long term reliability of the wire bond joints were measured by the destructive pull test at time zero and after 2000 hours of temperatures 85 degrees C and humidity at 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.  The alloy was Al, 99%, Si, 1%.  The tensile strength from 19-21 grams.  The samples tested ranged from 13.5 to 14.0 grams at time zero.  At the end of 2000 hours, at the specified temperature and humidity, all samples recorded a pull strength of 12-13 grams. (7)  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).

Thermocompression methods do not produce good bonds to electroless nickel.  Wire bonding to nickel phosphorus is not usually done unless specific thermal excursions are used to prepare the nickel surface as noted above.  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 conditions, proper selection of flux, joining time, temperature and the right furnace atmosphere make it possible to solder to any electroless nickel deposit regardless of the phosphorus or boron 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.