Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doí and Doníts of Soldering, Di and Wire Bonding to Electroless Nickel Plated Deposits


It is generally thought that it is not possible to solder to electroless nickel (EN) plated deposits unless a strong acid flux is used. The fluxes can be harmful to other components in an assembly, particularly if it is an electronic device. So, soldering is often avoided where electroless nickel could provide many advantages. Likewise Di attachment and or wire bonding are not considered due to the fear of the use of electroless nickel plated surfaces. Nickel and electroless nickel are good conductors compared with thick films used to metalize ceramic devices.
Electroless nickel deposits also provide good corrosion resistance, good adhesion and have excellent uniformity of thickness.

Soldering to electroless nickel

When soldering, intermetallic compounds form to provide high adhesion values. Compounds such as Ni3Sn, Ni3Sn2 and Ni3Sn4 are formed that provide excellent adhesion. Also nickel-gold, nickel-silver and nickel-copper compounds can form, although slowly.

Typical Surface films that interfere with adhesion are soils, phosphorus, and oxide. Electroless nickel deposits contain phosphorus or boron and oxides. To form good bonding to electroless nickel-phosphorus, soils, oxides and surface phosphorus must be removed. For electroless nickel-boron deposits the boron aids soldering. (Adding boron to the electroless nickel-phosphorus solution the NiB-P deposits aids soldering.) Nickel-boron electroless nickel solders easily, but is much more expensive. Low boron content EN-B works best. To make electroless nickel-phosphorous plated deposits easy to solder requires a strong acid flux or a heat-treat process. There are several heat-treat processes that can remove phosphorus from the surface along with oxide to make EN-P solderable. At 600C the Phosphorus begins to migrate to the surface. In moist hydrogen the phosphorus is removed at 600 to 800C. The moist hydrogen atmosphere is reducing for the nickel and phosphorus is oxidized by the oxygen supplied by the dissociation of the water and is vaporized from the surface leaving a pure nickel surface.

Solder Fluxes

Solder fluxes are many. They are classified by the degree of activity. Inorganic being the strongest and resin the weakest. Fluxes are used to remove surface oxides and sulfides, reduce the surface tension of molten solder and prevent oxidation during the heating cycle.

R fluxes are rosin, RMA are rosin-mildly activated and RA fluxes that are the most active. Soluble organic acid and inorganic acid fluxes complete the list.

A suitable inorganic flux is sulfamic acid. In tests a dilute solution of sulfamic acid did indeed act as a suitable flux alone. R, RMA, RA and organic acid fluxes can be used in addition to the sulfamic acid film that was left on the item for long times (several months.) Any water soluble flux can activate the sulfamic acid, including DI water. If sulfamic acid is applied the unit to be later soldered should be stored in a dry atmosphere. Sulfamic acid is a solid when dry and becomes an acid when water is added.

Di bonding

Di bonding to electroless nickel-phosphorus plated deposits requires a heat excursion above 700C, usually 800C for 8-10 minutes in a nitrogen/hydrogen atmosphere. This treatment leaves pure nickel on the surface that is needed for good bonding. An electroless gold or gold electroplating will minimize oxidation of the nickel so that Di bonding can take place by forming a nickel silicide intermetallic. Phosphorus inhibits the formation of nickel silicide, and/or gold-phosphorus-silicide. The result is a brittle joint that could cause delaminating from the silicon device.

Electroless nickel-boron deposits do not need the thermal excursion to form good quality bonding to silicon chips.

Wire bonding onto electroless nickel-boron plated surfaces.

Electroless nickel plated deposits wire bond well using aluminum wire and ultrasonic bonding systems. Higher ultrasonic energy is required for bonding to electroless nickel than for gold plated deposits. Bonding gold wire to electroless nickel-boron requires thermo sonic technique and higher energy. Long term reliability of wire bonded joints using aluminum wire 0.000125 inches in diameter wire bonded to electroless nickel boron deposits were run by destructive pull tests at time zero and after 2000 hours at 85C and 81% relative humidity. The Mil standard requires pull strength of 3 grams. The samples tested ranged from 13.5 to14 grams at time zero. At the end of 2000 hour test all samples recorded pull strength

Doís and Doníts of soldering to Electroless nickel

Do

Know that Phosphorus in the deposit causes poor soldering wetting and adhesion therefore must be removed from the surface before soldering. Boron in nickel-boron deposits does not have to be removed for soldering.

Do

To Solder,remove oxides on the surface must be removed from both nickel phosphorus and nickel boron deposits. Select active Rosins, or mild acid such as sulfamic acid to remove oxides, or heat at 600 to 800C (1110-1471F) in a moist hydrogen atmosphere for 12-15 minutes. Solder using RMA flux. The same is true for DI bonding except flux is not used. Wire bonding to EN requires higher energy, not more pressure.

DO

Add boron to aid to the EN solution for soldering of nickel phosphorus plating systems. This adds cost but aids activated rosin fluxes in oxide removal without the high temperature excursion. Boron is added by including a small amount of dimethyamine boron in addition to the usual sodium hypophosphite. This adds cost.

Use Electroless nickel-boron deposits for soldering, wire and DI bonding. ( more expensive than Ni-P)

Doníts

Delay attaching after the heat treat process.

Delay after fluxing for soldering.

Use Fluxes for wire bonding or for DI bonding.

You may download this article FREE in .pdf form, save it or share it with a colleague. Click here.