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.

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 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.

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