This specification covers the requirements for electroplated nickel coatings applied to metal products for engineering applications, for example, for use as a buildup for mismachined or worn parts, for electronic applications, including as underplates in contacts or interconnections and in certain joining applications.
Electroplating of nickel for engineering applications requires technical considerations significantly different from decorative applications.
Functional electroplated nickel coatings usually contain about 99 % nickel, and are most frequently electrodeposited from a Watts nickel bath or a nickel sulfamate bath. Typical mechanical properties of nickel electroplated from these baths, and the combined effect of bath operation and solution composition variables on the mechanical properties of the electrodeposit are given in Guide B 832. When electroplated nickel is required to have higher hardnesses, greater wear resistance, certain residual stress values and certain leveling characteristics, sulfur and other substances are incorporated in the nickel deposit through the use of certain addition agents in the electroplating solution.
Nickel is plated from a number of different electrolytes (watts, woods, sulfamate and electroless) and for a number of different purposes (decorative, engineering and barrier coats). Each coating has its own unique properties that can be selected to meet specific customer requirements.
In it simplest form decorative nickel is plated from a watts bath usually over a polished or bright substrate. As plated nickel deposited from a watts bath are yellowish in appearance and it has an ability to level out small imperfections in the base metal. The amount of leveling that a nickel bath provides is based on its organic makeup as well as the overall condition of the plating solution. In order to meet with a decorative appearance, the nickel is very often post plated with a layer of chrome, brass or gold, depending on the end users requirements. The decorative deposits that we are most accustomed with are nickel/chrome that is seen on car bumper and motorcycle parts, and nickel/gold that is used on the majority of costume jewelry.
|Electroless Nickel (Hi and Mid Phos)|
EN baths are unique in that they do not require any electrical current to deposit nickel metal. These baths are autocatalytic in nature. Because of this we do not have to be concerned with high and low current density areas. When a part is submerged in the plating solution the part will plate at the same rate anywhere in ht part surface. A good rule of thumb for a mid-phos EN is that it will deposit .0007" in one hour.
The difference in plating deposit between Electroless and Electroplating is showing on the figure bellow.
Besides the uniformity of the deposit, EN also has many other characteristics that can be used for engineering purposes such as ductility, lubricative, corrosion resistance, wear and abrasion resistance as well as solderability and hardness. The characteristics listed above can be altered by the phosphorous content of deposit. For example high and low phosphorous deposits have a much higher corrosion resistance than a mid-phos bath.
Electroless Nickel plating is essentially the process of producing a metallic layer on the substrate using a autocatalytic bath. Based on a Chemical Reduction, no electrical current is required to perform the metal deposition.
With excellent corrosion resistance the electroless nickel-phosphorus bath provides a uniform deposit without buildup at the edges or corners of the component being plated, a sharp edge receives the same thickness of deposits, as does a blind hole. This characteristic is very important on the field of metallic products for engineering (functional) uses. The coatings are generally deposited from acidic solutions operating at elevated temperatures.
The coatings are alloys of nickel and phosphorus produced by autocatalytic chemical reduction with hypophosphite. Because the deposited nickel alloy is a catalyst for the reaction, the process is self-sustaining. The chemical and physical properties of the deposit vary primarily with its phosphorus content and subsequent heat treatment (ASTM B733-04).
Different types of Electroless Nickel have been created to offer special properties, based on the end-use requirements.
Deposits from these baths can be identified by the phosphorus content.
Very low phosphorus
Applications requiring abrasion and wear resistance.
Electronic applications, solderability, bondability, increased electrical conductivity, and resistance to strong alkali solutions.
To meet the general purpose requirements of wear and corrosion resistance.
On beryllium and titanium parts for low stress properties
With low phosphorus content deposited 3 to 4%, the Alkaline Nickel-P is plated at moderately low temperature, making them suitable for plating on plastics and to many applications in the electronics industry, when good solderability is required.
Nickel-boron baths are normally formulated using an amine borane as the chemical reducing agent. Alloy deposits can be plated from acid or alkaline baths. Is often used in electronic applications to provide specific harder deposit and also produce a high electrical conductivity layer.
Types of Nickel-Boron Baths:
Solutions deposits having three or four elements designed to maximize qualities such as corrosion resistance, hardness, high-temperature resistance, electrical properties and magnetic or nonmagnetic characteristics.
Electroless Nickel can also be used for the restoration of worn pieces or salvage of undersized (mismachined) parts.
Woods nickel deposits are almost always used as an activator over stainless steel. The nickel deposited will remain active longer than the stainless steel allowing time for subsequent deposits to be applied without concern of adhesion problems. Woods nickel is used as an activator/nickel strike over many varieties of stainless steel.
This is because stainless steel is difficult to plate metal due to the formation of a passive oxide on the surface of the metal. Woods nickel baths are comprised of only two items, nickel chloride and hydrochloric acid used at a 1:1 ratio.
|Sulfamate / Dull|
Nickel sulfamate deposits have little or no stress even at heavy thickness. This solution is most popular for electroforming because of its high rate of deposition and its lack of stress.
Bright nickel solutions are similar to decorative nickel solutions but are considered not as HOT as bright nickel solutions. This means that they have less organic additives that tend to make the nickel deposit more ductile but not as bright as a decorative bath. These baths are used where parts may be crimped or bent during the assembly process.