Guest Editorial -For Plateworld.com
Don Baudrand, Don Baudrand Consulting, e-mail:firstname.lastname@example.org
Doís and Doníts of Heating Electroless Nickel Deposits
What happens when electroless nickel deposits are heated?
It is common practice to heat electroless nickel (EN) plating
to harden the deposit. Heating to 300 -400C for about 1 hour is usual. The
amount of phosphorus in the deposit and the exact temperature and time will,
determine the hardness of the deposit. For example an EN deposit containing
approximately 4% P will reach a higher hardness than higher phosphorus alloys.
For example about 4% P will show a hardness of about 700 Knoop hardness number.
A 9-11% P deposit will heat-treat to about 520-525 Knoop hardness number, 100 gm
The higher the P content the lower is the hardness. But the surface of high phosphorus EN will have a lower coefficient of friction. There is much information in the literature about hardening EN.
Electroless nickel boron deposits show a Knoop hardness number about 700 to770 as deposited for low boron content EN deposits of about 0.25% Boron. Wear resistance is considered to be very good for all types of electroless nickel. Comparing hard chrome and EN wear resistance shows favorable results depending on the type of wear instrument used. For example hard chrome shows better wear results when a Tabor wear instrument is used. But EN shows better wear resistance than hard chrome using a phalanx wear instrument. Speaking of hard chrome, the as- deposited hardness can vary over a large range depending on plating conditions such as plating solution temperature. The common misconception is that hard chrome deposits are always in the range of 1000 Knoop. But we know that the temperature of the chrome plating solution has a significant influence on the hardness of the deposit. For example, low solution temperature can result in a hardness of only 400Kn hardness. Maximum hardness is reached at about 120 degrees F (38C), plus or minus 8 degrees.
Interesting characteristics of EN include magnetic characteristics. For example, EN deposits of from about 3%P to 8% p are ferromagnetic. From about 9% to 12%P are not ferromagnetic, and remain non-magnetic when heated to 300 degrees for 1 hour. (An important characteristic for EN plated hard memory discs.)
What if the EN deposit temperature is raised to levels above 400 C? Higher temperatures cause interesting changes in the deposit characteristics. For example treating at 600 C, Phosphorus migrates to the surface and oxidizes the phosphorus. At 800 C and above phosphorus vaporizes from the Surface.
At 400- 450 degrees C for 10-15 minutes in a moist atmosphere will remove the phosphorus from the surface. This makes the deposit easier to solder, braze, and wire bond. An example is heating the "Forming Gas" (moist atmosphere) removes the phosphorus leaves a light nickel oxide on the surface that makes glass sealing bonds reliable, and soldering easy, a benefit to the electronics industry.
Heat treat to increase hardness in an inert atmosphere, such as nitrogen. Heat-treating in air can cause yellowing of the deposit.
Heat-treat for increased in air if yellowing is permitted. Sometimes heat-treating in air does not discolor the deposit. Most of the time High Phosphorous EN deposits do not discolor when heat-treated in air.
Heat-treat in forming gas or equivalent, or moist hydrogen to remove phosphorus from the surface. Removal of P makes the EN deposit easy to solder, glass to metal seal, and easy to wire-bond, all benefits to the electronics industry.
For soldering an acidic solder flux, such as RMA should be used. A dilute sulfamic acid works also.
Use nickel boron electroless nickel for its hardness, soldering ease, wire bonding ease, and high electrical conductivity (8-20 micro-ohm-cm).
Make large chemical make-up additions to the plating solution.
Allow dust or any particulate matter to enter the EN plating solution.
Particles cause rough deposits and can cause the solution to spontaneously plate
out as nickel particles, a costly event.
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