The final coating process for PCB manufacturing has undergone significant changes in recent years. These changes are the result of the constant need to overcome the limitations of HASL(Hot air cohesion) and the growing number of HASL alternatives.

The final coating is used to protect the surface of the circuit copper foil. Copper (Cu) is a good surface for welding components, but is easily oxidized; Copper oxide impedes wetTIng of solder. Although gold (Au) is now used to cover copper, because gold does not oxidize; Gold and copper will quickly diffuse and permeate each other. Any exposed copper will quickly form a non-weldable copper oxide. One approach is to use a nickel (Ni) “barrier layer” that prevents gold and copper from transferring and provides a durable, conductive surface for component assembly.

PCB requirements for non-electrolytic nickel coating

The non-electrolytic nickel coating should perform several functions:

The surface of a gold deposit

The ultimate purpose of the circuit is to form a connection with high physical strength and good electrical characteristics between PCB and components. If there is any oxide or contamination on the PCB surface, this welded joint would not occur with today’s weak flux.

Gold deposits naturally on top of nickel and does not oxidize during long storage. However, the gold does not settle on the oxidized nickel, so the nickel must remain pure between the nickel bath and the dissolution of the gold. Thus, the first requirement of nickel is to remain oxygen-free long enough to allow gold to precipitate. Components developed chemical leaching baths to allow 6~10% phosphorus content in nickel precipitation. This phosphorus content in the non-electrolytic nickel coating is considered as a careful balance of bath control, oxide, and electrical and physical properties.

hardness

Non-electrolytic nickel coated surfaces are used in many applications that require physical strength, such as automotive transmission bearings. PCB requirements are far less stringent than those for these applications, but a certain hardness is important for wire-bonding, touchpad contacts, edge-connetor connectors, and processing sustainability.

Lead bonding requires a nickel hardness. Loss of friction can occur if the lead deforms the precipitate, which helps the lead “melt” into the substrate. SEM images showed no penetration into the surface of flat nickel/gold or nickel/palladium (Pd)/ gold.

Electrical characteristics

Copper is the metal of choice for circuit formation because it is easy to make. Copper conducts electricity better than almost every metal (table 1)1,2. Gold also has good electrical conductivity, making it a perfect choice for the outermost metal because electrons tend to flow on the surface of a conductive route (the “surface” benefit).

Table 1. Resistivity of PCB metal

Copper 1.7 (including Ω cm

Gold (including 2.4 Ω cm

Nickel (including 7.4 Ω cm

Non-electrolytic nickel coating 55~90 µ ω cm

Although the electrical characteristics of most production plates are not affected by the nickel layer, nickel can affect the electrical characteristics of high frequency signals. Microwave PCB signal loss can exceed designer specifications. This phenomenon is proportional to the thickness of the nickel – the circuit needs to pass through the nickel to reach the solder spot. In many applications, electrical signals can be restored to the design specification by specifying nickel deposits of less than 2.5µm.

Contact resistance

Contact resistance is different from weldability because the nickel/gold surface remains unwelded throughout the life of the end product. Nickel/gold must remain conductive to external contact after prolonged environmental exposure. Antler’s 1970 book expressed nickel/gold surface contact requirements in quantitative terms. Various end-use environments have been studied: 3 “65°C, a normal maximum temperature for electronic systems operating at room temperature, such as computers; 125°C, the temperature at which universal connectors must operate, often specified for military applications; 200°C, that temperature is becoming more and more important for flying equipment.”

For low temperatures, nickel barriers are not required. As the temperature increases, the amount of nickel required to prevent nickel/gold transfer increases (Table II).

Table 2. Contact resistance of nickel/gold (1000 hours)

Nickel barrier layer satisfactory contact at 65°C satisfactory contact at 125°C satisfactory contact at 200°C

0.0 µm 100% 40% 0%

0.5 µm 100% 90% 5%

2.0 µm 100% 100% 10%

4.0 µm 100% 100% 60%

The nickel used in Antler’s study was electroplated. Improvements are expected from non-electrolytic nickel, as confirmed by Baudrand 4. However, these results are for 0.5 µm gold, where the plane usually precipitates 0.2 µm. The plane can be inferred to be sufficient for contact elements operating at 125°C, but higher temperature elements will require specialized testing.

“The thicker the nickel, the better the barrier, in all cases,” Antler suggests, “but the realities of PCB manufacturing encourage engineers to deposit only as much nickel as is needed. Flat nickel/gold is now used in cellular phones and pagers that use touch-pad contact points. The specification for this type of element is at least 2 µm nickel.

The connector

Non-electrolytic nickel/gold immersion is used in the manufacture of circuit boards with spring fit, press-fit, low-pressure sliding and other non-welded connectors.

Plug-in connectors require longer physical durability. In these cases, non-electrolytic nickel coatings are strong enough for PCB applications, but gold immersion is not. Very thin pure gold (60 to 90 Knoop) will rub away from the nickel during repeated friction. When the gold is removed, the exposed nickel oxidizes rapidly, resulting in an increase in contact resistance.

Non-electrolytic nickel coating/gold immersion may not be the best choice for plug-in connectors that endure multiple inserts throughout the life of the product. Nickel/palladium/gold surfaces are recommended for multipurpose connectors.

The barrier layer

Non-electrolytic nickel has the function of three barrier layers on the plate: 1) to prevent the diffusion of copper to gold; 2) To prevent the diffusion of gold to nickel; 3) Source of nickel formed by Ni3Sn4 intermetallic compounds.

Diffusion of copper to nickel

The transfer of copper through nickel will result in decomposition of copper to surface gold. The copper will oxidize quickly, resulting in poor weldability during assembly, which occurs in the case of nickel leakage. Nickel is needed to prevent migration and diffusion of empty plates during storage and during assembly when other areas of the plate have been welded. Therefore, the temperature requirement of the barrier layer is less than one minute below 250°C.

Turn and Owen6 have studied the effect of different barrier layers on copper and gold. They found that “… Comparison of copper permeability values at 400°C and 550°C shows that hexavalent chromium and nickel with 8-10% phosphorus content are the most effective barrier layers studied “. (table 3).

Table 3. Penetration of copper through nickel to gold

Nickel thickness 400°C 24 hours 400°C 53 hours 550°C 12 hours

0.25 µm 1 µm 12 µm 18 µm

0.50 µm 1 µm 6 µm 15 µm

1.00 µm 1 µm 1 µ M 8 µm

2.00 µm Non-diffusion non-diffusion non-diffusion

According to the Arrhenius equation, diffusion at lower temperatures is exponentially slower. Interestingly, in this experiment, non-electrolytic nickel was 2 to 10 times more efficient than electroplated nickel. Turn and Owen point out that “… A (8%) 2µm(80µinch) barrier of this alloy reduces copper diffusion to a negligible level.”

From this extreme temperature test, a nickel thickness of at least 2µm is a safe specification.

Diffusion of nickel to gold

The second requirement of non-electrolytic nickel is that nickel do not migrate through “grains” or “fine holes” impregnated with gold. If nickel comes into contact with air, it will oxidize. Nickel oxide is not soldable and difficult to remove with flux.

There are several articles on nickel and gold used as ceramic chip carriers. These materials withstand the extreme temperatures of assembly for a long time. A common test for these surfaces is 500°C for 15 minutes.

In order to evaluate the ability of flat non-electrolytic nickel/gold-impregnated surfaces to prevent nickel oxidation, weldability of temperature-aged surfaces was studied. Different heat/humidity and time conditions were tested. These studies have shown that nickel is adequately protected by leaching gold, allowing good weldability after long aging.

Diffusion of nickel to gold may be a limiting factor for assembly in some cases, such as gold thermalsonic wire-bonding. In this application, the nickel/gold surface is less advanced than the nickel/palladium/gold surface. Iacovangelo investigated the diffusion properties of palladium as a barrier layer between nickel and gold and found that 0.5µm palladium prevents migration even at extreme temperatures. This study also demonstrated that there was no diffusion of copper through 2.5µm of nickel/palladium determined by Auger spectroscopy during 15 minutes at 500°C.

Nickel tin intergeneric compound

During surface mount or wave soldering operation, atoms from the PCB surface will be mixed with solder atoms, depending on the diffusion properties of the metal and the ability to form “intermetallic compounds” (Table 4).

Table 4. Diffusivity of PCB materials in welding

Metal temperature °C diffusivity (µinches/ SEC.)

Gold 450 486 117.9 167.5

Copper 450 525 4.1 7.0

Palladium 450 525 1.4 6.2

Nickel 700 1.7

In nickel/gold and tin/lead systems, the gold immediately dissolves into loose tin. The solder forms a strong attachment to the underlying nickel by forming Ni3Sn4 intermetallic compounds. Enough nickel should be deposited to ensure that the solder will not reach underneath the copper.Bader’s measurements showed that no more than 0.5µm of nickel was required to maintain the barrier, even through more than six temperature cycles. In fact, the maximum intermetallic layer thickness observed is less than 0.5µm(20µinch).

porous

Non-electrolytic nickel/gold has only recently become a common final PCB surface coating, so industrial procedures may not be suitable for this surface. A nitric acid steam process is available for testing the porosity of electrolytic nickel/gold used as a plug-in connector (IPC-TM-650 2.3.24.2)9. Non-electrolytic nickel/impregnation will not pass this test. A European porosity standard has been developed using potassium ferricyanide to determine the relative porosity of flat surfaces, which is given in terms of pores per square millimeter (bugs /mm2). A good flat surface should have fewer than 10 holes per square millimeter at 100 x magnification.

conclusion

The PCB manufacturing industry is interested in reducing the amount of nickel deposited on the board for reasons of cost, cycle time, and material compatibility. The minimum nickel specification should help prevent copper diffusion to the gold surface, maintain good weld strength, and keep contact resistance low. The maximum nickel specification should allow flexibility in plate manufacturing, as no serious modes of failure are associated with thick nickel deposits.

For most of today’s circuit board designs, a non-electrolytic nickel coating of 2.0µm(80µinches) is the minimum nickel thickness required. In practice, a range of nickel thicknesses will be used on a production lot of the PCB (Figure 2). The change in nickel thickness will result from the change in the properties of the bath chemicals and the change in the dwell time of the automatic lifting machine. To ensure a minimum of 2.0µm, specifications from end users should require 3.5µm, a minimum of 2.0µm, and a maximum of 8.0µm.

This specified range of nickel thickness has proved suitable for the production of millions of circuit boards. The range meets the weldability, shelf life and contact requirements of today’s electronics. Because assembly requirements are different from one product to another, surface coatings may need to be optimized for each particular application.