[ Date Time : 2010-10-28 ] [ Comments : 0
Electroless nickel coating / Immersion Gold may not be those who undergo repeated throughout the product life of the connector plug into the best choice. Recommended Ni / Pd / Au surface for multi-purpose connector.
Non-electrolytic nickel barrier layer on the board has three functions: 1) to prevent the spread of copper on gold; 2) to prevent the spread of nickel gold; 3) Ni3Sn4 the formation of intermetallic compounds of nickel source.
Diffusion of copper on the nickel
The transfer of copper by nickel-copper on the surface the result will be the decomposition of gold. Copper will soon be oxidized, resulting assembly poor weldability, which occurred in the case of leakage of nickel. Ni needed to prevent an empty plate during storage and when the plates have been welded in other regions during the dispersal assembly. Therefore, the barrier layer is below the temperature requirements of less than 250 ° C under a minute.
Turn and Owen6 studied different Barrier on the role of copper and gold. They found that "... in the 400 ° C and 550 ° C the value of copper penetration comparison shows that there are 8 to 10% of the phosphorus content of hexavalent chromium and nickel is the most effective barrier layer." (Table III)
Table III, nickel-copper through the penetration of the gold
Ni thickness of 400 ° C 24 小时 400 ° C 53 小时 550 ° C 12 小时
0.25 μm 1 μm 12 μm 18 μm
0.50 μm 1 μm 6 μm 15 μm
1.00 μm 1 μm 1 μm 8 μm
2.00 μm non-proliferation non-proliferation non-proliferation
In accordance with the Arrhenius equation, the diffusion at lower temperatures is exponentially slow. Interestingly, in this experiment, electroless nickel plating nickel-efficient than the 2 to 10 times. Turn and Owen pointed out that "... one (8%) this alloy 2μm (80μinch) barrier to the diffusion of copper reduced to a negligible extent."
Seen from this extreme temperature tests, at least the thickness of 2μm nickel is a security specification.
Nickel diffusion of gold
The second requirement of non-electrolytic nickel immersion gold nickel, not through the "particles" or "pore" migration. If the Ni contact with air, it will be oxidized. Welding and nickel oxide is not removed with the flux difficult.
Several articles about the nickel and gold for the ceramic chip carrier. Assembly of these materials withstand extreme temperatures reach a very long time. These surfaces is a common test temperature of 500 ° C for 15 minutes.
To assess flat non-electrolytic nickel / immersion gold surface and the ability to prevent the oxidation of nickel, the surface temperature aging of weldability. Tested different thermal / humidity and time conditions. These studies have shown that Ni fully protected by the leaching of gold, after long time aging allows good weldability.
Nickel diffusion of gold in some cases may be a limiting factor on the assembly, such as gold wire bonding heat wave (gold thermalsonic wire-bonding). In this application, the nickel / gold surface than the Ni / Pd / Au surface shifts some. Iacovangelo of nickel and palladium as a gold diffusion barrier layer and found that the palladium 0.5μm even in extreme temperatures to prevent migration. This study also proved that the temperature in the 500 ° C within 15 minutes, there is no Auger electron spectroscopy (Auger spectroscopy) determined by the copper diffusion through the 2.5μm nickel / palladium.
In wave soldering, surface mount or during operation, the surface atoms from the PCB with solder atomic mixing, depending on the metal diffusion and the formation of "intermetallic compound" capability (Table IV).
Table IV, PCB materials, the diffusion rate in the welding
Diffusion rate of metal temperature ° C (μinches / sec.)
Gold 450 486 117.9 167.5
Cu 450 525 4.1 7.0
Palladium 450 525 1.4 6.2
Nickel 700 1.7
In the nickel / gold and tin / lead systems integration into the bulk of tin into gold immediately. Ni3Sn4 through the formation of solder to form intermetallic compounds of nickel on the following strong adhesion. Precipitation of nickel should be sufficient to ensure the solder will not reach the bronze below. Bader measurements show that nickel does not require more than 0.5μm to maintain this barrier layer, or even to go through more than six times the temperature scanning. In fact, the observed maximum thickness of intermetallic compounds is less than 0.5μm (20μinch).
Electroless nickel / gold have only recently become a common final PCB surface coating, so that the surface of industrial process may not be suitable. There is a test used as a plug-in connector for electrolytic nickel / gold process the porous nature of nitric acid vapor (IPC-TM-650 18.104.22.168) 9. Electroless nickel / immersion gold pass this test. Has been used to develop a porous ferricyanide European standards, to determine the relative porosity of the surface plane, the result is a unit of the number of holes per square millimeter (pores/mm2) given. A good flat surface should be under 100 times magnification factor of less than 10 holes per square millimeter.
PCB manufacturing industry due to cost, cycle time and material compatibility reasons, to reduce the precipitation of nickel in the number of board interest. Minimum size nickel should help prevent the spread of copper on gold surface, good weld strength, and low contact resistance. The maximum size of nickel should be allowed to board manufacturing flexibility, because there is no serious failure mode is the thickness of the nickel deposition.
For most of today's circuit board design, 2.0μm (80μinches) of the non-electrolytic nickel coating is the minimum required thickness of nickel. In practice, the PCB will be a production batch of nickel using a range of thickness (Figure II). Changes in the thickness of nickel bath chemical properties will change and the presence of auto-lifting machine time change the results. To ensure the minimum of 2.0μm from the end user's specifications should be required to 3.5μm, a minimum of 2.0μm, a maximum of 8.0μm.
The scope of this provision thickness of nickel has been shown to be suitable for the production of millions of circuit boards. The range to meet the solderability, shelf life and requirements of today's contact with electronic products. Because the assembly requirements are different from one product to another product, the surface coating may need to be optimized for each specific application.