简介概要

Morphology and electrical contact properties of electrical connection materials in corrosive atmosphere

来源期刊：Rare Metals2013年第2期

论文作者：Zhi-Gang Kong Liang-Jun Xu

文章页码：174 - 178

摘要：The impact mechanism of environmental factors, such as corrosive atmosphere, on connector materials was investigated, and the porosity of gold plating was tested. Series of inspections and analytical research methods were introduced in this article. The surface morphology of specimens after corrosion was observed by stereoscopic microscope and scanning electron microscope. Chemical constitution was examined by X-ray energy spectrum. The contact resistances were measured by four-point method. The experiment results show that after exposure to certain environment, the corrosion products, such as Cu2O, Cu（NO3）2·3H2O, and NiO, are observed on the surface of the specimens without gold coatings, whereas the corrosion products appear to have circle-shaped spots on gold-plating surface after corrosion test, which indicate that the gold plating has good corrosion protection. The porosity is increased with the increase of corrosion time for every kind of specimens gold plated, and the corrosion degree of goldplating specimens is decreased with the increase of the thickness of gold coatings. The static contact resistances of circle-shaped spots appear higher contact resistance than normal value, which can reach to 2,000 mX nearly. It is found that the high and unstable contact resistance of the pore and products is more likely to cause contact failure.

Rare Metals 2013,32(02),174-178+2

Morphology and electrical contact properties of electrical connection materials in corrosive atmosphere

Zhi-Gang Kong Liang-Jun Xu

Research Laboratory of Electrical Contacts,Beijing University of Posts and Telecommunications

作者简介：Zhi-Gang Kong e-mail:zgkong@bupt.edu.cn;

收稿日期：7 August 2012

基金：supported by the Chinese Universities Scientific Fund(No.2011RC0603);

Morphology and electrical contact properties of electrical connection materials in corrosive atmosphere

Abstract：

The impact mechanism of environmental factors, such as corrosive atmosphere, on connector materials was investigated, and the porosity of gold plating was tested. Series of inspections and analytical research methods were introduced in this article. The surface morphology of specimens after corrosion was observed by stereoscopic microscope and scanning electron microscope. Chemical constitution was examined by X-ray energy spectrum. The contact resistances were measured by four-point method. The experiment results show that after exposure to certain environment, the corrosion products, such as Cu2O, Cu(NO3)2·3H2O, and NiO, are observed on the surface of the specimens without gold coatings, whereas the corrosion products appear to have circle-shaped spots on gold-plating surface after corrosion test, which indicate that the gold plating has good corrosion protection. The porosity is increased with the increase of corrosion time for every kind of specimens gold plated, and the corrosion degree of goldplating specimens is decreased with the increase of the thickness of gold coatings. The static contact resistances of circle-shaped spots appear higher contact resistance than normal value, which can reach to 2,000 mX nearly. It is found that the high and unstable contact resistance of the pore and products is more likely to cause contact failure.

Keyword：

Electrical contact; Porosity; Corrosion; Temperature;

Received： 7 August 2012

1 Introduction

It is found that corrosion product due to the surrounding environment,such as temperature,humidity,mechanical vibration,dust contamination,and atmospheric corrosion[1–4],has long been known as one of the major degradation mechanisms for electrical contacts,which will cause high contact resistance,noise,and even poor reliability of electronic system[5–7].

Ni and Cu are good conductor materials,but their anticorrosion abilities are poor[8].Study shows that the intrinsic nobility of gold enables it to resist the formation of insulating oxide films that could interfere with reliable contact operation.Therefore,the gold coatings are often used in the contacts of separable electrical connectors and other devices[9–11].However,the porosity on the plating surface can deduce these functions since it provides openings through which atmospheric corrosion can attack the substrate metal and degrade the properties of the coating[12–14].Thus,porosity testing becomes one of the principal means of determining the quality of preciousmetal plating on contact surfaces[15,16].

Among these test methods,the HNO₃vapor test is a quick and convenient method to measure the porosity of the coating surface.The object of this article is to investigate the corrosion behavior of common electrical contact materials.From the test results,we can make some improvement on the contact surface or make a good control by the production process to lower the porosity of the metal-plating surface.

2 Experimental

In order to compare the performance of different contact materials,four contact materials were studied.The specimens are Cu,Ni1.0–Cu,Au0.3–Ni2.0–Cu,and Au0.9–Ni2.0–Cu.Ni1.0–Cu means that the top plating is Ni,and the thickness of Ni is 1 lm.Au0.3–Ni2.0–Cu and Au0.9–Ni2.0–Cu mean that the thickness of the hard Au surface plating is 0.3 and0.9 lm,respectively.The sizes of specimens are 12 nm 910 mm.The measurement area is defined to be the middle of the test exposure area and have a minimum distance 1 mm to the side of coating.The specimens are cleaned in absolute ethyl alcohol with supersonic wave.

Below is the procedure of HNO₃vapor corrosion test.The 500 ml fresh HNO₃are added to the bottom of the clean and dry test chamber.The volume of the chamber is10 L,and the concentration of HNO₃is(69±2)%.After(30±5)min,load the sample and replace the cover.For analyzing the effect of time on the corrosion results,the exposure time to HNO₃vapor shall be 0.5,1.0,1.5,and2.0 h,respectively.Then,remove the samples at the end of the test and dry in an oven at(125±5)°C for(30±5)min.Finally,place directly into a desiccator containing active desiccant,and cool to room temperature.

To study the corrosion mechanism and electrical contact behavior,series of inspections and analytical research methods are introduced.The surface morphology of specimens in gold coatings after corrosion is observed by stereoscopic microscope and scanning electron microscope(SEM).Chemical constitution was examined by X-ray energy spectrum(EDS).The contact resistances were measured by four-point method.

3 Results and discussion

3.1 Surface topography and EDS analysis

The surface topography of the Cu and Ni1.0–Cu specimens after corrosion test are shown in Figs.1 and 2.The chemical constitution of this corrosion product is listed in Table 1.The experiment results show that the corrosion products are discontinuous for Cu specimens.The EDS investigation concludes that they possesses Cu₂O,Cu(NO₃)₂?3H₂O.For Ni1.0–Cu specimens,the corrosion products are continuous and cover the substrate metal.The EDS investigation concludes that the corrosion products are NiO.

Figure 3 shows that the SEM morphology of Au-plated specimen after HNO₃vapor corrosion test for 1 and 2 h.It can be seen in Fig.3a that the corrosion products formed on the testing specimens are found as discrete circles.With the increase of corrosion time,the surface of pore is destroyed,and the whiskers corrosion products are found on the surface after 2 h,as shown in Fig.3b.

These pores will damage the protected effect of gold coating,which can be contributed to the pore that will provide a path for HNO₃vapor[17].When tiny water droplet forms on the pore region,gases in air,such as sulfur dioxide dissolve in the water to form electrolyte solution as shown in Fig.4a.The solution penetrates into the pore and reacts with either nickel or copper,which possesses lower electric potential than gold.A galvanic cell forms inside the pore and starts to create corrosion products.The reaction continues until the solution is saturated.Since corrosion product holds much larger volume than the loss of the metal,it starts to creep out of the pore and pile up at the vicinity area,which is shown in Fig.4b.When another water droplet/film concentrates on the same place,it partially penetrates into the pore as before,but most of it acts like a tide to push the corroded products away,like the seaweed,as illustrated in Fig.4c.When the solution evaporates in air,the corroded products acting like seaweed may stay on shore,which forms the first ring far away from the core,as shown in Fig.4d.Since the water droplet carries ions of both corroded gases and the metal,the solution saturates when evaporating,and precipitation of the corroded products thus spreads on the area between the core and the ring.When a smaller water droplet deposits on the same place again,another tide would push the excessive corroded products closer to the core than the first tide As the phenomena repeated again and again,an inner ring was formed.Naturally,precipitation of the corroded products will spread on the surface between the first ring and the second ring.Since the second ring is closer to the core,more corroded products are piled up on the surface than the first ring.

Fig.1 SE image of Cu after HNO3vapor corrosion test for 1 h

Fig.2 SE image of Ni1.0-Cu after HNO3vapor corrosion test for 1 h

Table 1 Energy spectrum analysis result of elements of corrosion products(points 1–4 in Figs.1 and 2)(at%) 下载原图

Table 1 Energy spectrum analysis result of elements of corrosion products(points 1–4 in Figs.1 and 2)(at%)

Fig.3 SE image of Au0.3–Ni2.0–Cu after HNO3vapor corrosion test:a 1 h,and b 2 h

Fig.4 Island growth of corroded products:a moisture becoming a water droplet/water film on the pore,b pore corroded product that creep out of the pore and water solution being evaporated,c corroded products being pushed away by another water droplet,and d islands of corroded products form due to evaporation of the water[17]

The chemical constitution of this corrosion product of Au0.3–Ni2.0–Cu specimen after HNO₃vapor corrosion test is listed in Table 2.The EDS results show that the concentration of Ni is higher than that of Au at point 1.At point 2,there is large quantity of O and Cu and very little of Au.It is indicated that element of Au is etched off,and the result shows that oxides of Ni and Cu exist.

3.2 Porosity of gold plating

The porosity was counted at 50 times magnification with stereoscopic microscope.Pore size shall be defined by the longest diameter of the corrosion product,and corrosion products less than 50 lm in diameter shall not be counted.The surface topography of Au0.3–Ni2.0–Cu specimens after corrosion test for different time is shown in Fig.5.It can be seen from the figure that there are no clear changes of the quantities of pores for Au0.3–Ni2.0–Cu specimens for 0.5 h,but the pores are increased obviously for 1.5–2 h.

The porosities of specimens coating gold after HNO₃vapor corrosion test for different time are shown in Fig.6The test results show that the corrosion time has obvious effect on the porosity of the Au0.3–Ni2.0–Cu specimens The corrosion degree is not enough,and the porosities are not resolvable when the corrosion time is 0.5 h,so the value of porosity is 0.83 only.After 2.0 h corrosion test the porosity of specimens reaches 2.40.The porosity of Au0.9–Ni2.0–Cu specimens is lower than Au0.3–Ni2.0–Cu specimens obviously,and the corrosion time has no obvious effect on the porosity of the Au0.9–Ni2.0–Cu specimens,which indicates that the corrosion degree decreases with the increase of the thickness of gold coatings.

Table 2 Energy spectrum analysis result of elements of corrosion products(points 1 and 2 in Fig.3a)(at%) 下载原图

Table 2 Energy spectrum analysis result of elements of corrosion products(points 1 and 2 in Fig.3a)(at%)

Fig.5 Surface OEM morphologies of Au0.3–Ni2.0–Cu after HNO3vapor corrosion test for different time:a 0.5 h,b 1 h,c 1.5 h,d 2 h

Fig.6 Porosity of specimens coating gold after HNO3vapor corrosion test for different time

3.3 Contact resistance

Contact resistance is measured on the specimens after corrosion by four-point method.The constant current is20 mA,and the normal force is set at 50,100,and 150 g,respectively.Contact resistance is measured for every0.1 mm along a straight line through the corroded products.The contact locations are controlled by a micrometer.

The relationship between contact resistance of Au0.3–Ni2.0–Cu specimen corroded and normal force are shown in Fig.7.The experiment results show that the contact resistance is found changing from outside point to inside point through the corrosion area.The resistance values are distributed under 500 m X for most test points.However,it increases rapidly in certain test points and the maximum value can reach to about 2,000 m X,so it can be concluded that the high contact resistance points are the region of corrosion products.The resistance decreased with the change of force from 50 to 150 g.When the force is increased to 150 g,the maximum contact resistance value is 1,345 m X.The resistance test results show that the resistance decreased with the increase of the contact force.

4 Conclusion

The effect of HNO₃vapor on connector materials was investigated.The experiment results show that after exposure to certain environment,the corrosion products,such as Cu₂O,Cu(NO₃)₂?3H₂O,and NiO are observed on the surface of the specimens without gold coatings.While the corrosion products formed on the testing specimens plated with Au are found as circles shaped.

The corrosion time has obvious effect on the Au0.3–Ni2.0–Cu specimens but no visible effect on Au0.9–Ni2.0–Cu specimens.The corrosion degree increases with the increase of corrosion time and decreased with the increase of the thickness of gold coatings.

Fig.7 Static resistance of Au0.3–Ni2.0–Cu after HNO3vapor corrosion test for 1 h:a 50 g,b 100 g;c 150 g

The contact resistance increases rapidly in corrosion fields and the maximum value can reach to 2,000 m X approximately.The resistance decreased with the change o force from 50 to 150 g.The resistance test results show tha the contact resistance became high and unstable due to the corrosion.