Corrosion of PC boards in contaminated industrial environments

applied surface science

ELSEVIER Applied Surface Science 74 (1994) 323-330

Corrosion of PC boards in contaminated industrial environments

S.H. Lin *, T.S. Chuang Department of Chemical Engineering, Yuan Ze Institute of Technology, Neili, Taoyuan 320, Taiwan, ROC

(Received 28 September 1993; accepted for publication 9 November 1993)

Abstract

Corrosion on the copper surface of a PC board is investigated. The corrosion materials were identified using SEM/EDX, XRD and SIMS to consist mainly of Cu,O along with a small amount of Cu,S and other components for the PC board exposed to the manufacturing environment. The chemical vapor in the manufacturing environment was found to accelerate the H,S corrosion process by lowering the H,S corrosion threshold from 0.5 to 0.3 ppm. Experiments were also conducted to test the effectiveness of vapor phase corrosion prevention. It was observed that 0.01 g of dicyclohexyl-ammonium nitrite (DHN) powder was observed to provide good protection against H,S corrosion up to 1 ppm H,S in an enclosed space. The efficiency of DHN vapor phase corrosion prevention appears to increase linearly with the amount of DHN powder provided.

1. Introduction

PC (printed circuit) board manufacturing is an integral part of the rapidly growing computer and electronic industries. The PC board, which could be of single-side, double-side or multi-layer types, is made of fiber glass and epoxy as the base substrate to which is attached on one side or both sides a layer of thin, high-purity copper film of thickness ranging from l/64 to l/8 inch depend- ing on its applications. After being printed on the board, the desired electronic circuit pattern is preserved while the remainder of the copper film is removed by etching processes. During the manufacturing process, the fresh copper surface of the PC board is exposed to the contaminated ambient environments at various stages. Very of-

* dorresponding author.

ten the copper becomes seriously corroded when the concentrations of SO,, H,S and other corrosive gases in the ambient air reach beyond certain levels. The corrosion would cause bad contact of the finished product. Due to a significant swell of the corroded metal, corrosion can sometimes ren- der the holes, which were precisely drilled during the manufacturing process, smaller than they were, causing serious problem in the later assem- bling process.

The PC board manufacturing is a fairly com- plex process which utilizes no less than twenty aqueous chemical compounds in various stages of the process for degreasing, etching, coating, washing and other purposes. The major aqueous chemicals include at least ammonium persulfate, sodium carbonate, cupric chloride, ammonium chloride, hydrogen chloride, ferric chloride, hydrogen peroxide, sulfuric acid, chloric acid and formaldehyde. Many of those aqueous chemical

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324 S.H. Lin, T.S. Chuang/Applied Surface Science 74 (1994) 323-330

compounds are not only volatile but also highly corrosive to the copper surface on the PC board. In industrial practices, minor corrosion caused by most chemical vapors can be removed by washing with dilute acid solution and hence does not pose any major problem to the manufacturers. How- ever, corrosion caused by contaminated ambient air containing H,S and SO, gases is rather serious. This is due to the difficulty in satisfactorily removing the corroded materials from the copper surface. Such a corrosion, when occurring would lead to the discard of the affected PC boards, causing a significant loss to the manufacturers. Hence prevention of such a corrosion is impera- tive during the manufacturing process.

Corrosion of copper by sulfurous gases has been investigated by many researchers [l-7]. The previous investigations were primarily concerned with measurements of the thickness of copper sulfate and copper oxide layers or with the kinet- ics of corrosion formation. Furthermore, those investigations were conducted in the laboratory with artificially high H,S and/or SO, concentrations. Little work has been done on the corrosion problem of a PC board in a real manufacturing environment which is very often contaminated with H,S, SO, and other corrosive gases. Hence it would be of considerable interest to the PC board manufacturers to look into this corrosion problem in such a contaminated environment. The purpose of this work is three-fold. First, it is attempted to identify the corrosion material(s) formed on the copper surface of the PC board in a manufacturing facility. Second, the approximate threshold of sulfurous corrosion is to be deter- mined. Third, the effectiveness of vapor phase corrosion prevention of the chemical inhibitor is to be tested. Experiments were conducted in real industrial environments and in the laboratory to address those issues.

The environment surrounding a PC board manufacturing facility is quite complicated due to the presence of H,S and SO, gases and mixed vapor from various volatile components of aqueous chemicals. H,S and SO, are two corrosive gases commonly found in a surroundings near heavy petrochemical complexes. In such an environment, the concentrations of SO, and H,S are

usually less than 0.5 and 0.1 mg/!, respectively, which are not sufficient to cause serious corrosion on the copper surface. On some occasions, however, the H,S and SO, concentrations in the ambient air can far exceed those levels and cause significant corrosion problems. Of these two gases, H,S is a more important one because of its high chemical activity toward pure copper. Hence the present study will focus on the H,S-related corrosion and explore the potential method for its prevention.

2. Identification of the corrosion compound

Primary reactions of H,S and copper can be represented by the following equations with the corresponding Gibbs free energy (AG) [2]

2 Cu + H,S + Cu,S + H,,

AG = - 11.416 cal/mol,

Cu + H,S + CuS + H,,

AG = -3.795 cal/mol,

Cu,S + H,S + 2 CuS + H,,

AG = 3.829 cal/mol.

Thermodynamically speaking, the lower the Gibbs free energy of a chemical reaction, the easier the reaction proceeds. Hence the first chemical reaction is the most likely reaction to occur if H,S is present in sufficient amount in the ambient air. This has also been experimentally confirmed by several previous investigators [l-5]. However, in the presence of chemical vapor in a highly contaminated manufacturing environment, some Cu,S formed on the copper surface by the above chemical reaction could be further oxidized to oxides according to the following:

2Cu2S+30,-,2Cu,0+2S0,,

cu,s + 2 0, + 2 cue + so,.

Those sulfides and oxides were believed to cause most problems in the copper surface. To ascertain the existence and type of sulfides and oxides, the corroded PC board samples gathered from a manufacturing facility were employed for identification analysis.

S.H. Lin, T.S. Chuang /Applied Surface Science 74 (1994) 323-330 325

Fig. 1. SEM image of an uncorroded PC board sample.

A batch of three PC board samples was first collected from the manufacturing facility. The three PC board samples were deliberately ob- tained to consist of a clean and apparently uncorroded one and two corroded ones. Fig. 1 shows the SEM (scanning electron microscope) micrograph of the uncorroded PC board sample with the protective coating being removed. The copper surface was visually very smooth, but the SEM micrograph did show streaks of shallow grooves. Figs. 2 and 3 demonstrate the two corroded samples with different extents of corrosion incurred in the manufacturing environment. The corroded copper surfaces corresponding to Figs. 2 and 3 looked light and dark brown in color, respectively. This indicates that different compounds might have been formed on the copper surface.

A Hitachi SEM/EDX (Model S-2300, Hitachi, Inc., Japan) was first employed for the corrosion material identification. The spectra of the one uncorroded and two corroded samples are demonstrated in Figs. 4, 5 and 6, respectively. The three distinct large peaks in Fig. 4 around 1, 8 and 9 eV were typical characteristics of copper ion. With quadruply amplified ordinate, Fig. 5 revealed the presence of sulfur ion around 2 eV for the slightly corroded sample. The sulfur peak was barely distinguishable from the background noise, indicating the presence of only a very small amount of sulfur in the corroded copper. Fig. 6 showed the presence of Cl in addition to S in the more seriously corroded sample. This perhaps

Fig. 2. SEM image of a slightly corroded PC board sample.

accounted for the darker color of the PC board on visual inspection. Again the fact that the Cl peak was barely noticeable indicates its presence in only a very small amount. It should be noted that the SEM/EDAX spectra are inherently un- able to detect the oxygen ion, hence Figs. 4,5 and 6 do not show any trace of this ion, although it could have existed in those samples in much larger quantity than S and Cl.

To further ascertain the nature of the corroded compound on the copper surface, a small amount was scraped off from a corroded PC board and subjected to bombardment by diffracted X-rays. (Philips model 1410, Philips Electronics, Inc., Netherlands). Fig. 7 displays the X-ray diffraction (XRD) spectrum of the sample.

Fig. 3. SEM image of a more severely corroded PC board

sample.


6-Now1991 15:15:14 Ixecutlon time = 9 seconds

ilert= 1645 counts Disp= 1

Fig. 4. SEM/EDX image of an uncorroded PC board sample.

The compound was identified to be Cu,O. This oxide could be the product of direct copper oxidation or further oxidation of copper sulfide formed in the first stage of the corrosion process. It primarily accounts for the characteristic color

in the corroded PC board sample. The sulfide or chloride of copper did not appear in sufficient amount in those samples for the XRD detection.

The same PC board samples were also ana- lyzed using a secondary ion mass spectrum (SIMS)

6-Nnu-1991 15:32:25 Ixecutlon time = 9 seconds

/ert= 500 counts 1

n

Disp= Preset= 80 scci

80 SecI Elapsed=

m

I- 0.000 Range= 10.230 kcV 10.110 -) Integral 8 3 2152

Fig. 5. SEM/EDX image of a slightly corroded PC board sample.

S.H. Lin, T.S. Chuang/Applied Surface Science 74 (1994) 323-330 327

6-Nav-1991 15:38:52 Execution time = 14 seconds

Vert= 500 counts Dlsp- 1 Preset= Elapsed=

Fig. 6. SEM/EDX image of a more severely corroded PC board sample.

(Cameca model IMS4F, Cameca, Inc., France). The SIMS is capable of revealing the counts of various ions as a function of the depth from the sample surface. Fig. 8 demonstrates a typical SIMS spectrum of a corroded sample. The two upper curves show, respectively, the oxygen and chlorine ions and the two lower ones the copper and sulfur ions. It is of interest to note that the

ratio of oxygen to sulfur ions is of the order of magnitude of 100, indicating the significantly larger amount of oxide than of the sulfide in the corroded sample. This further confirms the previous observations which show only Cu,O appear- ance in significant amount. In fact, as noted by Sharma [2], the standard Gibbs free energies of Cu,O, Cu,S and CuS at room temperature are

lCOYnt61 -

400-

Fig. 7. XRD spectrum of a slightly corroded PC board sample.


- 34.98, - 20.6 and - 11.7 cal/mol, respectively. Hence Cu,O is thermodynamically more stable than Cu,S and CuS and is more likely to exist as the final product in the corroded copper. This further confirms our analytical results.

3. Determination of approximate corrosion threshold

Experiments were also conducted in the laboratory to determine approximately the corrosion threshold of H,S on the copper surface. A glass reactor of 3.5 cm in diameter and 15 cm high was employed for the corrosion test. The reactor had a volume of 141 cm3 when the Teflon stopper was firmly in place. The Teflon stopper permits easy injection of H,S into the reactor for H,S concentration control.

Before the experiment was started, a small piece of fresh, uncorroded PC board sample of 1 x 2 cm was suspended in each reactor. The sample was held in place by a metal wire ex- tended from the top Teflon stopper. Glycerol solution of fixed concentration was employed to

control the relative humidity at 70% in the sealed reactor according to the ASTM standards. The H,S concentration was controlled by injecting an appropriate amount of pure H,S gas by a high- precision micro-needle. The H $ concentration ranged from 0.1 to 20 ppm. The sealed reactors were then placed in a steel rack which in turn was put in a constant-temperature shaker. In each experimental run, sixteen such reactors were employed. At various times, one reactor was re- trieved from the steel rack and opened for close inspection of corrosion formation. The experiments were conducted according to this procedure in the laboratory environment which was free of any chemical vapor except for H,S added and in the manufacturing environment with the presence of chemical vapor in addition to injected H,S.

After the sample was removed from the reactor, it was first inspected visually for any sign of corrosion. It was then placed under a microscope for more careful scrutiny for assurance of corrosion formation. It was observed that the occur- rence of corrosion depends strongly on the H,S concentration level and exposure time. At a low

0 0 0.2 0.4 06 0.8

Depth from Surface, microns

Fig. 8. SIMS spectra of a more severely corroded PC board sample.

S.H. Lin, T.S. Chuang/Applied Surface Science 74 (1994) 323-330 329

H,S concentration below 0.3 ppm, the corrosion process was rather slow. Only light pitted corrosion on the copper surface was observed after 1 h of exposure when the PC board was exposed to the clean air with 0.3 ppm H,S. However, when exposed to the manufacturing environment with the presence of chemical vapor, similar pitted corrosion was observed at 0.1 ppm H,S concentration.

Patch corrosion was observed to occur when the H,S concentration exceeded 0.5 ppm in the laboratory environment. With the presence of chemical vapor in the manufacturing environment, patch corrosion on the PC board started to appear at 0.3 ppm. The patch corrosion however did not occur instantaneously either, but over a period of at least one half hour. During that transition period, only pitted corrosion was found. After one half hour, the corrosion spread all over the PC board sample, although not uniformly. Such observation reveals that the presence of chemical vapor tends to accelerate the corrosion process of H,S. Furthermore, corrosion on the PC board seemed to be generally more severe around the small holes and near the edge of the board. This indicated that the stress which the PC board sustained from drilling and cutting has a strong and negative influence on corrosion.

4. Vapor phase corrosion prevention

The fresh copper surface of the PC board has been observed to be easily corroded by oxidation when exposed to contaminated manufacturing environment. To avoid or minimize this corrosion problem, preventive measures have been ex- plored here. Several chemicals have been re- ported in the literature to be effective for corrosion prevention of pure metals, but the available information has been rather sketchy [g-11]. Here attempts were made to test more quantitatively the effectiveness of those chemical(s) for this corrosion prevention of PC board.

In general, there are several aqueous chemical retardants known to provide corrosion inhibition for copper against H,S or other corrosive gases. 2-mercapatobenzothiazole (MBT) and 1,2,3-ben-

zotriazole (BTA) are the best known corrosion inhibitors, When in contact with copper, these chemicals are adsorbed onto the surface and form a protective film. The protection provided by the film against corrosion was confirmed in our laboratory tests to be very good indeed. However, the film could affect the electrical contact of the copper surface. Hence it needs to be removed in the manufacturing process to warrant the electrical integrity of the finished PC board. Such an added procedure does not lend much support for using the aqueous corrosion retardants.

The vapor phase corrosion prevention was primarily provided by dicyclohexyl-ammonium nitrite (DHN). The DHN powder sublimates slowly at room temperature. Sublimation could be con- siderably sped up above 40°C. To test the effectiveness of such vapor phase protection against H,S corrosion, several small pieces of fresh PC board sample were suspended in a completely sealed reactor of 5 ! with DHN powder under ambient conditions (63% relative humidity). The reactor was heated to 40°C for a few minutes to vaporize DHN. H,S was then injected into the sealed reactor. One hour later, the reactor was opened for visual examination of H,S corrosion.

It was observed that 0.01 g of DHN powder is sufficient to provide very good protection against PC board corrosion for a H,S concentration of up to 1 ppm. In no less than four tests, there was essentially no visual trace of H,S corrosion on the PC board. The amount of DHN powder was increased to 0.05 and 0.1 g in two different tests and the corrosion protection was found to be effective approximately for 5 and 10 ppm of H,S, respectively, indicating the linear relationship be- tween the vapor phase corrosion protection against H,S and the amount of DHN powder employed. The DHN vapor phase protection was so remarkable that it could be recommended for general or specific industrial applications.

5. Conclusions

Analyses of the corroded PC board sample by various instruments (SEM/EDX, XRD and SIMS) have indicated that the corrosion of cop-

330 S.H. Lin, ES. Chuang /Applied Surface Science 74 (1994) 323-330

per on a PC board is primarily in the form of Cu,O with presence of a small amount of Cu,S and other compounds. This indicates that the corrosion of copper by H $ proceeds in a two-step reaction.

Experiments in the laboratory show that the corrosion of PC board is of pitted type when the H,S concentration is below 0.3 and 0.1 ppm in clean and contaminated environments, respectively. Serious patch corrosion was observed to occur at a H,S concentration of 0.5 ppm. In the contaminated manufacturing environment, the same serious patch corrosion was observed to start at a lower H,S concentration of 0.1 ppm. This indicates the chemical vapor in the manufacturing environment can significantly speed up the corrosion process.

Vapor of dicyclohexyl-ammonium nitrite (DHN) is found to provide good corrosion inhibition of copper against H,S in an enclosed space. Laboratory experiments have indicated that chemical vapor generated from 0.01 g of DHN power is sufficient to prevent copper corrosion up to 1 ppm H,S in an enclosed space of 5 e. The effectiveness of vapor phase corrosion prevention against H,S was found to increase essentially linearly with the amount of DHN powder employed.

6. Acknowledgements

The authors wish to sincerely thank the Chi- nese Petroleum Corporation for the financial support of this project and Mr. Y.W. Chang of the Chemical Engineering Department for tech- nical assistance.

7. References

[l] W.H. Abbott, IEEE Trans. Parts, Hybrids Packag. 10

(1974) 24. [2] S.P. Sharma, J. Electrochem. Sot. 127 (1980) 21.

[3] T.E. Graedel, J.P. Franey and G.W. Kamlott, Corros.

Sci. 23 (1983) 1141. [4] G. Sick and K. Schwerdtfeger, Mater. Sci. Technol. 2

(1986) 159.

[5] N.B. Jemaa, J.L. Queffelec, D. Travers and D. Simon,

IEEE Trans. Component, Hybrids Manuf. Technol. 13

(1990) 1063.

[6] G.W. Walters, Corros. Sci. 32 (1991) 1331.

[7] G.W. Walters, Corros. Sci. 32 (1991) 1353.

[8] A. Wachter, T. Skei and N. Stillman, Corrosion 7 (1951)

284.

[9] H.R. Baker, Ind. Eng. Chem. 46 (1954) 2502. [lo] K. Agrawala and K.C. Tripathi, Mater. Prot. 5 (1966) 26.

[ll] B.A. Miksic, Chem. Eng. 84 (1977) 115.

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Corrosion of PC boards in contaminated industrial environments