ECA CB 2-1959 Contamination of Printed Wiring Boards《印制电路板的污染》.pdf

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1、I f -, EIA CB2 59 3234600 OOOOLbO O W COMPONENTS BULLETIN NO. 2 JUNE 1959 .- CO N TAMI NATI ON OF PRINTED WIRING BOARDS Published by ELECTRONIC INDUSTRIES ASSOCIATION Engineering Department 2001 Eye Street, N. W., Washington, D. C. 20006 - I - EIA CB2 59 m 3234600 OOOOLbl 2 m , ABSTRACT Contaminatio

2、n of printed wiring boards given of the causes of such contamination. is defined, and examples are The electrical effects of such contamination are then reviewed. Specifications are given for tests which may be used to evaluate the causes of contamination. Electrical tests for evaluating the effects

3、 of contamination are detailed. This report was prepared by Messrs. Edward B. Saubestre* and Saul W. Chaikin*, Chairmen of Eastern and Western Subcommittees on Contamination of Printed Wiring Boards. The report was subsequently reviewed and formally approved by the full membership of EIA Committee 4

4、0C on Printed Wiring. Mr, R. A. Geshner*M, Chairman, and recommended for issuance as an EIA Engineer- ing Bulletin. *Enthone Inc. , (work done while employed with Sylvania Electric Products Inc., Flushing, New York). 0 *Stanford Research Institute, Menlo Park, California. *Stromberg-Carlson Company,

5、 Rochester, New York v 6113 29-29 i 6/59 (C) - c) i- EIA CB2 59 - 3234600 OOOOLb2 4 . TABLE OF CONTENTS a 1 . Introduction 1.1 Definiti.n.o . 1 l02 Sources of Contamination.a . 1 . 1.2.1 Raw Material . 1 102.3 Environment 1 .2.2 Processing . 1 1.3 Effects of Contamlnatim . 2 2 . Tests for Determinin

6、g the Sources of Contamination 2.1 General TestsCO 2 2,l.l Extraction Tests . 2 2,1.2 Copper Corrasion Test 3 2.2 Specific Tests . 4 2.2.1 Test Papers for Cu and Fe 4 2,2.2 Other Tests for Cu and Fe.o 6 2.Z03 Autoradiography 6 2.2.4 Organic Decomposition 7 2.2.5 Fingerprints; perspiration . 7 2.2,6

7、ZXZst . 7 2.2.7 Detection of Ag Migration 7 3 . Tests for Determining the Effects of Contamination 3.1 Surface and Bulk Resistivity . 8 3.10i Standard Pattern. . 8 3.1.2 Attaching Leads- . 8 3.1.3 Standard Test Conditionsoa.O . 9 3.1.4 Measuring Equipment 9 3.1.5 Reporting Results.aO.O . 9 3.2 A.C.

8、Measurements. . 10 . EIA CB2 59 = 3234b00 00001b3 b 4 . Correlation Between Cause and Effect 4.1 Water Extraction Test . 10 4.2 Copper Corrosion Test 11 4.3 General Teats vs . Specific Tests . 12 4.4 Spot Tests for Cu and Fe . 12 4.5 Autaradi.graphy 13 4.6 Fingerprints 13 4.7 Cleaning and Plating .

9、13 4. 9 Silver Migration- 14 4.8 Dust, Fluxes. Protective Coatings 14 5. Elimination of Contamination a 5.1 Cleaning Residueso . 15 5.2 Etching Residues. 16 5.3 Plating Residues. . 17 5.4 Soldering Residues . 17 5.5 Potting Compo.dso . 17 5.6 Silver Migration . 18 - 1 . EIA CB2 59 = 3234600 0000164

10、B CONTAMINATION OF PRINTED WIRING BOARDS 1. Introduction 1.1 Definition For the purpose of this paper, contamination will be defined as a chemical or metallurgical material change or deterioration on the board., occurring before or after assembly. These c,hangessmay arise in (a) the raw material, (b

11、) processing or (c) from environmental con- ditlons. 1.2 Sources of Contamination 1.2.1 Raw Material . Organic decomposition of the laminate may occur. This is es- pecially true of partially cured, thermplastic materials. If the cop- per-coated board is manufactured by a plating technique, rather th

12、an by use of a pre-clad laminate, contapination may arise as a result of the cleaning and plating procedures used. Tk;e susceptibility of 3 board to the effects of contamination is dependent on the type of iamihate used, since some react with, or absorb contaminants more readily than others, and/or

13、absorb. moisture more readily. 1.2.2 Processing In producing etched boards, the principal sources of contam- ination are etching solution residues. . This is particularly true with ferric.chloride etchants, buk is also true to varying degrees for other etchantS.as well. Residual electrolyte.from cle

14、aning, sensitizing, chemical plating, and electroplating solutions is often a major factor in contamination of the printed circuit board Among lesser sources of contamination are solder flu resi- dues, metallic migration of silver, and growth of whisker formations on cadmium, zinc, and tin plated su

15、rfaces, extraneous contamination, asfrom fingerprints, spillages, etc., and.decomposition of organic protective coatings. 1.2.3 Environment After the printed wiring board has been completely processed, and is ready to be placed in servlce, further contamination of the board may arise as a result of

16、the environment to which the completed assembly is subjected. For example,.oxldation and tarnishing of the metal surfaces may occur, making subsequent soldering operations more difficult, and further metal migration may occur as a result of humid- ity conditions to which the board is exposed. In tro

17、pical or other high-humidity, high-temperatu.re environments, fungus and similar growths may give rise to troublesome contamination problems. Corona discharges give rise to ozone and nitrogen oxides .in the air which can also cause deterioration of the electrical properties of the wiring assembly. 9

18、 -1 - - EIA CB2 59 6 3234600 00001b5 T W 1.3 Effects of Contamination Contamination of a printed wiring board may cause many un- O desirable effects, but the principal one by far is associated with electrical performance changes, such as bridging and leakage between adjacent conductors on the assemb

19、ly, and changes in both the surface and bulk conductivity of the laminate. In extreme case$, actual mechanical failure of the board can qcour as a result of contamination tamination on the board is decreased solderability of copper or plated surfaces., and increased resistance of contact tabs. Final

20、ly, some processing procedures may weaken the adhealve band between the laminate and the copper conductor. Another important epfect of con- 2. Tests for Determining the Sources of Contamination Such tests can be divided into two types: Those which d.e- termine the total amount of contamination prese

21、nt on the board, and those which determine the presence of specific contaminants on the board. 2.1 General Tests 3 Tests which determine the total amount of contamination on the board are based on the assumption that contamination which leads to altered electrical properties of the board is due to t

22、he presence of ionized or ionizable matter. The presence of such matter may be detected in two ways: the ionized matter may be extracted in boiling water, and the resulting change in conductivity of the water measured, or, a potential may be applied between adjacent conductors at high humidity, so a

23、s to cause corrosion of the copper conductor, and the amount of dissolved copper determined analgtically. 2.1.1 Extraction Tests The following procedure may be employed to carry out the water extraction test: Thoroughly clean five i80 ml. electrolytic beakers, five watch glasses for beaker covers, o

24、ne 50 ml, graduated cylinder. Clean- ing shall be performed as follows: wash in a hot water detergent solu- tion, wash several times with tap water and rinse at least five times with distilled water. Perform all of the following operations in a laboratory maintained at 23OC 2 2Oc. Pour 50 ml. of dis

25、tilled water at 23OC into each of the five beakers and cover with watch glasses. Measurements of the resistivity of the water in each beaker shall be made to determine if the beakers have been thoroughly cleaned. The specific resistance of the distilled water in each beaker shall be equal to or abov

26、e 500,000 ohm-cm at 23 2%. If the resistivity of the water in any beaker is less than 5OO,OOO ohm-cm, the beakers shall be re-washed, 50 ml. of distilled water again added, and the resistiv- ities re-measured. This procedure shall be repeated until the resis- tivity of the water in each beaker is eq

27、ual to or more than 5OO,OOO ohm-cm. Cover all beakers. 6 -2- - EIA Ci32 59 m 3234600 OOOOLbb II m 2 1 * 1 Extraction Tests (Cont fd. ) A conductivity cell, whose cell constant is approximately 0.1 and a conductivity bridge similar to Model RC-1C (Industrial Instruments Co.) may be used for these mea

28、surements. The conductivity cell should be thoroughly washed in distilled water prior to use. The distilled water employed shall have a s ecific resistance of approxi- mately 500,000 ohm-cm or higher, at 23 2OC. In each of the five beakers, each containing 50 ml. of distilled water, place one l*“ x

29、13“ test specimen. Cover the beakers with the watch glasses. Heat the five beakers at one time on a hot plate until the distilled water begins to . boil and continue boiling for one minute. Remove the beakers from the hot plate and allow to cool at room temperature for 10 minutes. Place the beakers

30、in a bath of cool (15-180) running tap water and allow contents of beakers to cool to24oC. Remove beakers from the bath im- mediately after reaching 24OC. Remove specimens from the beakers using clean forceps and determine the resistivity of the water extracts in the following manner. Thoroughly was

31、h the conductivity cell with distilled water and immerse it in the water extract of one sample. Make instru- ment reading. Thoroughly wash conductivity cell in distilled water and immerse in a water control. Make instrument reading. Thoroughly wash conductivity cell in distilled water and immerse in

32、 a water extract. Make instrument reading. Wash the conductivity cell in distilled water and continue measuring resistivities of the remaining controls and water extracts. The specific resistance of each of the controls shall not be below 500,000 ohm-cm at 23 2 2%. If the latt tank. tion through fun

33、nel in the top. casional gentle agitation. re-use. t0 stand 45 minutes with occasional gentle agitation. ferrocyanide solution, saving it also for re-use. Zinc be decanted and used again.) and emptying the tank with distilled water three times. paper and air dry it face up on a clean glass plate. Cl

34、ase the tank and fill with the zinc acetate solu-. Pour the zinc! acetate off, saying it for Fill the tank with potassiwn ferrocyanide solution, and allow Pour off the (The precipitate of ferrocyanide will settle out and the potassium ferrocyanide can Rinse the paper by alternately filling Remove th

35、e When ready to use the paper on a board, moisten the dried paper for 30 seconds in 0.1 M HC1 in a developing tray. against the sample. Place a resilient surface which is flat and iron- free against the paper and apply a pressure of 25-50 psi for 5 minutes (t“ neoprene sheet covered with Saran Wrap

36、has been found suitable for this purpose). Drain and blot the excess acid with filter paper. Place the gelatin side of the paper Remove the paper from the sample. A positive test for iron is indicated by a blue color, and CO per by a red-brown color. m8 of both iron and copper. This test is sensitiv

37、e to 0.001 micrograms/ A variation of the above test is the following:* The test area on the board is first moistened with iO$ (wt) HC1 (with low iron analysis). Then, the moistened areas are immersed : iti 10% (wt) potassium ferrocyanide solution or iO$ (wt) ammonium Thiocyanate solution (the latte

38、r is used for iron detection only). The board is removed from the test solution, and blotted with filter *Westinghouse Spec. T552065, 5/8/56. - - *. PRi38LEM HARD COPY EIA CB2 59 = 3234600 OOOOLb 7 W paper. The latter is then examined for evidence of iron or copper. Iron ia indicated by a blue color

39、, copper by a red-brown color. If the thiocyanate paper is used, Iron is indicated by a red color. 2.2.2 Other Tests for Copper and ;:ron The above test paper is very convenient in that it will simultaneously reveal the presence of iron and copper. the other impurity is being investigated, it is als

40、o possible to use reagents specific to one metal ion. reagents are specific for iron in the ferric state:* soaked in thioglycolic acid (mercaptoacetic acid), and, immediately before use, Is moistened with 6N ammonia, rather than.HC1. , The presence of ferric ion is revealed by a purple-red color, th

41、at the paper is soaked in an aqueous salution of 8-hydroxy-quinoline- 7-iodo-5sulfonic acid (ferron), and, immediately before use, ,is moisten- ed with HC1 as before. green color. If only one or For example, the fallowing two The test paper is prepared as above, except that the paper is Alternativel

42、y, the test paper is prepared as above, except The presence of ferric ion is revealed by a The following neagents can be used for copper:* The paper is prepared as above, except that it is soaked in 0.1 g/l solution of dithizone, (diphenylthiocarbazone) in CC1 and, im- Pres- ence of copper is indica

43、ted by a yellow-brown color. dithiocarbamate dissolved in cc14, this reagent giving a brown-yellow reaction with cupric ion of great selectivity. tests. One Involved fixing the reagent on suitable paper-by means of an insoluble zinc salt, the other involved conducting the reaction di- rectly on the

44、board, and applying filter paper to the board to detect the reaction. There are other, less satisfactory ways to accomplish such spottests. For example, the board could be washed in water or suitable solvent, and a drop of test solution can then be reacted with a drop of the reagent solution on a sp

45、ot plate, in a test tube, or on filter paper. Needless to say, such methods would provide no indica- tion of the distribution of residual contamination, on the board. mediately before use, is moistened with dilute ammonia soluti 8 n, Alternatively, the test paper is impreknated with diethyl- e Two m

46、ethods have been mentioned above for using these spot 2.2.3 Autoradiography Radioactive iron and copper compounds can be used in making studies of etching residues and the like. If this is done, then trace contamination of copper and iron can be detected by autoradiographic techniques, In this metho

47、d, a polyester film is placed over the specimen and Kodak Autoradio raphic Stripping Film T e NTB is used to develop the image. produce a measurable image. If CU is used (3000 mc/g, where 1 mc = 3.7 x 107 disintegrations/sec), and Fe59 is used (1 mc/g), a one hour exposur and lo- g/cm2 of iron. Abou

48、t 10 dis1 egrations per cm F are needed to O would detect about 2,s x 10-9 g/cm2 of copper on the surface, “ .- * Fa Feigl, Spot Tests, Vol. I (Inorganic), Elsevier Publ. Co. (1954). -6- - PROBLEM HARD COT EIA CB2 57 m 323YbOO O000170 3 m 2.2.4 Organic Decomposition Operating temperatures of equipme

49、nt may reach levels at which polymeric materials decompose, liberating vapors which condense on cooler portions of the equipment. The following method may be used to detect such decomposition of organic materials: * which time any vapors liberated are allowed to condense on a cool glass plate. The presence of such matter is indicated by fogging of the plate. The deposit on the glass plate is made visible by dust- ing with a fluorescent powder, and photographs are then taken by ex- posure in ultra-violet light. 0 The board is exposed to heat for a period of 16 hours,