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JOSEPH F. DYTRT Naval Research Laboratory, Washington 25, D. C.
Electroplating and Metalizing Printed Wiring Design possibilities are offered by new physical properties and simplified fabrication T H E first forms of printed wiring were soon inadequate. As uses increased, systems having more diverse characteristics were needed. Changes in production methods brought changes in electroplating procedures, with problems generally foreign to shop platers. When the electroplate was used as the etchant resist, damage to photo-resist coatings during cleaning and plating was of immediate concern. Bond deterioration was of primary concern to those engaged in processing etched wiring systems and was further investigated.
Electroplating Processes
Bond damage seemed to result from reactions with cleaners and electroplating solutions. Alkaline cleaning baths were modified using milder reagents. Various detergents were added to baths, the most successful being the sodium sulfosuccinate class, preferably Aerosol OT. Operating temperatures were lowered to prevent catalytic heat effects, and strong acid mixtures were modified by dilution or buffering. However, modifications often retarded cleaning effectiveness. Electroplating provides solderability, corrosion resistance, wear resistance, and appearance. Figure 1 shows the relation of hardness (DPH) to plating thickness, indicative of the wear resistance that can be had by using base plate to
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0.0005
0.001
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THICKNESS
0.0025
(in]
Figure 1. Relation of diamond pyramid hardness (Vickers) to plate thickness indicates wear resistance Two-ounce copper-clad laminates were used as plating bases. A Wilson Tukon microhardness tester was used to make the hardness measurements
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back u p thin overlays. Electroplates common to printed \+iring fabrication are copper, tin-lead alloys, tin-nickel alloy, silver, gold, rhodium, and nickel. Copper deposits are used primarily to build up thin conductor areas, as in plated-through-hole areas: and are similar regardless of solution type. Pyrophosphate deposits are finer grained, brighter, and more dense than those from conventional acid copper baths. Fluoroborate deposits have characteristics similar to acid copper plates. Bond deterioration is slight from exposure to acid baths. High speed cyanide deposits are useful, but seriously damage the foil-laminate bond. Fluoroborate tin-lead alloy deposits improve solderability and simplify production soldering operations. Bond deterioration is slight. Because of critical operating conditions, tin-nickel alloy deposits have been neglected except for special apFlications. Hardness! conductivity, corrosion resistance, and solderability indicate a variety of uses. Because deposition occurs in almost neutral baths, bond damage is slight. Internal stresses, normally found in nickel and nickel alloy plates, are absent. Gold and silver baths seriously damage foil-laminate bonds. Their alkaline cyanide content is the contributing factor to bond deterioration. Silver migration problems make use of silver deposits debatable. Gold deposits serve as etchant resists and corrosion inhibitors, and provide solderable surfaces. In selectively plated systems, gold acts as a protective overlay for copper or nickel. In general, deposits from alkaline cyanide baths are most useful as etchant resists. Rhodium deposition occurs in sulfuric or phosphoric acid baths. Bond damage is slight in these baths. Applications requiring high degrees of wear and corrosion resistance dictate the use of rhodium deposits (Figure 2). Nickel deposits are, perhaps, the most versatile electroplates used in printed wiring. Solution types are many, providing a variety of deposits. Addition agents and variations in temperature and in p H offer a variety of deposits from each basic bath. Sulfamate and fluoborate baths have the least effect on the bond. Other nickel baths attack the bond slightly. Addition agent concentrations are critical only when in-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ternal plating stresses are increased, Because the hardness values realized from various nickel platings cover a wide range, they are useful as bases for thin precious metal overlays (Figure 2). Bond Deterioration
Individual actions of cleaning cycles, electrolytic processes. and internal plating stresses combine to cause bond deterioration. Tensile test results give wide ranges of values. Peel tests are direct and reliable. Several devices used for peel testing are shown in Figure 3 (3). Nominal peel strengths of unprocessed clad laminates are generally within ranges of manufacturers' specifications. Generally, alkaline cyanide baths are the most destructive. -41though some deterioration is evident after acid baths, peel strengths remain within nominal strength ranges. Soak tests have been made in processing solutions to attempt to determine the nature of bond deterioration. When soak times are equivalent to processing times, little or no bond damage results. More concentrated solutions and prolonged immersion cause varying degrees of deterioration. However, the time required for deterioration, even in more concentrated solutions, is in excess of the actual time of exposure. Synthetic resins are relatively inert to a variety of chemical solutions. The complexity of some systems permits only supposition, but certain reactions seem likely. For example, epoxy resins contain active ethylene oxide groups throughout the polvmer. The hydrogen liberated
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generally deposited from modified Fehling’s solutions. Copper utilizes immersion processing, while silver deposits by immersion or spray processing. Copper deposits slowly and adheres poorly. However, deposits from fluoborate solutions have properties similar to silver. Again the use of silver is discouraged because of migration problems. Silver is resistant to most etchants and must be removed. Metalized holes are frequently damaged during removal. Result : discontinuity and bond failure in the hole areas. Nickel can be deposited from both acid and ammoniacal hypophosphite solutions on several metals as well as sensitized nonconductive surfaces. As most etchants attack nickel, processing cycles can be completed without removing the coating. A t present, molten metal spraying has only limited application to printed wiring. Zinc, copper. and aluminum can be used, because their deposition temperatures are low, preventing burn damage to the laminate T h e metal is sprayed through a stencil to give a firmly adherent pattern on the laminate.
during electrolysis can react in acid solutions with these active groups to form soluble polyalcohols or diethers. .4lkaline cyanides form unstable nitriles or intermediate compounds. Ammoniacal solutions form soluble amino alcohols. Addition agents may cause subsequent reactions, further complicating the problem. Electrolytic processes also may be catalytic to reactions involving resins, adhesives, and processing solutions. T h e reaction products exist in such small amounts that analysis is difficult. Compounds have not been isolated, because of interference from bath constituents and solution complexes. Metalizing Processes Several methods of metallizing nonconductors may be used in printed !\.iring: chemical reduction, molten meral spraying, silk screening and stenciling metallic suspensions, and serographic deposition of metal powders. Copper, silver? and nickel are commonly deposited by chemical reduction. Reduction is important because of estensive use in plated-through-hole production. Copper and silver are
Properties of Baths a n d Deposits Used in Printed Wiring Fabrication Tensile Hardnessa Ductilityb Gtrengt 11‘ Bath Type Copper Acid sulfate (6) Fluoborate (2) High-speed cyanide (2)
40-85 40-75 100- 160
Watts ( 2 )
100-250 350-500 125-300 200-550 300-600 350-700 500-780 350-700
15-40 620 30-50
33-68 17-40 50-60
10-35 4-10 5-32 6-30 10-40
50-80 140- 160 55-120 60-130 35-115
Nickel Hard ( 2 ) Fluoborate ( 2 ) Sulfamate ( 2 ) Ni-Co alloy Plus addition agents Electroless Sn-Ni alloy
...
3-10 10-45
... ... ...
10-25
35-50
30-45
18-20
Silver Cyanide ( 2 )
55-130
Gold 24k industrial
35-65 35-100
Room temp. cyanide Acid sulfate
...
...
Rhodium 700-1 100
100-300
Chromium Industrial ( 2 )
300- 1000
0- 1
Diamond pyramid hardness or Tickers hardness number. 1000 p.s.i.
Laminate Type Phenolic-paper Phenolic-nylon Melamine-glass Silicone-glass Teflon-glass Epoxy-glass British epoxy-glass
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inches.
Peel, Lb./In., after Completion of
Plating Cycles
Peel. lb./in.
Tensile, p.s.i.
Alkaline cyanide
Acid
8-15 5-7 6-8 2-4 3-9 9-17 6-12
660-4270 363&4640 1420-1690
2-4 1-5 2-4
6-12 4-7 4-7
790-2030
3-6 4-6
5-9 6-12
... 800- 1450 ...
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Figure 3. Three devices were used for peel testing Method C. Force, FI, is applied to the loading bar, causing the pulleys and drum to move clockwise rolling the copper foil onto the drum. The force required to roll up the foil is read on a meter or recorded on a moving chart a, b.
c.
Simple methods Climbing peel method (3)
Silk screening and stenciling of metallic suspensions are particularly adaptable to ceramic systems where firing can be used. KO further build-up is required after deposition. Room temperature curing suspensions often present discontinuities, because binders can insulate the metal particles from each other. Hence, fire-on suspensions are more reliable. T h e serographic process is limited to ceramic-base systems, especially those used in high temperature work. A photoconducting selenium plate is electrostatically charged. T h e plate is exposed to light through a photopositive plate. Selective areas are discharged by exposure to light, leaving a charged latent image. Development is accomplished by cascading a charged micronized powder mixture over the plate. T h e powder pattern is transferred electrostatically to the base material and fused by firing. Suitably compounded powders of properly sized metal and glass particles are difficult to obtain commercially, limiting the usefulness of this process ( 7 ) .
10-30
* Elongation 5 in 2
Bond Strength Ranges of Laminates Unprocessed Laminates
&
SAMPLE,
... ...
... ...
literature Cited
(1) Mellon Institute of Industrial Research, Computer Components Fellowship 347, Quarterly Reports, Second Series, 1953-57. (2) Rubinstein, M , .$fetal Finishing 54, 52-6 (1956). (3). Stanford Reseaich Institute, “Preparation of Standards and Test Procedures for Printed Circuits,” Quarterly Reports, 1955-57. RECEIVED for review April 13, 1958 ACCEPTED December 19, 1958 Division of Industrial and Engineering Chemistry, Symposium on Chemical Aspects of Printed Wiring, 133rd Meeting, ACS, San Francisco, Calif., April 1958. VOL. 51,
NO. 3
MARCH 1959
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