U.S. patent application number 13/187248 was filed with the patent office on 2011-11-17 for solderability enhancement by silver immersion printed circuit board manufacture.
This patent application is currently assigned to ENTHONE INC.. Invention is credited to Peter Thomas McGrath, Andrew McIntosh Soutar.
Application Number | 20110279991 13/187248 |
Document ID | / |
Family ID | 10765791 |
Filed Date | 2011-11-17 |
United States Patent
Application |
20110279991 |
Kind Code |
A1 |
Soutar; Andrew McIntosh ; et
al. |
November 17, 2011 |
SOLDERABILITY ENHANCEMENT BY SILVER IMMERSION PRINTED CIRCUIT BOARD
MANUFACTURE
Abstract
A process used during manufacture of printed circuit boards
comprises protecting metal pads and/or through-holes to provide a
tarnish-resistant and solderable coating. In the method, the pads
and/or through-holes are bright-etched, metal plated, preferably by
an immersion process, and treated with a tarnish inhibitor. The
tarnish inhibitor may be incorporated into the immersion plating
bath. The metal plating is usually with silver or bismuth and the
pads and/or through-holes comprise copper.
Inventors: |
Soutar; Andrew McIntosh;
(Singapore, SG) ; McGrath; Peter Thomas; (Mission
Viejo, CA) |
Assignee: |
ENTHONE INC.
West Haven
CT
|
Family ID: |
10765791 |
Appl. No.: |
13/187248 |
Filed: |
July 20, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10099936 |
Mar 13, 2002 |
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13187248 |
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08939656 |
Sep 29, 1997 |
6395329 |
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10099936 |
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08567885 |
Dec 8, 1995 |
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08939656 |
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Current U.S.
Class: |
361/782 ;
106/1.19; 174/257; 361/760; 427/436 |
Current CPC
Class: |
H05K 3/244 20130101;
Y10T 29/4913 20150115; C23C 18/42 20130101; B05D 5/12 20130101;
Y10T 29/49144 20150115 |
Class at
Publication: |
361/782 ;
427/436; 106/1.19; 174/257; 361/760 |
International
Class: |
H05K 7/00 20060101
H05K007/00; C09D 5/00 20060101 C09D005/00; H05K 1/09 20060101
H05K001/09; B05D 1/18 20060101 B05D001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 9, 1994 |
GB |
9425031.3 |
Claims
1. A process for improving the solderability of a metal surface,
said process comprising treating the metal surface with an
immersion silver plating solution, said solution comprising: a). a
soluble source of silver ions; b). an acid; c). an additive
selected from the group consisting of fatty amines, fatty amides,
quaternary salts, and ethoxylated versions of any of the
foregoing.
2. A process according to claim 1 wherein the silver plating
solution also comprises material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives and
benzimidazole derivatives.
3. A process according to claim 1 wherein the metal surface
comprises copper.
4. A process according to claim 3 wherein the silver plating
solution also comprises a material selected from the group
consisting of imidazoles, benzimidazoles, imidazole derivatives,
and benzimidazole derivatives.
5. An immersion silver plating solution comprising (i) a soluble
source of silver ions, (ii) an acid and (iii) an additive selected
from the group consisting of fatty amines, fatty amides, quaternary
salts, and ethoxylated versions of any of the foregoing.
6. An immersion plating solution according to claim 5 also
comprising a material selected from the group consisting of
imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
7. A printed circuit board having metal pads, metal through holes
or combination thereof, the metal pads, metal through-holes or
combination thereof being formed of copper and comprising an
immersion silver plate thereon, the silver plate having been formed
by a process comprising treating the metal surface with an
immersion silver plating solution comprising a soluble source of
silver ions, and acid, and an additive selected from the group
consisting of fatty amines, fatty amides, quaternary salts, and
ethoxylated versions of any of the foregoing.
8. The printed circuit board as set forth in claim 7 comprising
components attached to silver plated copper metal pads and/or
through holes thereof.
9. The printed circuit board as set forth in claim 8, said
components having been attached subsequent to immersing said silver
plated copper metal pads in said plating solution comprising said
additive.
10. The printed circuit board as set forth in claim 8 wherein said
component(s) are selected from the group consisting of resistors
and transistors.
11. The printed circuit board as set forth in claim 9 wherein said
component(s) are selected from the group consisting of resistors
and transistors.
12. The printed circuit board as set forth in claim 7 comprising a
bare board.
13. The printed circuit board as set forth in claim 7 wherein said
silver plate has been deposited from a silver immersion plating
solution comprising a material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
14. The printed circuit board as set forth in claim 8 wherein said
silver plate has been deposited from a silver immersion plating
solution comprising a material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
15. The printed circuit board as set forth in claim 9 wherein said
silver plate has been deposited from a silver immersion plating
solution comprising a material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
16. The printed circuit board as set forth in claim 10 wherein said
silver plate has been deposited from a silver immersion plating
solution comprising a material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
17. The printed circuit board as set forth in claim 11 wherein said
silver plate has been deposited from a silver immersion plating
solution comprising a material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
18. The printed circuit board as set forth in claim 12 wherein said
silver plate has been deposited from a silver immersion plating
solution comprising a material selected from the group consisting
of imidazoles, benzimidazoles, imidazole derivatives, and
benzimidazole derivatives.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of pending U.S. patent
application Ser. No. 10/099,936, filed Mar. 13, 2002, which is a
continuation of U.S. patent application Ser. No. 08/939,656, filed
Sep. 29, 1997, now U.S. Pat. No. 6,395,329, which is a continuation
of U.S. patent application Ser. No. 08/567,885, filed Dec. 8, 1995,
now abandoned, which claims priority to Serial No. 9425031.3, filed
on Dec. 9, 1994, in Great Britain. The entire text of all of the
above applications and patents are hereby incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] In the production of a printed circuit board (PCB), in a
first (multi-step) stage a "bare board" is prepared and in a second
(multi-step) stage, various components are mounted on the board.
The present invention relates to the final steps in the manufacture
of the bare board, in which the bare board is coated with a
protective layer prior to passing to the second production
stage.
PRIOR ART
[0003] There are currently two types of components for attachment
to the bare boards in the second stage referred to above: legged
components e.g. resistors, transistors, etc., and, more recently,
surface mount devices. Legged components are attached to the board
by passing each of the legs through a hole in the board and
subsequently ensuring that the hole around the leg is filled with
solder. Surface mount devices are attached to the surface of the
board by soldering with a flat contact area or by adhesion using an
adhesive.
[0004] In the first stage referred to above, a board comprising an
insulating layer, a conducting circuit pattern and conductive pads
and/or through-holes is produced. The board may be a multi-layer
board having more than one conducting circuit pattern positioned
between insulating layers or may comprise one insulating layer and
one conducting circuit pattern.
[0005] The through-holes may be plated through so that they are
electrically conducting and the pads which form the areas to which
the surface mount components will be attached in the subsequent
component-attachment stage, are also electrically conducting.
[0006] The conductive areas of the circuit pattern, pads and
through-holes may be formed from any conductive material or
mixtures of different conductive materials. They are generally
however, formed from copper. Since over time copper tends to
oxidise to form a copper oxide layer with poor solderability, prior
to passing to the second, component-attachment stage, a protective
layer is coated over the pads and/or through-hole areas where it is
desired to retain solderability to prevent formation of a poorly
solderable surface layer of copper oxide.
[0007] While there is more than one way for preparing the bare
boards, one of the most widely used processes for making the bare
boards is known as the "solder mask over bare cooper" (SMOBC)
technique. Such a board generally comprises an epoxy-bonded
fiberglass layer clad on one or both sides with conductive
material. Generally, the board will be a multi-layer board having
alternate conductive layers which comprise circuit pattern, and
insulating layers. The conductive material is generally metal foil
and most usually copper foil. In the SMOBC technique, such a board
is obtained and holes are drilled into the board material using a
template or automated drilling machine. The holes are then "plated
through" using an electroless copper plating process which deposits
a copper layer on the entirety of the board: both on the upper foil
surfaces and on the through-hole surfaces.
[0008] The board material is then coated with a light sensitive
film (photo-resist), exposed to light in preselected areas and
chemically developed to remove the unexposed areas revealing the
conductive areas which are the plated through-holes and pads.
Generally, in the next step, the thickness of the metal foil in the
exposed areas is built up by a further copper electroplating step.
A protective layer of an etch resist, which is usually a tin lead
alloy electroplate composition is applied over the exposed and
thickened copper areas.
[0009] The photo-resist is then removed exposing the copper for
removal and the exposed copper surface is etched away using a
copper etching composition to leave the copper in the circuit
pattern finally required. In the next step, the tin-lead alloy
resist is stripped away.
[0010] Since components will not be attached to the copper circuit
traces, it is generally only necessary to coat the solder for
attaching the components over the through-hole and pad areas but
not the traces. Solder mask is therefore applied to the board to
protect the areas where the solder coating is not required, for
example using a screen printing process or photo-imaging technique
followed by development and, optionally curing. The exposed copper
at the holes and pads is then cleaned and prepared for solder
coating and the protective solder coating subsequently applied, for
example by immersion in a solder bath, followed by hot air leveling
(HAL) to form a protective solder coating on the areas of copper
not coated with solder mask. The solder does not wet the solder
mask so that no coating is formed on top of the solder mask
protected areas.
[0011] At this stage, the board comprises at least one insulating
layer and at least one conductive layer. The conductive layer or
layers comprise a circuit trace. The board also comprises a pad or
pads and/or through-hole(s) protected from tarnishing by a layer of
solder. A single conductive layer may comprise either a circuit
trace or pad(s), or both. Any pads will be part of a conductive
layer which is an outerlayer of a multi-layer board. The circuit
traces on the board are coated with solder mask.
[0012] Such a board is ready to proceed to the second stage for
attachment of the components. In this second stage, generally
attachment of the components is achieved using solder: firstly a
layer of solder paste (comprising solder and flux) is applied onto
the boards, generally by printing and the components are positioned
on the printed boards. The board is then heated in an oven to
produce fusion of the solder in the solder paste, which forms a
contact between the components and the board. This process is known
as reflow soldering. Alternatively a wave soldering process is used
in which the board is passed over a bath of molten solder. In
either case additional solder is used over and above any protective
solder coating.
[0013] The additional complications of attaching both legged
components and the surface mount devices and the special
requirements for mounting many small closely spaced components have
resulted in increased demands on the surface protection coating for
the conductive metal to which the components will be attached on
the PCB's. It is essential that the finish applied by the bare
board manufacturer does not leave a pad with an uneven surface as
this increases the risk of electrical failure. It is also essential
that the protective coating does not interfere with the subsequent
solder step, thereby preventing formation of a good, conducting
bond between the bare board and components. An extra step in which
the protective coating is removed would be undesirable.
[0014] As explained above, the conductive metal surfaces are
generally formed of copper and the protective surface must be
applied at the end of the first stage to prevent the formation of
non-solderable copper oxide on the copper surfaces prior to the
component attachment. This is particularly important because,
generally speaking, the first stage and the second,
component-attachment stage will be carried out at completely
different sites. There may therefore be a considerable time delay
between formation of conducting pads and/or through-holes and the
component-attachment stage, during which time oxidation may occur.
Therefore, a protective coating is required which will retain the
solderability of conducting material and enable a soldered joint to
be made when the components are attached to the bare boards.
[0015] The most common protection coating presently used is
tin/lead solder, generally applied using the "HAL" (hot air
leveling) process, an example of which is described in detail
above.
[0016] HAL processes are limited because it is difficult to apply
the solder evenly and the thickness distribution produced by the
use of HAL processes makes it difficult to reliably attach the very
small and closely spaced components now being used.
[0017] Several replacement treatments for the HAL coating of a
solder layer are being introduced. The coatings must enable
formation of a reliable electrical contact with the component. They
should also be able to stand up to multiple soldering steps. For
example, as described above, there are now both legged and surface
mount components for attachment to the bare boards and these will
generally be attached in at least two soldering operations.
Therefore, the protective coatings must also be able to withstand
at least two soldering operations, so that the areas to be soldered
in a second operation remain protected during the first
operation.
[0018] Alternatives to the tin/lead alloy solder used in the HAL
process, which have been proposed include organic protection,
immersion tin or tin/lead plating and nickel/gold plating. In the
nickel/gold process electroless plating of the copper surfaces is
carried out in which a primer layer of nickel is applied onto the
copper followed by a layer of gold. This process is inconvenient
because there are many process steps and in addition, the use of
gold results in an expensive process.
[0019] Organic protection for the copper pads during storage and
assembly prior to soldering have also been effected using flux
lacquer. Its use is generally confined to single-sided boards (i.e.
boards which have conductive pads on only one side). The coating is
generally applied by dip, spray or roller coating. Unfortunately,
it is difficult to provide a consistent coating to the board
surfaces so limited life expectancy, due to the porosity of the
coating and to its inconsistent coating thickness, results. Flux
lacquers have also been found to have a relatively short shelf
life. A further problem is that in the component-attachment stage,
if reflow soldering is to be used to attach the components, the
components are held in place on the underside of the boards with
adhesive. In cases where the flux lacquer is thick, the adhesive
does not bond the component directly to the printed board, but
instead forms a bond between the adhesive and the lacquer coating.
The strength of this bond can drop during the fluxing and soldering
step causing components to be lost during contact with the solder
baths.
[0020] One other alternative currently being used is
passivation/protection treatment based upon the use of imidazoles
or triazoles in which copper-complex compounds are formed on the
copper surface. Thus, these coatings chemically bond to the surface
and prevent the reaction between copper and oxygen. However this
process is disadvantageous because it tends to be inadequate for
withstanding successive soldering steps so that the high
temperatures reached in a first soldering step tend to destroy the
layer which cannot withstand a subsequent soldering operation
needed to mount further components. One example of such a process
is given in EP-A-0428383, where a process is described for the
surface treatment of copper or copper alloys comprising immersing
the surface of copper or copper alloy in an aqueous solution
containing a benzimidazole compound having an alkyl group of at
least 3 carbon atoms at the 2-position, and an organic acid.
[0021] Processes are also known which provide coatings using
compositions which comprise silver.
[0022] The three common complexing systems for electroless silver
plating processes are either ammonia-based, thiosulphate-based or
cyanide-based.
[0023] The ammonia systems are disadvantageous because the
ammonia-containing silver solutions are unstable and explosive
azides may tend to form. Thiosulphate systems are disadvantageous
for use in the electronics industry because sulphur compounds in
the silver coatings formed result in poor solderability so that in
the subsequent component-attachment step, a poor electrical contact
may be formed between the bare board and the component.
[0024] The cyanide-based systems are disadvantageous due to the
toxicity of the plating solutions.
[0025] In U.S. Pat. No. 5,318,621 an electroless plating solution
containing amino acids as rate enhancers for depositing silver or
gold onto a nickel coating overlying copper on a circuit board is
disclosed. It is described that neither gold nor silver electroless
plating baths based on thiosulphate/sulphate will plate directly
onto copper because the copper rapidly dissolves without allowing a
silver or gold coating to form. In the introduction of the
reference, "Metal Finishing Guidebook & Directory" (1993
edition), silver plating solutions comprising silver nitrate,
ammonia and a reducing agent such as formaldehyde are
mentioned.
[0026] U.S. Pat. No. 4,863,766 also discloses electroless silver
plating, using a cyanide-based plating solution. In Metal Finishing
(1983) 81(i), pp 27-30 Russev described immersion silvering of
copper powder from a plating solution containing silver nitrate and
a nitrogen complexing agent. In Metal Finishing (1960) August, p 53
Geld described a silver coating process involving an initial bright
dip of the brass or copper substrate, followed by a silver plating
step in which a thick coating of silver is plated from a solution
of silver nitrate and potassium iodide. The process is for plating
of electrical contacts to increase conductivity.
[0027] In JP-A-04-110474 a base material is plated with silver,
dried and subsequently treated with a mercaptan compound to prevent
tarnish.
[0028] In DE-C-4316679 base metals such as copper are coated with
palladium in a two-step process including a first step in which the
surface is contacted with a bath containing a palladium salt and an
oxidizing agent, and in the second step with a bath containing a
palladium salt, a complexing agent and formic acid or formic acid
derivative. The latter bath may also contain stabilizers for the
bath itself, which stabilize the bath against decomposition or
"plating-out". It is suggested that the copper substrate should
previously be etched using a non-bright etch bath including
persulphate. However, such pretreatment steps tend to produce
relatively porous coatings. The inventors there minimize the
porosity of the coating by using the two-step process in the first
of which a very thin coating is formed. This reference warns
against using silver as corrosion protection due to migration.
[0029] The present invention relates to a displacement immersion
metal plating in which a more electropositive metal displaces a
less electropositive metal at the surface to be coated. Ions of the
more electropositive metal oxidize the substrate metal. A
displacement plating process differs from an electroless process
because the silver coating forms on the surface of a metal by a
simple displacement reaction due to the relative electrode
potentials of the oxidisable metal of the surface to be protected
and of the silver ions respectively.
[0030] It is reported in for example "Modern Electroplating" by F.
A. Lowenheim, published by J. Wiley & Sons (1963) that silver
will plate by displacement on most base metals but that immersion
plated silver is poorly adherent. F. A. Lowenheim there suggests
that when electroplating base metals with silver, it is necessary
to deposit first a thin film of silver on the work piece from a
high-cyanide strike bath to ensure good adhesion of the subsequent
electroplated silver layer.
SUMMARY OF THE INVENTION
[0031] The present invention aims to provide an alternative to the
solder protection coating for the copper or other conducting
surfaces of bare boards which require protection from tarnishing
between bare board manufacture and the component-attachment
stage.
[0032] In accordance with the present invention, there is provided
a method for coating a PCB comprising an insulating layer and a
conducting layer, with metal pads and/or through-holes in which the
pads and/or through-holes are provided with an anti-tarnish
coating, the process comprising contacting the pads and/or
through-holes with a bright-etch composition in a bright-etch step;
and subsequently immersion plating the etched pads and/or
through-holes in a metal-plating step to form solderable plated
metal surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 is a graphical representation illustrating an example
described in the present application.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] The insulating layer and conducting layer of the PCB will be
as described immediately above. They may comprise the insulating
layer and conducting circuit pattern of any conventional PCB,
respectively. The pads and/or through-holes for plating are those
areas of the PCB for which solderability must be maintained for
attachment of components in the subsequent soldering steps for
component attachment.
[0035] The bright-etch step comprises contacting the pads and/or
through-holes with a bright-etch composition. Such compositions are
already known in the industry for other applications and they
produce a bright smooth cleaned surface on the conducting metal
from which the pads and/or through-holes are formed. In contrast,
non-bright etch compositions, such as those which are based on
persulphate provide microroughened, cleaned surfaces. The use of
the bright-etch step allows the formation of a dense, nonporous
metal coating, which is particularly suitable for a subsequent
soldering step.
[0036] Suitable bright-etch compositions are generally aqueous and
may be based for example on one or mixtures of more than one of
hydrogen peroxide, sulphuric acid, nitric acid, phosphoric acid or
hydrochloric acid. The bright-etch compositions generally include
at least one component which will tend to modify the dissolution or
copper in bright-etch compositions.
[0037] Particularly preferred bright-etch compositions where the
metal surface of the pads and/or through-holes comprises copper or
a copper alloy are, for example, as described in JP 62-188785 A2
(comprising 5.1-10.2 moles/l nitric acid, 4.6-9.2 moles/l sulphuric
acid, 0.01 moles/l zinc nitrate and 0.4 moles/l copper nitrate in
aqueous solution); JP 60-190582 (comprising for example 20-50% by
weight sulphuric acid (96%), 10-25% by weight nitric acid (67.5%),
0.5-1% by weight hydrochloric acid (35%) and 0.5-1% by weight
nonionic surfactant); U.S. Pat. No. 3,668,131 (comprising hydrogen
peroxide, sulphuric acid and urea additives); Metal Finishing
(February 1986), 84, (2), 67-70 (comprising sodium dichromate,
sulphuric acid, hydrochloric acid, sodium diethyldithio carbonate);
Trans Inst. Metal Finishing (Summer 1983), 61, (2), 46-49
(acidified hydrogen peroxide comprising hydrogen peroxide,
sulphuric acid and stabilizer); Oberflache Surf, (August 1979) 20,
(8), 178-179 (comprising nitric acid and dodecyl pyridinium
chloride); U.S. Pat. No. 4,510,018 (comprising sulphuric acid,
hydrogen peroxide, fatty acid amine and ammonium compound); U.S.
Pat. No. 4,459,216 (comprising 5-100 g/l hydrogen peroxide and
100-300 g/l sulphuric acid and aromatic stabilizer); JP 84-038308
(comprising 0.15-0.3 moles/l hydrochloric acid; 0.2-0.4 moles/l
phosphoric acid and 0.02-0.1 moles/l sulphuric acid). Where the
conducting material of the pads and/or through-holes comprises
stainless steel, particularly preferred bright-etch compositions
may be as described for example in WO 93-08317; JP 62-238379 A2; DE
1928307; or Tr. Gos. Nauchno-Issled. Proektn. Inst. Osnovn. Khim
(1974), 36, 93-97. Where the conducting material is aluminium, a
suitable bright-etch is as described in Met. Finishing (July 1986)
84, (7), 55-59.
[0038] Thus, any etch composition which provides a bright, cleaned
surface may be used. In the bright-etch step, contact with the
bright-etch composition may be by immersion, spray or any other
coating technique, such as is described in any of the references
above, for sufficient time and at a suitable temperature to enable
a bright surface to form on the conducting material of the pads
and/or through-holes. Generally the temperature for contact with
the bright-etch composition will be ambient and the contact time
will be from 5 seconds to 10 minutes, preferably at least 30
seconds, or even at least 2 minutes, and preferably for no greater
than 5 minutes.
[0039] Generally after the etching step, there will be a post-rinse
step comprising rinsing with deionized water and generally without
drying, the bare boards then proceed directly to the plating step.
Alternatively, an acid rinse step may be included, after the
aqueous rinse.
[0040] The plating step is an immersion (or displacement) plating
step. In an immersion plating step, the plating composition
comprises metal ions of a metal which is more electropositive than
the conducting material. The choice off metal ions in the immersion
plating solution, therefore depends on the metal to be plated.
Since the pads or through-holes generally comprise copper or
nickel, suitable examples of plating metals include bismuth, tin,
palladium, silver and gold; silver and bismuth ions are
particularly preferred.
[0041] A particularly preferred immersion silver plating method is
described in our copending British application filed on even date
herewith under the application number 9425030.5, and subsequent
U.S. Ser. No. 08/932,392.
[0042] As sources of plating metal ions, any water soluble metal
salt may be used, for example nitrates, acetates, sulphates,
lactates or formates. Preferably silver nitrate is used.
[0043] The metal plating ions are generally in the plating
composition at a concentration of from 0.06 to 32 g/l (based on
metal ions), preferably from 0.1 to 25 g/l, most preferably from
0.5 to 15 g/l.
[0044] Contact of the metal surface with the plating solution will
generally be at temperatures of from 10 to 90.degree. C.,
preferably 15 to 75.degree. C., more preferably 20 to 60.degree. C.
For example, the temperature of contact with the plating solution
will be from 15 to 50.degree. C., most usually from 20 to
40.degree. C.
[0045] Contact can be by any method, usually dip, spray or
horizontal immersion coating. Spray coating is preferred. Such
contact may be part of a substantially continuous coating
process.
[0046] The contact time of the plating solution with the metal
surface is sufficient to form plated metal surfaces over the metal.
Generally the contact time will be from 10 seconds to 10 minutes. A
contact time of less than 10 seconds has generally been found to
give insufficient coverage with silver coating and although the
contact time may be longer than 10 minutes, no additional benefit
has been found from a contact time of longer than 10 minutes.
[0047] The preferred plating process is an immersion displacement
process and not a true electroless plating process. In the
preferred plating compositions of the present invention, metal
atoms on the surface of the metal are oxidized by the metal plating
ions in the solution, so that a layer of plated metal deposits on
the pads and/or through-holes. The process is self-limiting because
when plated metal covers all of the surface sites of metal
oxidizable by the plating metal no further reaction and therefore
no further deposition occurs.
[0048] In a second aspect of the invention, there is provided for
coating a PCB comprising an insulating layer and a conducting
layer, with metal pads and/or through-holes in which the pads
and/or through-holes are provided with an anti-tarnish coating, the
method comprising metal plating the etched pads and/or
through-holes by contact with a plating composition in a metal
plating step to form solderable plated metal surfaces and
contacting the plated metal surfaces with a solution of a tarnish
inhibitor.
[0049] In this aspect of the invention, preferably, prior to
contacting the metal surface with the plating composition in the
plating step, the metal surface is cleaned. Cleaning may be using
an acidic cleaning composition, such as any cleaning composition
known in the art. Examples are copper conditioner PC 1144 supplied
by Alpha Metals Limited of the United Kingdom. Where there is a
cleaning step using an acidic cleaning solution, generally there
will be a rinsing step prior to contacting the metal surface with
the plating solution. Preferably any pre-cleaning will include a
bright-etch step. In both aspects of the invention, the tarnish
inhibitor may be present in the plating solution itself so that the
plating solution comprises the solution comprising tarnish
inhibitor. Thus, in a preferred method of the invention, the plated
metal surfaces are contacted with a solution comprising a tarnish
inhibitor during the plating step (i.e., contact may be during
formation of the plated metal surfaces).
[0050] Alternatively, the metal surfaces are formed in the plating
step and subsequently the formed metal surfaces are contacted with
a solution comprising a tarnish inhibitor in a further step. The
solution is preferably aqueous, being made up from deionized or
otherwise purified water. The composition comprising tarnish
inhibitor may additionally comprise solubilizers, for example
non-aqueous solvents, surfactants and/or pH buffers.
[0051] Contact of the composition comprising tarnish inhibitor with
the plated metal surfaces will be for at least 5 seconds,
preferably for at least 20 seconds. Where the tarnish inhibitor is
present in the plating solution, the time of contact is generally
determined by the duration of the plating step. Generally, the
contact time will be from 1 to 5 minutes. The temperature of
contact is most usually from 10 to 90.degree. C., preferably 15 to
75.degree. C., more preferably 20 to 60.degree. C. For example the
temperature of contact with the plating solution may be from 15 to
50.degree. C., most usually from 20 to 40.degree. C. Contact may be
any conventional means, for example by dip, spray or horizontal
immersion coating.
[0052] The most appropriate pH depends to some extent on the
particular tarnish inhibitor used but primarily on the metal ions
present in a plating bath which contains the tarnish inhibitor.
Where the tarnish inhibitor is contacted in a separate step with
the plated metal surface, the pH should be appropriate for the
tarnish inhibitor and selected so that it does not attack the
plating. Where the solution is a silver plating composition, a
convenient pH is in the range of 3 to 10. Where the solution is a
bismuth plating composition, the pH may be 1 or lower.
[0053] The solution comprising the tarnish inhibitor may be a final
rinse solution, applied to the boards prior to drying of the
boards. The board may undergo subsequent treatment steps after
contact with the composition comprising tarnish inhibitor. However,
generally, after contact with the solution, comprising a tarnish
inhibitor and drying, they are at the end of the first bare board
manufacturing stage, and are ready for the second
component-attachment stages. Optionally, for example, there may be
a deionized water rinse step, prior to drying.
[0054] The concentration of tarnish inhibitor in the solution
comprising tarnish inhibitor, will generally be from 0.0001% to 5%
by weight, i.e. 0.001 to 50 g/l. Preferably, the amount of tarnish
inhibitor will be from 0.005 to 3% by weight, and most preferably
from 0.01 to 2% by weight, or even below 1% by weight.
[0055] The method of the second aspect of the invention may
surprisingly also be used on precious metals such as gold, platinum
or ruthenium where it will improve solderability.
[0056] In the second aspect of the invention, the metal plating
step is preferably an immersion/displacement plating or electroless
plating step. It consists of a single step using a single plating
composition. Most preferably the plating step will be an
immersion/displacement plating step comprising contacting the metal
of the pads and/or through-holes with an immersion plating
composition.
[0057] Where the plating is other than by the preferred
immersion/displacement process, for example, if it is by
electroless plating, the plating composition may comprise
alternative plating metal ions, such as nickel.
[0058] The use of tarnish inhibitor in the invention has been found
to provide metal coatings which have good tarnish resistance
(resistance to humidity and oxidation) even when stored at
40.degree. C. and 93% RH for 96 hours or at 150.degree. C. for 2
hours. The porosity inherent in immersion coatings is reduced by
the provision of a level surface using the bright-etch step and
using, so that the anti-tarnish properties are considerably
improved, even at the high temperatures reached in reflow soldering
processes. Concern over the use of silver plating as described for
example in DE-C-4316679 due to migration of silver ions is overcome
as it has been found that the present invention substantially
prevents silver migration by providing a barrier to moisture.
[0059] In both the above aspects of the invention, an immersion
plating composition preferably contains a complexing agent for the
ions of the more electropositive metal.
[0060] In a further aspect of the present invention there is
provided a displacement metal plating process in which a relatively
less electropositive base metal is plated with a relatively more
electropositive coating metal by contact with an aqueous plating
composition containing ions of the more electropositive metal, a
complexing agent for such ions and a tarnish inhibitor for the more
electropositive metal so as to form a coating of the more
electropositive metal.
[0061] In this aspect of the invention there is also provided a new
plating composition containing ions of a metal which can be
displacement plated, a complexing agent for the ions, preferably
present in higher than equimolar amounts as compared to the metal
ion, and containing a tarnish inhibitor for the metal, and being
substantially free of reducing agent capable of reducing the ions
to the metal.
[0062] This aspect of the invention has been found to be
particularly useful for silver or bismuth plating. Therefore
preferably, the plating composition described contains silver or
bismuth ions.
[0063] The plating composition used in this aspect of the invention
may be an immersion plating composition based on any plating
composition used in the PCB industry.
[0064] In this embodiment of the invention, preferably, prior to
contacting the metal surface with the plating composition in the
plating step, the metal surface is cleaned. Cleaning may be
accomplished using an acidic cleaning composition, such as any
cleaning composition known in the art. Examples are copper
conditioner PC 1144 supplied by Alpha Metals Limited.
[0065] Where there is a cleaning step using an acidic cleaning
solution, generally there will be a rinsing step prior to
contacting the metal surface with the plating solution.
[0066] Preferably any pre-cleaning will include a bright-etch
step.
[0067] The plating composition may also comprise a complexing
agent. If so, the complexing agent is preferably present in an
amount of from 0.1 to 250 g/l, preferably from 2 to 200 g/l and
most preferably from 10 to 100 g/l, especially around 50 g/l. The
complexing agent may be any complexing agent for the plating metal
ions which does not form a water insoluble precipitate under the
aqueous and pH conditions of the composition. Mixtures of
complexing agents may also be used. It is desirable to use
complexing agents which are bi-dentate or higher dentate ligands
since the stability constants of such complexes are higher than
mono-dentate ligands.
[0068] Examples of suitable complexing agents have
oxygen-containing ligands, for instance amino acids and their
salts, preferably having at least 2 and up to 10 carbon atoms,
polycarboxylic acids, usually amino acetic acids, such as
nitrilo-triacetic acid or, usually, alkylene polyamine polyacetic
acids including ethylene diamine tetra-acetic acid (EDTA),
diethylene triamine penta-acetic acid, N-hydroxyethyl ethylene
diamine triacetic acid,
1,3-diamino-2-propanol-N,N,N,'N,'-tetra-acetic acid,
bishydroxyphenylethylene diamine diacetic acid, diamino cyclohexane
tetra-acetic acid or ethylene
glycol-bis-[(.beta.-aminoethylether)-N,N'-tetra-acetic acid)] and
N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylene diamine, citrates
and/or tartrates, N,N-di-(hydroxyethyl)glycine, gluconates,
lactates, citrates, tartrates, crown ethers and/or cryptands.
[0069] Particularly preferred complexing agents for silver are
EDTA, DTPA and N,N,N',N'-tetrakis-(2-hydroxypropyl)ethylene
diamine. The complexing agent should form a soluble complex with
plating metal ions in aqueous solution under the pH conditions of
the plating solution.
[0070] A suitable complexing agent for bismuth is chloride, and it
is generally unnecessary to use a multidentate (i.e., bi- or higher
dentate) ligand complexing agent for bismuth.
[0071] The complexing agent is preferably used either in
stoichiometric equivalent amounts or in a stoichiometric excess so
that all the plating metal ions may be complexed. By stoichiometric
we mean eguimolar. Preferably the complexing agent is present in a
higher molar concentration than the silver ions, the molar ratio
preferably being (at least 1.2):1, more preferably (at least
2.0):1, more preferably (at least 3):1.
[0072] Suitable tarnish inhibitors for use in all aspects of the
present invention include for example:
[0073] (a) fatty acid amines, preferably having at least 6 carbon
atoms, most preferably at least 10 carbon atoms and generally no
greater than 30 carbon atoms, they may be primary, secondary,
tertiary, diamines, amine salts, amides, ethoxylated amines,
ethoxylated diamines, quaternary ammonium salts, quaternary
diammonium salts, ethoxylated quaternary ammonium salts,
ethoxylated amides and amine oxides. Examples of the primary,
secondary and tertiary amine-type corrosion inhibitors are
ARMEEN.TM. to (.TM. denotes trademark). Examples of the subsequent
amine-type corrosion inhibitors are respectively DUOMEEN.TM.,
ARMAC.TM./DUOMAC, ARMID.TM., ETHOMEEN.TM., ETHODUONEEN.TM.,
ARQUAD.TM., DUOQUAD.TM., ETHOQUADT.TM., ETHOMID.TM., AROMOX.TM.,
all supplied by Akzo Chemie.
[0074] (b) purines and substituted purines.
[0075] (c) N-acyl derivatives of sarcosine, such as the SARKOSY
range of products supplied by Ciba-Geigy.
[0076] (d) organic polycarboxylic acids such as Reocor 190 supplied
by Ciba-Geigy.
[0077] (e) substituted imidazoline in which substituents are for
example hydroxyl C.sub.1-4 alkyl amino or carbonyl-containing
groups. Examples include AMINE 0, produced by Ciba-Geigy,
especially in combination with a N-acyl sarcosine of category
(c).
[0078] (f) alkyl or alkyl benzyl imidazoles, e.g. undecyl imidazole
in which the alkyl group has up to 22 carbon atoms, preferably no
greater than 11 carbon atoms and in which the alkyl or benzyl
groups are optionally substituted.
[0079] (g) benzimidazoles, especially alkylaryl benzimidazoles in
which the alkyl group has up to 22 carbon atoms, preferably no
greater than 10 carbon atoms and in which the alkyl or benzyl
groups are optionally substituted, for example 2-(p-chlorobenzyl)
benzimidazole which is particularly preferred.
[0080] (h) phosphate esters such as EKCOL PS-413, supplied by
Witco.
[0081] (i) optionally substituted triazole derivatives such as
REOMET 42, supplied by Ciba-Geigy. Examples are benzo triazole,
tolyl triazole and alkyl substituted triazole derivatives having a
carbon number on the alkyl group of from 1 to 22, preferably from 1
to 10.
[0082] (j) substituted tetrazoles, such as 5(3(trifluoromethyl
phenyl)) tetrazole, is also a preferred example.
[0083] The choice of tarnish inhibitor will depend to some extent
upon the metal of the plated metal surfaces, but this will be clear
to a person skilled in the art. For example, if the tarnish
inhibitor is to be incorporated into a gold plating bath, the
tarnish inhibitor may be a chloride salt, however, in contrast,
using a silver plating bath, chloride salts may not be used as they
will result in formation of an insoluble silver chloride
precipitate.
[0084] The tarnish inhibitor is preferably water soluble so that
the solution is an aqueous solution. However, water immiscible
tarnish inhibitors may be used although it may be necessary to
include a surfactant/cosolvent in the solution.
[0085] This invention has been found to provide considerable
advantages in preventing tarnishing and conferring humidity
resistance on the bare boards produced so that additional
protection is provided between the bare board manufacture stage and
the component-attachment stage. Solderability is found to be
enhanced.
[0086] A suitable pH for a silver plating composition may be from 2
to 12, but is preferably from 4 to 10. Thus, the composition may be
acidic, up to pH 7. Alternatively, the composition may be alkaline,
and have a pH of greater than 7, or even greater than 7.5. A
bismuth plating solution usually has a low pH of 1 or less.
[0087] A buffering agent may be included in the plating composition
to ensure that the pH of the composition is within the desired
range. As the buffering agent, any compatible acid or base may be
included. A compatible acid or base is an acid or base which in the
amounts required in the composition does not result in the
precipitation out of solution of the silver ions and/or complexing
agent. For example hydrogen chloride is unsuitable for a silver
plating composition as it forms an insoluble silver chloride
precipitate. Suitable examples include sodium or potassium
hydroxide or a carbonate salt, or where acids are required,
suitable acids may include citric acid, nitric acid or acetic acid.
Borates, phthalates, acetates, phosphonates may be used but the
buffer should not result in precipitation of the metal salts and
preferably does not inhibit the plating raze. An appropriate buffer
will be dependent on the desired working pH.
[0088] The plating composition may include optional ingredients
such as surfactants or wetting agents to improve the coating
uniformity. Where surfactants are included, preferably they are
introduced into the composition in an amount such that in the
plating bath, they will be present at a concentration of from 0.02
to 100 g/l. Preferably they will be incorporated at a concentration
of from 0.1 to 25 g/l and most preferably at a concentration of
from 1 to 15 g/l. Any standard surfactant or wetting agent useful
in a plating bath may be used. Nonionic surfactants are preferred.
Particularly preferred surfactants are alkyl phenol ethoxylates,
alcohol ethoxylates and phenol ethoxylates such as *Synperonic NP9
(ex. ICI), *Synperonic A14 (ex. ICI) and *Ethylan HB4 (ex.
Harcros), respectively (*denotes trade name).
[0089] A further optional ingredient which can be included in the
plating baths of the present invention are grain refiners. Grain
refiners improve the appearance of the plated metal surfaces by
causing formation of smaller crystals of plated metal having a more
densely parked structure. Suitable examples of grain refiners
include lower alcohols such as those having from 1 to 6 carbon
atoms, for example isopropanol and polyethylene glycols, for
example PEG 1450 (Carbowax* Union Carbide). Grain refiners may be
incorporated in the composition in amounts from 0.02 to 200 g/l.
More preferably, if a grain refiner is included, it will be at
concentrations of from 0.05 to 100 g/l and most preferably from 0.1
to 10 g/l. Any nonaqueous solvent should be present in amounts
below 50% by weight of the composition, preferably below 30% by
weight or even below 10% or 5% by weight of the plating
composition.
[0090] Other non-active, non-interfering components may be included
such as defoamers especially for spray applications (e-g., A100
supplied by Dow), dyes, etc.
[0091] The balance in the composition is water. Deionized water or
other purified water which has had interfering ions removed, is
used in the plating composition used in the process of the
invention.
[0092] In order to form the plating composition for use in the
processes of the present invention, preferably a solution is
firstly prepared comprising deionized water, complexing agent as
defined above, and any buffering agent, optionally with the other
optional ingredients, and a salt of the more electropositive metal
is added as an aqueous solution to the other components which have
been formed into a pre-mix. It has been found that this is the most
advantageous way to prepare the solution because trying to dissolve
the metal salt directly into the plating composition is relatively
time consuming and, where the metal is silver, tends to be more
vulnerable to photo-reaction which results in precipitation of
silver ions out of solution, as a dark precipitate.
[0093] Preferably the pH of the composition to which a silver salt
is added will be from pH 3 to 10, most preferably from 4 to 8.
[0094] The components are mixed until they have substantially
dissolved. The use of heat for silver dissolution is
disadvantageous because again, it may tend to cause the formation
of a dark silver precipitate.
[0095] After contact of the bare board with the solution comprising
tarnish inhibitor, the board is dried. Preferably, there will be no
post-rinse step between contact of the board with the solution and
drying.
[0096] Drying may be by any means, but is generally using warm air,
for example treated metal may be passed through a drying oven
[0097] The coating obtained using the method of the present
invention produces a surface which is considerably more uniform and
even than that obtained in the conventional HAL processes and,
compared with organic protection, the coating is more resistant to
soldering operations. Furthermore, the process of this invention is
less expensive and simpler than use of the nickel/gold process.
[0098] In the subsequent component-attachment stage, the components
are soldered onto the plated pads and/or through-holes of the bare
board. The metal of the pad(s) and/or through-holes (generally
copper) and plating metal, usually silver, and/or the plating metal
and solder may tend to intermix. The bond formed with the
components has good electrical conductivity and good bond
strength.
[0099] After component attachment, finished boards having
components attached over the plated layer of the present invention,
do not suffer joint reliability problems as do those boards formed
using a nickel/gold step.
Example 1
[0100] A composition was prepared in which 50 g EDTA and 20.4 g of
solid sodium hydroxide were mixed with sufficient water to dissolve
them. A solution comprising 1 g silver nitrate in deionized water
was subsequently added to the premixture comprising EDTA and sodium
hydroxide solution and deionized water was added to 1 litre. Copper
double-sided circuit boards, having a variety of surface mount
feature and plated through-holes of various diameter were coated
with the silver solution using the following procedure.
[0101] Boards were chemically brightened in an aqueous solution of
20% v/v H.sub.2O.sub.2 (35%), 0.5% v/v H.sub.2SO.sub.4, (96%), 2.5%
1,4-butanediol for 1 minute. A tap water rinse was then employed,
followed by an acid rinse in 10% H.sub.2SO.sub.4, for 1 minute. The
boards were given a further water rinse, then immersed in the
silver plating solution at 40.degree. C. for 4 minutes. After
removal from the bath, the boards were rinsed with water and warm
air dried. Copper areas of the board were coated with a bright,
even silver deposit.
[0102] Coated boards were subjected to three passes through a
typical IR silver paste reflow profile (see FIG. 1), then wave
soldered using NR300, an Alpha Metals VOC free, no clean flux. 100%
filing of the plated through-holes with solder was achieved.
[0103] Further boards were stored in a humidity cabinet at
40.degree. C./93% RH for 24 hours before being passed through 3 IR
reflow profiles. These boards showed a slight degree of tarnishing
on the silver coating. However 100% hole filling was still achieved
during subsequent wave soldering with NR 300 flux.
Example 2
[0104] A silver plating solution was prepared by forming a solution
comprising 50 g EDTA, 20.4 g NaOH, 14 g Ethylan HB4 (Akros
Chemicals), 3 g Crodamet 02 (Croda Chemicals) in 800 mls deionized
water. To this solution was added a solution of 1 g AgNO.sub.3 in
100 mls deionized water. The pH was adjusted to 6.8 by addition of
dilute NaOH/HNO.sub.3, then made up to 1 litre with deionized
water.
[0105] Double sided bare copper boards were coated with the above
solution using the procedure as described in Example 1. 100%
filling of plated through-holes with solder was achieved during
wave soldering with NR300 flux after passage through 3 IR reflow
profiles.
[0106] Boards stored at 40.degree. C./93% RH for 24 hours prior to
passage through 3 IR reflow profiles showed no evidence of
tarnishing and soldered well during wave soldering trials, giving
100% hole filling.
Example 3
[0107] Double-sided bare copper boards were coated using the bath
composition and procedure as described in Example 1. Following
removal of the boards from the silver plating solution and rinsing,
the boards were immersed in a solution of 4 g Reomet 42
(Ciba-Geigy) in 1 litre deionized water (pH 7) for 1 minute at room
temperature. The boards were then rinsed in tap water and warm air
dried. A bright even silver coating was produced.
[0108] The coated boards were stored at 40.degree. C./93% RH for 24
hours then passed through 3 IR paste reflow profiles. The boards
showed no evidence of tarnishing, and soldered well when wave
soldered using KR 300 flux.
Example 4
[0109] Coupons of copper strip (5 cm.times.1 cm) were coated with
the silver coating as described in Example 2. In addition, further
samples were coated with immersion tin, 63/37 Sn/Pb and two
competitors solderability preservative coatings based on
substituted benzimidazole chemistry. The following coating
procedures were applied for the various samples:
[0110] Immersion Tin Coating
[0111] Coupons were etched in an aqueous solution of
Na.sub.2S.sub.2O.sub.8 (5%), H.sub.2SO.sub.4 (5%) for 2 minutes,
rinsed with tap water, then rinsed with 10% H.sub.2SO.sub.4, for 1
minute and then rinsed with deionized water. The coupons were then
immersed in an immersion tin plating solution comprising 0.33 g/l
Sn(BF.sub.4).sub.2, 150 g/l thiourea, 20 g/l fluoroboric acid and 5
g/l Synparonic NP9 (ex. ICI) in deionized water, for 1 minute at
room temperature. The coupons were then rinsed with deionized water
and warm air dried.
[0112] Sn/Pb Coating
[0113] Coupons were etched in an aqueous solution comprising
Na.sub.2S.sub.2O.sub.8, (5%) and H.sub.2SO.sub.4 (5%), rinsed with
tap water then with 10% H.sub.2SO.sub.4 and then with deionized
water. The coupons were warm air dried. Alpha NR 300 flux was then
applied to each coupon. The coupons were then coated 63/37 Sn/Pb by
immersion in molten solder at 250.degree. C. for 3 seconds.
[0114] Azole 1 and Azole 2
[0115] Coupons were etched and rinsed as for the immersion tin
samples. Coupons were then immersed in the solution containing the
azole at 40.degree. C. for 90 seconds. After removal from the azole
containing solution, the coupons were rinsed with deionized water,
and warm air dried.
[0116] The coupons were subjected to a variety of different
pretreatments. [0117] A. No pre-treatment. [0118] B. Passage
through 3 solder paste reflow profiles. [0119] C. Storage at
40.degree. C./93% RH for 96 hours. [0120] D. Storage at 40.degree.
C./94% RH for 96 hours, then 3 solder paste reflow profiles. [0121]
E. Storage at 150.degree. C. for 2 hours. Samples were then
soldered using a meniscograph with NR300 flux.
[0122] The meniscograph test method monitors the solderability by
measuring the net force acting between specimen and solder. The
coatings are assessed by the length of time to reach zero wetting
force, and the size of the equilibrium wetting force. To achieve
good results in wave soldering a short wetting time and high
equilibrium wetting force are preferred.
[0123] The table below shows the wetting times in seconds and
wetting forces after 2 seconds immersion in mN/mm for various
copper coated samples.
[0124] As can be seen from above, the silver coatings prepared
according to this invention have shorter wetting times and higher
wetting forces than the Sn and benzimidazole alternative and retain
these properties more readily after humidity and heat
treatment.
TABLE-US-00001 TABLE 1 Wet Wetting Force at 2 Coating Pre-Treatment
Time/Sec Seconds Example 2 A 0.7 0.429 Example 2 B 0.8 0.444
Example 2 C 0.7 0.429 Example 2 D 0.7 0.441 Example 2 E 0.8 0.438
Tin A 0.9 0.488 Tin B >5 -0.028 Tin C >5 0.008 Tin D >5
-0.148 Azole 1 A 0.8 0.439 Azole 1 B 0.9 0.412 Azole 1 C 0.9 0.443
Azole 1 D 0.9 0.426 Azole 1 E 1.0 0.421 Azole 2 A 0.9 0.449 Azole 2
B 1.0 0.417 Azole 2 C 0.9 0.466 Azole 2 D 1.1 0.310 Azole 2 E 1.2
0.296 Sn/Pb A 0.8 0.475 Sn/Pb B 0.8 0.501 Sn/Pb C 0.8 0.492 Sn/Pb D
0.8 0.474 Sn/Pb E 0.8 0.492
Example 5
[0125] A displacement bismuth plating composition was prepared
comprising 3.9 g bismuth oxide, 183.1 g hydrogen chloride (as 37%
solution), 490.5 g glycolic acid (70% solution), 265.4 g (50%
sodium hydroxide solution), 0.077 g potassium iodide, 0.003 g
Synperonic NP9 (ex. ICI) and 4 g 2p-chlorobenzyl benzimidazole,
were added to deionized water to make 1 liter of product solution.
Bare boards having copper pads and copper through-holes were
chemically brightened as described in Example 1, then immersed in
the plating baths for 2 minutes at 70.degree. C. A coating of
bismuth was formed on the surface of the copper having a thickness
of 0.05 .mu.m. Subsequent solderability and tarnish resistance
tests carried out on the plated bare boards showed good results for
solderability and tarnish resistance.
Example 6
[0126] Double-sided bare copper boards were bright etched in an
aqueous solution of 50% v/v HNO.sub.3, 10% H.sub.2SO.sub.4, 10%
H.sub.3PO.sub.4, 1% HCl for 1 minute at room temperature. Boards
were then rinsed in tap water followed by 10% H.sub.2SO.sub.4 for 1
minute. After a further water rinse, boards were immersed in silver
plating bath described in Example 2 for 4 minutes at 45.degree. C.
Boards were then water rinsed and warm air dried.
[0127] The coated boards were stored at 40.degree. C./93% RH for 24
hours then passed through 3 IR paste reflow profiles. The boards
showed no evidence of tarnishing when wave soldered using NR 300
flux.
Example 7
[0128] A silver plating bath was prepared by forming a solution
comprising 64.8 g diethylene triamine penta-acetic acid, 23.0 g
NaOH, 24 g surfactant Ethylan HB4 (Akros Chemicals), 2.5 g Crodamet
02 an ethoxylated 3.degree. amine compound (Croda Chemicals) in 800
mls deionized water. To this solution was added a solution of 1 g
silver nitrate in 100 mls deionized water. The pH of this solution
was adjusted to 6.9 by addition of dilute NaOH solution or nitric
acid. The volume was then made up to 1 litre using deionized
water.
[0129] Double-sided bare copper boards were coated using the above
solution using the procedure as described in Example 1. 100%
filling of the plated through-holes was achieved during
wave-soldering of the coated boards using Alpha Metals MR300 flux
after passage through 3 IR reflow profiles showed no evidence of
tarnishing, and soldered well during wave-soldering trials giving
100% hole-fill.
Example 8
[0130] An immersion silver plating solution was prepared comprising
98.2 g deionized water, 1 g of nitric acid, 0.1 g of silver
nitrate, 0.3 g of Chemeen C2 (antitarnish) and 0.4 g Mazawet DF
(solubilizer). The pH was adjusted to 6 using a 50% solution of
ethylene diamine. The bath produced an adherent silver deposit on
copper coupons which showed good solderability and humidity
resistance.
Example 9
[0131] A bismuth plating solution was prepared containing bismuth
trioxide 2.1% weight, hydrochloric acid (22.degree. Be) 46.73%
weight, glycollic acid (70%) 49.5% weight, potassium chloride 0.07%
weight, polyethylene glycol 600 0.1% weight, Chemax Chemeen C2 0.2%
weight, distilled water 1.2% weight and tartaric acid 0.1% weight.
A further solution was prepared from which the Chemeen C2 was
omitted. Samples of copper clad printed circuit material were
plated in each of the solutions. These plated samples were then
placed in a humidity chamber for 16 hours at 600.degree. C. and 95%
relative humidity.
[0132] After this exposure the samples were examined, and those
prepared in the solution without the Chemeen C2 were heavily
tarnished. The samples prepared in the solution containing the
tarnish inhibitor had a good appearance with minimal oxidation, and
when tested showed good solderability.
[0133] Other embodiments of the invention are to be considered
within the scope of the appended claims.
* * * * *