Note: Descriptions are shown in the official language in which they were submitted.
7~
The present invention relates to a prOGeSS for -the
reliable desmearing and etching as well as roughening of plastic
layers in holes drilled in basic material of printed circuit
boards for adhesive metal-coating of the same.
When holes are drilled in plastic boards, e.g. glass-
fiber boards impregnated with epoxy resin, the drill smears the
wall of the hole since the material heats up during drilling and
becomes soft.
The functional capacity of multilayer circuits of
printed circuits boards in which the electrical connections
between the two sides or between the different layers of inter-
connections are established via feedthrough in the through-plat-
ing process depends to a greater degree on whether the smearingof the plastic layers that occurs during drilling of the holes
can be reliably removed.
Processes for the removal of this smearing are already
known, examples being the chromic-acid or sulfuric-acid process.
But these processes work unsatisfactorily in as much as
the copper that is deposited without the use of electric current
adhered poorly on the walls of the holes, especially on resin~
coated zones, which leads to defectlve feedthrough as a result of
poor coating.
The present invention provides a process of the type
initially mentioned, which permits reliable removable of the
plastic smears contained in the holes of basic boards due to the
drilling, as well as crack-free roughening of the walls of the
holes.
According to the present invention there is provided a
process for the reliable desmearing and etching as well as crack-
free rougheniny of plastic layers in holes drilled in basic mate-
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r~" ~
~ 267826
rial of printed circuit boards for adhesive metal-coating of the
same, in which the holes are treated with a solution containing
sulfuric acid and nitrate anions.
The process in accordance with the invention leads, in
hitherto unattained fashion, to bo-th complete removal of the
layer smears in the walls of ths holes and ideal microroughening
of the surface of these holes.
The consequence thereof is that the metal deposited
chemically and galvanically, e.g. a metal in groups I B and VIII,
such as copper, after subsequent activation and reduction adheres
especlally well to the walls of the holes and is superbly suit-
able for the through-plating required to build up the conductive
pattern.
Another important advantage is to be found in the fact
that the etching rate can be lowered considerably.
The process can be applied in the generally ~nown fash-
ion by the spray-on or wave-flow method, but unlike the known
methods it can also be advantageously applied by the immersion
method.
By preference concentrated sulfuric acid containing 92
to 98 percent H2S04 by weight is used.
An acid containing 85 percent H3PO4 by weight is used
by preference as the phosphoric acid.
3~
Nitrate anions N03 are contained in the sulfuric-acid
solution in the form of nitric acid and~or its salts, possible
salts of the nitric acid being alkali-metal nitrates, e.g.
sodium, potassium, lithium or ammonium nitrate, or heavy-metal
nitrates, e.g. chromic nitrate or ferric nitrate.
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The solution preferably contains the designated consti-
tuents in concentrations of 9 to 17 moles/liter of sul-
furic acid (d = 1.84 g/ml) corresponding to 500 to 950 ml/
liter, and 1.2 to 1.4 10 moles/liter of NO3 anions,
corresponding for example to 0.1 to 120 g/l or sodium
nitrate. Phosphoric acid is contained in concentrations
of 0.7 to 7.4 moles/liter corresponding to 50 to 500 ml/l
of H3PO4 (d = 1.71 g/ml)
It is advantageous to add hydrofluoric acid and, possibly,
a wetting a~ent to the solution in accordance with the in-
vention, by preference a fluoridated hydrocarbon, expediently
in concentrations of 0.01 g to 0.5 g/l. The addition of
alkali-metal phosphates has also proved to be advantageous.
It is understood that the process in accordance with the
invention is also suitable for desmearing and etching as
well as roughening of the plastic layer on the surface of
the basic material.
As a rule, treatment takes place at room temperatures of
20 to 45 C, by preference at 25 to 40 C and can, depending
on the desired effect, take frorn 2 to 5 minu-tes, shorter or
longer reaction times being possible if needed.
By varying the concentrations and reaction times it is pos-
sible, for example, to superficially corrod~ a surface
coating of less than 10 ~m (so-called desmearing) and etch-
back a surface coating of more than 10 ~m.
A suitable basic material is, in par-ticular, glass-fiber
reinforced epoxy resin, but i-t is also possible to use
other common materials such as non-coated basic material,
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26
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phenolic paper, epoxy paper, p~e~h~, polysulfone, poly-
imide, polyamide, polyphenylene odixe polystyrene, fluoro-
carbon, polycarbonates, polyether imide or, as a coated
basic material, a carrier, the surface of which has been
provided with an adhesive coating which can be hardened
by heat and which contains at least one material that be-
longs to the group comprising modified rubber and/or ar-ti-
ficial rubber and which can be oxidized and/or decomposed
by suitable oxidants, or, as the basic material, a carrier
whose surface is coated with a metal, copper by preference,
on one or both sides.
The individual operations up to the drilling and further
treatment performed in accordance with the invention such
as rinsing processes, activation and metal-coating by
chemical and, possibly, galvanic methods are already known
in the art and, depending on the task, can be performed
without major modifications by an expert.
The process in accordance with the invention is suitable
~or both the semisubtractive and subtractive tec~miques
of manufacturing printed circuits.
The following examples serve to explain the process in ac
cordance with the invention.
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EX~MPLE 1
A basic material consisting of glass-fiber reinforced
epoxy resin and coated with copper on both sides is dril-
led and deburred in the customary fashion.
The board is subsequently immersed in a solution consisting
~ of
i 850 ml of sulfuric acid (96 percent by weight)
150 ml of phosphoric acid (85 percent by weight)
1 g of sodium nitrate
I for 1 to 5 minutes at a temperature of 25 to 40 C.
. .
! After dripping off, rinsing, subsequen~ alkaline cleansing,
I treatment with a solution of hydrochloric acid and ammonium
fluoride as well as further, customary follow-up treat~
ment, it is activated and chemically plated with a copper
coating of 0.2 ~m or 3 ~m in thickness.
The copper cQating is free of pores and features excellent
adhesion, is capable of being soldered (solder-resistance
test 3 x 5 sec. at 260 C). The desmearing ~etching rate)
in this connection is greater than 10 ~m (e-tch-back).
~6782~
EXAMPLE 2
l
Similar to Example 1 a board is treated with a solution con-
sisting of `,
850 ml of sulfuric acid (96 percent by weight)
150 ml of phosphoric acid (85 percent by weight)
10 g of sodium nitrate.
The copper deposit displays the same outstanding properties
as those described in Example 1.
1~ 6
EXAMPLE 3
Similar to Example 1 a board is treated with a solution
consisting of
850 ml of sulfuric acid (96 percent by weight)
150 ml of phosphoric acid (85 percent by weight)
10 g of potassium nitrate.
The copper deposit displays the same outstanding properties
as those described in Example 1.
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EXAMPLE 4
Similar to Example 1 a board is treated with a solution
consisting of
850 ml of sulfuric acid (96 percent by weight)
150 ml of phosphoric acid (85 percent by weight)
10 g of ammonium nitrate
The copper deposit displays the same outstanding properties
as those described in Example 1.
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o
EXAMPLE 5
Similar to Example 1 a board is treated with a solution
consisting of
850 ml of suluric acid (96 percent by weight)
150 ml o phosphoric acid (85 percent by weight)
10 g of chromic ni-trate
The copper deposit displays the same outstanding p.roperties
as those described in Example 1.
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EX~MPLE 6
Similar to Example 1 a board is treated with a solution
consisting of
850 ml of sulfuric acid (96 percent by weight)
150 ml of phosphoric acid (85 percent by weight)
1 g of sodium nitrate
0.05 g of wetting agent (fluorocarbon).
The copper deposit displays the same outstanding properties
as those described in Example 1.
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EXAMPLE_7
The processes descri~ed in Examples 1 to 6 are repeated
with an immersion time of 20 to 60 seconds.
This results in a desmearing (etching rate) of less than
10 ~m, and the copper deposits display the same outstanding
properties as those described in Example 1.
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EXAMPLE 8
The basic material made of glass-fiber reinforced epoxy
resin and coated with copper on both sides is drilled
and deburred in the customary fashion.
The board is subsequently immersed in a solution con-
sisting of
1000 ml of sulfuric acid (96 percent by weight)
1 g of sodium nitrate
for 20 to 60 seconds at a temperature of 25 to 40 C.
After dripping off, rinsing, subsequent alkaline cleansing,
treatment with a solution of hydrochloric acid and ammonium
bifluoride as well as further, customary follow-up treat-
ment, it is activated and chemically plated with a copper
coating of 0.2 ~m or 3 ~m in thickness.
The copper coating is free of pores and displays excellent
adhesion, is capable of being soldered (solder-resistance
test 3 x S sec at 260 C~. The desmearing (etchin~ rate)
is this connection is greater than 10 ~m 5etch-back).
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o
EXAMPLE 9
Similar to Example 8 a board is treated with a solution
consisting of
1000 ml of sulfuric acid ~96 percent by weight)
10 g of sodium nitrate.
The copper deposit displays the same outstanding properties
as those described in Example 8~
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EXA~IPLE 10
Similar to Example 8 a board is treated with a solution
consisting of
1000 ml of sulfuric acid (96 percent by weight)
10 g of potassium nitrate.
The copper deposit displays the same outstanding properties
as those described .in Example 8.
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EXAMPLE 11
Similar to Example 8 a board is treated with a solution
consisting of
1000 ml of sulfuric acid (96 percent by weight)
10 g of ammonium nitrate.
The copper deposit displays the same outstanding properties
as those describedin Example 8.
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EXAMPLE 12
Similar to Example 8 a board is treated with a solution
consisting of
1300 ml of sulfuric acid (96 percen-t by weight)
10 g of chromic nitrate.
The copper deposit displays the same outstanding properties
as those described in Example 8.
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EXAMP~E 13
Similar to Example 8 a board is treated with a solution
consisting of
1000 ml of sulfuric acid ~96 percent by weight)
1 g of chromic nitrate.
The copper deposit displays the same outstanding properties
as those described in Example 8.
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.
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EXAMPLE 14
Similar to Example 8 a board is treated wi-th a solution
consisting of
1000 ml of sulruric acid (96 percent by weight~
1 g of potassium nitrate
0.05 g of wetting agent (fluorocarbon)
The copper deposit displays the same outstanding properties
as those described in Example 8.
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