Note: Descriptions are shown in the official language in which they were submitted.
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595-RG-059
1 The present invention relates to a process for the manu-
2 facture of p~inted circuit boards provided wi~h holes havlng
3 metallized walls and, particularly, to such boards provided
4 with copper conduc~ors and coppe~-~plaeed hole ~al}s.
6 For manufacturing such printed circuit boards, a number of
7 processes have been suggested. In one of ~hose processes the
8 conductors as well as the hole walls are provided with a
9 metal layer serving as an etch-resist during the subsequent
etching step and being removed afterwards. Preferred metsls
11 or metal alloys to be used as etch-resistant metal layers are
12 such metals which can easily be removed without adversely
13 affecting the copper surface as, for example, tin or tin/lead
14 alloys.
16 Independent of the poor economy of such process, its high
17 reject rates, too, make it unsuitable for mass production;
18 especially, it has proven to be difficult to achieve a copper
19 surface with good solderability.
21 Furthermore, it has been suggested - for two-sided copper clad
22 boards - to provide the holes in a first step and, subsequently,
23 to deposit copper on the hole walls by electroless copper
24 deposition or by electroless copper deposition followed by
2S electroplating until a desired thickness of the copper layer
26 on the hole walls and the surface of the base material is
27 achieved. Subsequently, the surface of the base material i~
2B covered on both sides with a photoresist dry film of suitsble
29 thickness. ~pon exposure through a positive of ~he desired
printed circuit pattern and developing, an etch-resistant mask
31 is formed which, on the one hand, covers the copper surface
32 corresponding to ~he desire~ circuit pattern and, on the other
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595-RG-059
1 hand, the holes thereby sealing them hermetically. The copper
2 in the exposed areas is removed in a subsequent etching step.
3 After re~oval of the masking layer, the circuit board shows
4 copper conductors and copper-clad hole walls well suited for
soldering.
7 The high costs of suitable photoresist dry films and the
8 photoprinting process itself constitute a considerable dis~
9 advantage of this process whenever conductor width and density
would allowscreen printing methods.
11
12 Thereforf, it nas been suggested, instead of using a photo-
13 resist dry film, to fill the holes with an etch-resistant ink
14 using, for example, a squeegee. Before or after the removal
of the said ink from the surface, the ink inside the holes
16 is hardened by heat curing. Subsequently, an etch-resistant
17 mask corresponding to the positive of the conductor pattern is
18 applied by screen printing. After etching, the said etch-
19 resistant mask as well as the hole filIings have to be removed.
The removal of the dried and hardened ink from the surf~ce
21 prior to screen printing has prsven to be a very tedious
22 process, normally to be performed by precision polishing.
23 Furthermore, it has also proven to be costly and difficult to
24 remove all hole-fill left overs fro~ the hole walls after
etching. Therefore, this process is not suited for mass
26 production purposes as it requires very ~horoughful
27 processing to achieve acceptable reject ~a~es.
28
29 The process in accordance with the present i~vention avoids
the problems and disadvanta~es described hereinabove and pro-
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595-RG-0.
1 vides a safe and economic process for producing printed
2 circuit boards with copper conductors and coppPr-clad hole
3 walls.
In accordance with the present invention there is provlded
6 a process for the manufacture of printed circuit boards with
7 metallized hole walls on any suitable insulating base
8 material provided on one or both sides with a metal layer
~ comprising the steps of providing said base material with those
holes whose walls are metal-coated in the finished circuit
11 board, and depositing a metal layer of desired thickness
12 on the walls of said holes and thP surface or selected areas
13 of the surface of said base material in known manner,
14 characterized in that a masking layer is applied to said
metal-coated surface(s~ of said base material by screen
16 printing a positive image of the desired circuit pattern;
17 and that said holes provided with metallized walls are sub-
18 sequently filled with a resinous ink composition employing
19 a screen printing stencil provided on the side facing the
surface of said base material during use with a foil securely
21 fixed to the screen of said stencil; and that said screen and
22 said foil are provided with holes at locations corresponding
23 to the holes with metallized walls in the circuit pattern;
24 and that said resinous ink composition is pressed into the
holes of said base material by means of a squeegee; and that
26 the unmasked areas of the metal layer on the surface of said
27 base material are etched away in known ~anner; and, finally,
28 that the masking layer applied by screen printing and the said
29 ink composition in said holes are removed by the agency of
one or more suitable solvents.
95-RG-059
673~
1 A metal-clad base material, for example a two-sided copper-
2 clad insulating base like, e.g., a laminate, hereinafter
3 called base material, of suitable size, is provided with holes
4 to be coated with a metal layer. Subsequently, a eopper
layer of desired thickness is deposited in a well known manner
6 on the surface of the base material and the hole walls.
7 Then, a masking layer corresponding to the desired printed
8 circuit pattern is screen printed onto the base material,
9 preferably using a scratch-resistant screen printing ink which
is hardenable by W radiation. Subsequently, the holes are
11 filled with an etch-resistant ma,erial using a screen printing
12 s~encil in accordance with the present invention, said stencil
13 being manufactured by fixing a suitable screen in a frame,
14 said screen being provided on the side facing the surface
to be printed with a foil of metal or plastic material of
16 a thickness of Q,1 mm or less. In accordance with one
l? e~bodiment of the present invention an aluminum foil is fixed
18 to the screen by means of an adhesive.
~
Screen and foil are provided with holes, preferably by drilling,
21 in a pattern which corresponds to the hole pattern of the
22 finished circuit board. Preferably, the diameter of the
23 holes in the screen and foil is larger than the diameter of
24 the holes in the base material, but should not exceed the
limit of the etch-resistant mask applied by screen printing.
26 As a rule, the diameter of the holes in the screen and foil
27 is only slightly in excess of the diameter of the holes in
28 the base material.
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1 In a further screen printing step, the ink for protecting the
2 hole walls is pressed into the holes through ehe screen by
3 means of a squeegee. Preferably, a protective ink is used
4 which - under the influence of dry air and the oxygen contained
S therein - forms an etch-resistant surface film covering the
6 holes, while the ink inside the holes remains almost or
7 completely viscous. Thereby, the removal of the hole filling
8 in a later process step is easily achievable using a solven~
9 or a thinner fs~ the respective ink.
10 '
11 For a better understanding of the scope of the invention,-
12 referenc~ is made to the following description of the drawings.
13
14 Figures lA to lF show in a diagrammatic view the base material
during the different process steps of the process in accordance
16 with the invention.
17
18 Fig. lA shows a partial view of the base material 1 covered
19 on both sides with a copper foil 2. The walls of the holes
10 as well as the surface of th~ copper foil 2 are provided
21 with a copper layer 3 deposited by methods well kno~m in the art.
22
23 Fig. lB is the partial view of the board of Fig. lA after
24 applying an etch-resistant mask 4 by screen printing.
26 Fig. lC is a diagrammatic view of the screen printing stencil
27 5 with the holes 9 in proportion to the holes 10 of the base
28 material 1. The screen printing stencil 5 comprises the
29 carrier screen 50 and the thereto adhered stencil foil 51.
The hole 9 is already filled with the ink 7 by means of the
31 squeegee 60
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1 Fig. 1~ shows the board 1 with the hole 10 filled with the
2 ink 7.
4 Fig. lE shows the board of Fig. lD after drying; the viscous-
fluid ink filling 7 of hole 10 is covered by an etch-resistant
6 film 70 formed on the surface of the filling 7.
8 Fig. lF shows the finlshed printed circuit board in accordance
9 with the present invention after removalof the etch-resistant
mask 4 (Fig. 1~) and the ink filling 7 including ~he film 70
11 from the hole 10. The copper conductors as well as the copper
12 layer on the hole walls is of excellent solderability.
13
14 The present invention is neither limited to copper-clad base
material nor to copperized hole walls. Furthermore, the
16 metal layer on the surface of the insulating base material
17 has not necessarily to be a laminated copper foil~ but can
18 be produced by electroless metal deposition as well as
19 electroless metal deposition followed by electroplating.
21 The scope of the invention will be still better understood
22 from the following examples.
23
24 Exa~ple 1
The base material used in this example is a glassfiber re-
26 inforced epoxy resin laminate provided on both sides with a
27 35 ~m thick copper foil. After the boards are cut to size,
28 the manufacturing process comprises the followlng steps:
29
595-RG-05~ 37~DIL
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1 (1) Providing those holes in the base material
2 whose walls are to be metallized;
4 (2) Brushing to remove the burr surrounding the
hole wall edges;
7 (3) Cleaning with a cleaner condi~ioner at 70C
for 5 minutes;
(4) Rinsing with water at 50C for 5 minutes;
11 t5) Slightly etching the copper foil surfare
12 ` with a solution of ammoniumpersulf~te at 50C
13 for 1 minute;
14
(6) Carefully rinsing in wate~;
16
17 (7) Immersing into a sodiumchloride preca~alysing
18 solution;
(8) Catalysing by immersing into a Sn(II)Pd(II)C1
catalysing solution at room temperature for
21 2 minutes;
22
23 ~9) Rinsing;
24
(10) Immersing into a commercially available elec~ro-
2fi less copper deposition bath at roo~ temperature
~7 for 45 minutes;
28
29 tll) Building-up the copper layer on the hole wall
to 35 ~m by electroplating~
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1 (12) Rinsing and drying;
3 (13) Applying ~he etch-resistant mask corresponding
4 to the desired circuit pattern by screen
printing and air dry;
7 (14) Filling the holes by screen printing using the
8 same ink as in step (13) and a screen printing
g stencil comprising a screen having adherPd
to its lower suriace an aluminum foil of
11 0,8 mm thickness. The screen and the foil
12 are provided with holes corresponding to the
13 pattern of the finished board consisting of
14 holes with metallized walls;
(15) Air-drying to form an etch-resistant film
16
17 sealing the hole fillings;
18 (16) Etching in a hydrogenperoxyde-con~A;nln~
19 hydrochloric ~cidic copper chloride solution;
21 (17) Removing the etch-resistant mask as ~ell as
~2 the hole fillings with trichlorethylene and
23 a spray etcher.
24
Example 2
26 The base material is an epoxy resin paper provided on
27 both sides with an adhesive layer. The board is f~r~t
28 provided with those holes whose walls are to be metallized.
~9 The adhesive layer is rendered wettable and microporous in
a known manner. Subsequently, the surface including ~he hole
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595-RG-059
1 walls are catalysed for the deposition of co?per from electro-
2 less copper deposition baths; then, a thin copper layer is
3 deposited from a commercially available electroless deposl-
4 tion bath. The further process steps are as described in
Example 1, steps (11) through (17).
7 Exa~Dle 3
8 The process of Examples 1 or 2 is usedi in step (13), however,
9 a W -curable screen printing ink is employed and the printed
mask is cured by W radiation.
11
12 Example 4
13
14 The process of Example 3 is used, with the copper layer being
produced solely by electroless copper deposition.
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17
18
19
21
22
23
24
26
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