Method for producing printed circuit boards with punched holes having metallized walls

A method for producing printed circuit boards with holes having walls provided with a metal coating on base material provided with an adhesive layer which, in turn, is covered by a masking foil composed of plastic material or metal. Following the preparation of the pattern of holes and prior to the removal of the masking foil, the layer of adhesive coating is removed at the edges of the holes by chemical action, so that an annular zone free of adhesive is formed underneath the masking foil. The masking foil is subsequently removed and the metallization of the pattern of conductors and walls of the holes is produced by currentless deposition alone or combined with galvanic precipitation.

This invention relates to circuit boards and, more particularly, to a 
method for producing such boards having holes with metallized walls 
therein. 
Circuit boards of the printed circuit type and having holes with metallized 
walls are finding increased use. In such circuit boards, the metal coating 
on the walls of the holes enhances the quality of the soldered junctions 
between wires inserted in such holes connecting the components with the 
associated circuit pattern. The metal coating on the hole walls connects 
the holes to the circuit pattern both electrically and mechanically. In 
circuit boards having circuit patterns on both sides of the board, the 
metal wall coating on the holes which pass through such boards serve at 
the same time or solely for electrically connecting the circuit pattern on 
one board side with the circuit pattern on the other side. 
A particularly useful method of producing printed circuit boards is based 
on the use of a base material having a surface provided on one or on both 
sides with an adhesive coating which coating, in turn, is provided with a 
masking foil. Such a base material is prepared, for example, by coating a 
laminated pressboard with an adhesive layer. The adhesive layer is applied 
by dipping or casting or by other known process. The surface of the 
adhesive coating is then provided in a later process step with a removable 
masking layer composed of plastic material or metal, preferably aluminum. 
Another widely used method for preparing base material comprises the use of 
a foil, for example aluminum, which is coated with the adhesive material. 
The foil is combined with the impregnated layers of the laminated 
pressboard with heat and pressure to form a base material with masking 
foil and an adhesive layer. Suitable adhesive materials are, for example, 
phenolic resins modified with synthetic rubber. This is done before holes 
are provided in the board. 
Following the punching or drilling of the holes and the removal of the 
masking foil, the adhesive coating is rendered microporous and wettable in 
a known way. This might be accomplished, for example, by subjecting the 
coating to the action of chromosulphuric acid. This facilitates the build 
up of the conductor pattern on the adhesive coated surface by means of 
currentless or electroless metallization alone or in combination with 
galvanic deposition. Such printed circuit boards have excellent adhesive 
power or strength between the board and the circuit pattern, including 
excellent strength during soldering bath temperature conditions. 
In connection with the constantly increasing requirements on printed 
circuit boards because of miniaturization of components, it is found that 
beads of adhesive material build up on the outlet side of the punched hole 
during the punching or drilling step by which such holes are prepared. 
This formation of beads is more severe the smaller the diameter of the 
holes. In the finished printed circuit board, the metal coating within the 
zone of the beads results in a weak point which, particularly under the 
conditions of a thermal shock as in the soldering operation in a mass 
production, often leads to the formation of cracks in the metal during 
soldering or the later formation of a defective junction. 
In printed circuit boards for use under conditions where strong temperature 
fluctuations are encountered, for example in the fields of aeronautics and 
astronautical applications, it is generally necessary to improve the 
quality of printed circuit boards having so-called fully metallized hole 
walls. Increased resistance to the formation of fine cracks within the 
zone of the metallization of the walls of the holes or within the zone of 
transition connecting between the wall and the circuit pattern on the 
circuit board surface and the avoidance of operational breakdowns or 
failures as a result of cyclic temperature loads are of particular 
importance to a high degree of safety. 
It has been discovered that, by the method of the instant invention, the 
beads of adhesive material around punched or drilled holes in adhesively 
coated circuit boards might be removed. It has also been discovered that, 
to a limited extend, the removal of the layer of adhesive material 
radially starting at the edge of the hole results in circuit boards having 
distinctive features of extremely high quality and insensitivity with 
respect to temperature loads. 
The method of the instant invention is particularly adapted to circuit 
board base material for supporting on one or both sides an adhesive layer 
having a surface provided with a masking foil, for example an aluminum 
foil. 
After the hole pattern has been drilled or punched, the board is subjected 
to or exposed to an etching agent or solvent for the adhesive coating. An 
etching agent or solvent which will not attack the masking foil or will 
attach such foil to an inconsequential extent, is employed. Depending on 
the type of masking foil material used, suitable solvents or etching 
agents are, for example, chromic acid and alkaline permanganate solutions, 
as well as alkali metal hydroxide solutions. The duration of treatment is 
selected in a way such that the coating of adhesive material is removed 
underneath the masking foil, preferably up to a radial spacing from the 
edge of the respective hole conforming at least with the thickness of the 
adhesive coating. 
The masking foil is subsequently removed. Depending on the type of foil 
material used, such removal can be achieved with the help of suitable 
etching agent or solvent or by mechanical stripping. 
The board blank is subsequently treated in the known way to render the 
adhesive coating microporous and wettable. The board surface, including 
the walls of the holes, is subsequently sensitized catalytically for the 
currentless metal precipitation or electroless metal deposition. A 
solution containing the reaction product of Pd(II)chloride and 
Sn(II)chloride may be employed to sensitize the board blank catalytically. 
The board blank is then immersed into a bath for the currentless 
precipitation of metal, for example of copper or nickel. The board blank 
is left in the bath until a deposit of desired thickness is achieved or of 
a thickness sufficient for subsequent galvanic metallization has been 
obtained if subsequent galvanic plating is to be used. A covering mask 
corresponding with the negative of the desired conductor circuit pattern, 
for example by phototype or screen stencil printing, is applied. The 
conductor pattern and, at the same time, the metallization of the walls of 
the holes, are built up on the board blank to the desired thickness by 
means to galvanic metal deposition. The covering mask is then removed. A 
thin, currentlessly or electrolessly deposited metal layer is now exposed 
on the board blank surface between the circuit pattern and is removed. 
If the circuit pattern is to be formed exclusively by currentless metal 
precipitation or electroless metal deposition, the covering mask is 
applied immediately after catalytic sensitizing. The board blank is then 
left in the currentlessly or electrolessly metallizing bath until the 
circuit pattern has reached the desired thickness. 
The method of the instant invention might also be employed with base 
materials having interspersed therein a substance which acts catalytically 
on the currentless metal deposition. In this case, the treatment with a 
sensitizing solution can be omitted.