SE451778B - Metallised PCB mfg. method - Google Patents

Abstract

The method provides a punched metallised pattern card, using a copper layered pattern card (11) which is drilled with the required hole pattern, and the conduit pattern is etched out of the foils (12,13). The etched conduit pattern, with the exception of the holes (14) and their surrounding collars (18), is produced with a permanent mask (15,16) resistant to thermic and chemical effect, and then the hole walls and their surrounding collars are metallised by chemical deposit. The mask used is of the type normally used on a pattern card, and is of epoxy-film, whilst the basic material is of plastic laminate (11) with copper foils (12,13) on both sides, each having a thickness between 17 and 35 micrometres. (Provisional Basic advised Week E26)

Description

15 20 25 30, 451 778 usually gold is usually hot tinned to the holes and collars to facilitate component soldering.

The pattern plating method is also based on a laminate with relatively thin copper foils on the card's surfaces (eg 17 μm). After drilling and an initial chemical plating of the order of 2-3 μm, the surfaces of the plate are provided with a resist layer which leaves conductors and holes free but covers other parts. By electrolytic means, conductors and cavity walls are first coated with a 25-35 μm thick copper layer and then with a 5-10 μm thick layer of a tin-lead alloy. After removing the resist, the card is etched briefly, whereby the tin-lead layer forms an etching mask. In this case, what is to be etched away is relatively thin, whereby etching time and under-etching are considerably reduced.

DESCRIPTION OF THE INVENTION In the panel plating method, a relatively thick copper layer must be etched away, which is a disadvantage from an economic point of view. This layer also varies considerably in thickness along the surface. To meet the minimum requirements for the thickness of the hole metalation, the copper layer becomes unnecessarily thick in some places. Since the etching time must be adapted to the thickest portion, the under-etching is considerable and it is difficult to maintain the intended conductor width.

These disadvantages are avoided with the pattern plating method, which, however, causes other problems. One is that the adhesion between the solder and insulation mask and the conductors is affected when the tin-lead plating melts during component soldering, which can cause the varnish to flake off. Another problem is that the resist layer used as a plating mask is affected by the plating bath and can easily be damaged so that short-circuit bridges are formed which remain after etching.

When it is found that a conductor thickness of 17-35 μm is completely sufficient in normal cases, according to the invention a method can be used for producing printed circuit boards with plated holes starting from a copper foil-covered laminate which is drilled with the intended hole pattern. Conductor pattern 10 15 20 25 451 778 and holes are covered by an etching mask and excess copper foil is etched away.

The invention is characterized in that the surface of the plate with the exception of the heel and surrounding collars (solder surfaces) is coated with a heat and chemical resistant mask, after which the uncovered surfaces and holes are copper-plated by chemical precipitation to a thickness sufficient from a mechanical point of view.

With this method a homogeneous copper layer with good ductility is obtained.

The saving of copper is considerable as the thick copper plating is only carried out in places where it is needed. The solder and insulating mask used as a plating mask covers most of the card's surface, which is why the risk of short-circuit bridges due to unintentional copper deposition in any scratches is small. No special plating mask that must be removed after the metallation is required either.

DESCRIPTION OF FIGURES The invention will be described in more detail in connection with the accompanying figures, in which Fig. 1 shows a section and Fig. 2 a plan view of a part of a pattern card according to the invention.

PREFERRED EMBODIMENT As a starting material, short blanks of a plastic laminate (11), usually glass fiber-reinforced epoxy which is copper foiled (12, 13) on both sides, are used.

The foil thickness is usually 1? -35 μm but can of course be thicker or thinner. The short blank is drilled with the desired hole pattern (14). After per se known preparation of the surface of the copper foil and the holes, a chemical precipitation of a thin copper layer (usually 2-3 μm) takes place on the short surface and the walls of the holes. This chemical precipitation is of a conventional nature.

In the next step, the surface of the card is covered with a photoresist in the form of a film and the desired wiring pattern is exposed. After development, all areas where the copper foil should remain (conductors, collars around the heel) and the heel are covered with resist. The card is etched in a conventional manner whereby the original foil (eg 17 or 35 μm) plus the layer (2-3 μm) applied by chemical precipitation is etched away in the places which are not covered with resist. Since the foil and the applied layer have an even thickness, the etching time can be calculated with tight tolerances and the etching will be as small as possible.

After etching, an insulating and soldering stop mask 15, 16 is applied over the entire surface of the card with the exception of the holes 14 and the surrounding collars as well as any other surfaces where soldering is desired. The insulating and solder stop mask can be produced in a known manner, for example by screen printing, or by applying a photosensitive film which is exposed in the same way as the iotoresist. The material in the solder stop mask is suitably heat- or UV-cured epoxy plastic which has good resistance to the baths for chemical copper precipitation that are normally used. The resistors used in connection with etching, on the other hand, do not withstand the long time required for chemical precipitation of a copper layer of the order of 25 μm.

The printed circuit board provided with solder and insulation mask 15, 16 is placed in a bath for chemical copper plating of a kind known per se. Such a bath contains a copper salt, for example copper sulphate, a complexing agent, for example EDTA, a reducing agent, for example formaldehyde, an agent for adjusting the pH value of the bath to the order of 12, for example sodium hydroxide and also stabilizers, wetting agents, etc. The printed circuit board is immersed in the copper plating bath for a period of the order of 15 hours and then a layer about 17 μm thick of copper has precipitated on the walls of the holes 14 and on the collars 18 around the holes which are not covered by the mask, as these surfaces are the only ones act as a catalyst in the precipitation process.

After any tinning, the printed circuit board is ready for component assembly. Unlike previously known methods, the mask used in plating does not need to be removed, but it can be used directly as an insulating and soldering mask. This copper layer has good adhesion to the conductor pattern 12 and the ductility of the layer is sufficient to withstand the heat shock that occurs during soldering (test parameters - 10 sec in molten sn-Pb at 60 ° C>. The processes above are only indicated schematically Between the various main steps, certain degreasing and cleaning operations are performed which are completely v, 451 778 conventional.

The method according to the invention can be varied in a large number of ways within the scope of the inventive idea. Thus, for example, the treatment of the surfaces to be provided with chemically precipitated copper layers can take place in different methods known per se.

Claims (5)

A 451 778 CLAIM PROCEDURE Method for producing straw-metallized printed circuit boards in which a copper-foiled printed circuit board (II) is drilled with the intended hole pattern, after which the conductor pattern is formed from the foil characterized by the following step 1 The drilled copper-foiled board covers foils and holes. 2. The surface of the card is coated with photoresist over all areas where the copper foil (12) is to remain and over the holes (14). 3. The copper foil is etched away on all surfaces that are not covered by photoresist. 4. Remove the photoresist and the entire surface of the card with the exception of the holes (14) and the surfaces (18) where solder is desired are covered with a permanent insulating and solder stop mask (15) with good resistance to baths for chemical copper precipitation. 5. The hollow walls and the surfaces (18) intended for metallization are coated by chemical precipitation with a copper layer of the same size as the copper foil (12). au u;