Metal working – Method of mechanical manufacture – Electrical device making
Patent
1996-10-15
1998-06-02
Vo, Peter
Metal working
Method of mechanical manufacture
Electrical device making
29DIG12, 427 97, 427 98, H05K 342
Patent
active
057584125
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to a method of making a printed circuit board of the plate through hole type.
Plate through hole (PTH) boards have conductive circuit elements formed on both sides and interconnected by holes drilled through the board from a conductive element on one side to an opposed element on the other side, the holes having copper or other conductive metal plated therethrough.
BACKGROUND TO THE INVENTION
Conventionally, PTH boards are manufactured by a process of the type in which holes are first drilled in the correct locations through a substrate with copper coated on both sides, and the board is then given an initial treatment with an electroless copper plating solution after preparation. A negative resist image of the circuit elements is then formed by a conventional imaging process, which is followed by an electroplating step. The copper areas are protected by an application of tin, or possibly gold, and then the resist is removed. Etching of the exposed copper areas can now take place. Finally, the tin or gold is removed, leaving the required printed circuit board.
The conventional process has the following disadvantages: same time as the plating in the holes. It is therefore usual to start with a substrate having copper layers thinner than required for the eventual conductive tracks and to attempt to build up to the required thickness during the plating processes. The result is typically a wide variation in track thickness, possibly leading to difficulties with subsequent treatment stages or to excessive track resistance locally. Further, the electroplating is costly, increasing the board cost, and can give rise to inadequate bonding of the plated copper, leading to delamination in subsequent processing or in use, again leading to localised high resistance of the tracks. to build a continuous layer, and this can cause the creation of voids during electroplating. These voids can contain some of the plating solution which, during subsequent soldering operations, can boil, causing failure of the copper plating in the hole. This is referred to as "outgassing". after the formation of the holes, and this can lead to contamination of the surface in the holes before the electroplating. This can again cause outgassing or lead to an inadequate connection through the holes. the board, and because a negative resist image is required, the use of screen printing is not practical, and dry film techniques are typically used. These are relatively complex and costly, and require solvent treatment to remove the dry film. treatment. operating it. The resultant cost of manufacturing boards is high, often unacceptably so.
GB-A-1 056 814 discloses a process for making a plate through hole printed circuit board in which the circuit elements can be formed on the opposed faces of the board, and a de-sensitizing etch resist is coated over the board. The holes are drilled or punched through, and a sensitizing bath is then used to treat the exposed surfaces in the hole, followed by removal of the resist layer, for example by dissolving it in a solvent. Electroless plating then follows.
SUMMARY OF THE INVENTION
The present invention provides a method of making a plate through hole printed circuit board, comprising the steps of: non-conductive substrate; material; element on each of the opposed faces of the board; receptive to the action of a metallic plating solution; deposit conductive metal in the holes to the desired thickness to provide an electrical connection through each hole between two opposed conductive circuit elements, characterised in that the de-sensitizing material is an alkali-strippable material, and in that step (e) comprises treating the board with an alkaline accelerator solution to accelerate the action of the sensitization and simultaneously to remove the de-sensitizing material.
Preferably, the de-sensitizing material is a plating resist coating material. The conductive metal may be copper, but it has been found especially advantageous to plate nickel. This may suit
REFERENCES:
patent: 4574031 (1986-03-01), Dorey, II et al.
patent: 4605471 (1986-08-01), Mitchell
patent: 4782007 (1988-11-01), Ferrier
patent: 5218761 (1993-06-01), Maniwa et al.
Cordani John L.
MacDermid Incorporated
Vo Peter
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