Metal working – Method of mechanical manufacture – Electrical device making
Patent
1995-05-10
1997-04-08
Arbes, Carl J.
Metal working
Method of mechanical manufacture
Electrical device making
156150, 427437, H05K 302
Patent
active
056176295
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the production of printed circuit boards and a use thereby.
Normally a printed wiring pattern is used for the production of printed circuit boards in the electronics industry. This technique means the use of so called printed circuits, which usually are manufactured using a copper clad plastic laminate as starting material. A copy of the wiring pattern is transferred to the copper layer by e.g. printing or by a photochemical process. The applied pattern, the so called etch resist, acts as a protection during the subsequent removal by etching of superfluous copper. The electronic components are thereafter mounted on the laminate holding the finished wiring pattern and a complete printed circuit board of the most plain kind is thus obtained. The copper conductors provide the necessary electrical connections and the laminate provides mechanical support. The possibilities of a space and weight saving build up of the electronic unit are good. The method gives a high reliability and a rational production.
The most common bases are paper reinforced phenolic laminates which are used for rather plain circuits and glass cloth reinforced epoxy resin laminates which are used when the technical requirements are high. Other types of fibre reinforced laminates are also used. Base materials of the type plastic films and plastic coated metal plates are, furthermore, used to a certain extent.
It is common when preparing the copper coating to use copper foils, which are placed on a base forming fibre impregnated material impregnated with a partially cured resin (so called prepreg), followed by moulding at high pressure and elevated temperature. The final curing of the resin is hereby effected and the fibre material is compressed to a sheet to which also the copper foil is bonded. The copper foil usually has a thickness of 35 .mu.m, but thicker or thinner foils also exist. A further common process is the gluing of a copper foil onto a plastic film using heat and pressure.
Due to the rapid development in the field of electronics, the need for printed circuits with good dimensional precision especially in circuitry with a small line width and a space between the conductors, is increasing. Presently printed circuits with a line width of 0.1-0.2 mm and with an equally small conductor space are already required. Dimensions that are yet smaller will be required in the future. This development has led to a demand for laminates with thinner copper layers. During the last years laminates with a copper foil thickness of 6-12 .mu.m have increasingly come into use. By using a thinner copper foil advantages, such as a reduced so called "undercut", are obtained. Undercut is understood as the removal of copper underneath the etch resist caused by the etch solution which while dissolving the unprotected parts of the copper layer also eats into the copper covered by the etch resist. Undercut is a difficult problem causing an unacceptably low dimensional precision especially at small line widths. The effect of undercut on small conductor widths will below be further explained. It is by means of an accurate technique possible to obtain a high precision in the application of the etch resist itself.
However, owing to the undercut, difficulties arise in maintaining the good dimensional precision of e.g. the line widths and conductor spaces, which the technique of masking with an etch resist per se makes possible.
It is not solely with small line widths and small conductor spaces that a thinner copper layer is of advantage. Thinner copper layers result in an improved dimensional precision also with larger line widths and conductor spaces, which is an advantage, e.g. in those cases in which the electrical influence between the conductors must be considered when building up the electronic construction. These demands are expected to increase in the future i.a. in electronic systems operating at high frequences.
Further advantages by using thinner copper layers are that the etching time is considerably red
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Arbes Carl J.
Metfoils AB
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