1. Field of the Invention
The present invention relates to a varnish and a prepreg, in particular to a varnish containing with inorganic mineral powders, a prepreg made from the varnish, and a substrate made from the prepreg.
2. Description of Related Art
A Printed Circuit Board (PCB) is made by laminating and curing the prepregs, copper clad laminate (CCL), or copper film. The prepregs are generally made by dipping glass fabrics into vanish and then curing the fabrics to form thin prepregs. Regarding environmental protection (such as RoHS, WEEE), lead-free solder is more and more widely used for electronic application. However, the process temperature is increased in 30 to 40° C. for the lead-free solder processes and it is necessary to improve the heatproof property of the PCBs so that the PCBs can be used in higher temperature process
Traditionally, the inorganic fillers and fillers of the anti-flammability such as SiO2 and aluminium hydroxide powders are added into the varnish to improving the heat-proof and anti-flammability of the prepregs/substrates. However, the above-mentioned powders are harder materials and the manufactured prepregs/substrates are too hard and brittle to be machined. For example, while drilling on the hard and brittle substrates, the surfaces of the drilled holes are roughness, or the laminated layers of the substrate are de-laminated. There are some cracks inside the substrate. On the other hand, the drill bit is easily broken and the manufacturing cost is increased.
On the other hand, the smaelite powders which is often used for manufacturing insulating layer of the electrical wires are also applied to the varnish. Generally, the smaelite powders not only have SiO2 but also aluminum compound in weight above 30% to improve the properties of heat-proof and anti-flammability of the prepregs/substrates. Furthermore, the smaelite powders are softer materials so that the manufactured prepregs/substrates have improved machining properties comparing to the prepregs/substrates manufactured the varnish having higher fillers of SiO2. Thus, the drilled holes have smooth surface and have no cracks thereon. However, the smaelite powders have less composition of SiO2 (less than 40%) and the electrical properties such as of Dk, Df of the prepregs/substrates cannot meet the requirement of applications. In addition, the peeling strength of the laminated substrates is decreased.
Still further, talcum powders can be used as fillers to improve the electrical properties such as of Dk, Df of the prepregs/substrates for the communication application. Talcum powders have high composition of SiO2 and composition of magnesium oxide. The composition of magnesium oxide can be provided for improve the anti-flammability of the prepregs/substrates. However, magnesium oxide materials are condensed and suspended in the alkaline solution. Thus, talcum powders merely can be used in a varnish with limited condition. In other words, the talcum powders cannot be used in general applications.
Therefore, in view of these difficulties, the inventor has developed ways to overcome these difficulties to yield dependable product with reliable results in production; the present invention addresses these difficulties and allows reliable production at high speed and overcomes the above problems.