This invention relates to the field of circuit boards and, in particular, to aluminum alloys for use in the manufacture of printed circuit boards.
It is well known in the art that an elementary printed circuit board includes conductive foil sheets laminated on opposite sides of a prepreg (a glass fabric reinforced with epoxy resin). Subsequently, the copper is etched to produce conductive paths on the surface of the prepreg layer. It is not uncommon to assemble a stack, called a book, of such boards in the manufacturing process. Separator sheets are used to keep the boards in the book from touching.
To function properly, it is important to have a copper foil free from defects such as dents, scratches, and bumps. If defects appear in the copper foil, proper conductive paths cannot be etched in the image transfer process, leading to open circuits or electrical shorts in the finished product. Therefore, the copper foil surface must be even. Such an even surface can only be achieved by using separator sheets with a completely smooth surface.
Separator sheets are often made of stainless steel. However, this approach includes a major drawback. Stainless steel is a poor conductor of heat, meaning it takes more time and thus requires more energy to heat the books.
As a consequence, aluminum sheets are used more and more frequently, since aluminum conducts the heat better and more evenly than steel. For example, U.S. Pat. No. 5,256,474 issued to Johnston disclose a separator-release plate for use in the lamination and drilling of printed circuit boards comprising a substrate sheet of aluminum foil, rather than a stainless steel plate. Similarly, U.S. Pat. No. 5,725,937 issued to Johnston discloses a laminate of copper and aluminum foil for use in the manufacture of printed circuit birds.
Other benefits of using aluminum foil include a lower cost than stainless steel plates.
Problems still exist with the CAC approach. The aluminum is softer than stainless steel, causing a xe2x80x9cgravestone rubbingxe2x80x9d effect. In turn this affects the image transfer process and can lead to dysfunctional circuitry in the final product.
Therefore, what is needed is a separator sheet that is less expensive that stainless steel and can help smooth out any small flaws in the copper foil surface. Also, an aluminum alloy that is not too soft to be effective is needed.
The present invention presents a hardened aluminum alloy for use in the manufacture of printed circuit boards. While this alloy is used in the aircraft construction industry, it is not used in the manufacture of printed circuit boards as a separator sheet. As a separator sheet, the hardened alloy allows less image transfer and smoother copper foil surfaces in the circuit board.
The alloy is hardened relative to standard material used in the art due to elevated levels of magnesium and manganese in the alloy. The alloy is made into separator sheets, or a foil, by a cold rolling process. The optimal thickness of such foil is well known in the art.
Therefore, it is an aspect of this invention to provide a hardened. aluminum alloy for use in the manufacture of printed circuit boards.
It is another aspect of this invention to provide a separator sheet that is less expensive than stainless steel sheets and which due to it""s reduced thickness allows more boards to fit each to each press load.
It is another aspect of this invention to provide a separator sheet that can be recycled.
It is a further aspect of this invention to provide a separator sheet that helps smooth out small wrinkles in the surface of the copper foil.
It is a further aspect of this invention to provide a separator sheet that does not react chemically with copper.