Electric heating – Metal heating – By arc
Reexamination Certificate
1999-12-03
2001-04-24
Heinrich, Samuel M. (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06222156
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to methods of repairing printed circuit board wiring and in particular to laser ablation repair methods of repairing shorts on printed circuit boards and both organic and inorganic chip carriers.
BACKGROUND OF THE INVENTION
Printed Circuit (PC) boards, also called printed wiring boards, are known in the art. Integrated Circuit (IC) chips are mounted on PC boards and interconnected through PC board wiring. These IC chips may be packaged and, the packaged chips mounted on a board or, to achieve higher chip density, chips may be mounted directly onto the board.
To further increase the number of chips that may be mounted on PC boards, such as state of the art Surface Laminar Circuit (SLC) boards, narrower wires are crowded closer together on each wiring layer to wedge more and more wires into smaller and smaller areas. Currently, the space between wires is about 5 mils (12.5 &mgr;m). One result of increasing wiring density is a corresponding increase in PC board costs.
So, to control PC board costs and to reduce waste, defective PC boards are repaired, whenever possible. Excimer lasers, which are expensive and difficult to maintain, are normally used to repair surface wiring on ceramic substrates.
However, a typical excimer laser used to repair a short on a ceramic surface would pass through an organic surface layer of an SLC board, damaging underlying wiring. Also, Xenon lasers with a wavelength in the blue-green range (450-570 nm) have been found to pass through an organic surface layer, damaging any underlying wires.
Thus, these types of lasers may not be used to repair defects (especially shorts) in dense SLC surface wiring. Instead, shorts are repaired on SLCs, typically, by cutting them with a knife (manually) or with an ultrasonic cutter.
Unfortunately, cutting the short may also damage the PC board being repaired. The knife may slip and damage the SLC. Further, as wiring density increases, cutting becomes very difficult. So, for dense wiring on an organic insulator, such as an SLC, manual repair is inconsistent at best.
Thus, there is a need for a method of removing shorts on densely wired PC boards, such as SLCs.
PURPOSES OF THE INVENTION
It is therefore a purpose of the present invention to improve densely wired PC board yield;
It is another purpose of the present invention to reduce the cost of densely wired PC boards;
It is yet another purpose of the present invention to simplify wiring short removal on densely wired PC boards.
SUMMARY OF THE INVENTION
The present invention is method of selectively trimming metal from a surface layer of an insulator material without damaging wiring buried in or beneath the material. A laser is selected with a wavelength such that it is partially absorbed between 1-10% by the dielectric material. Preferably, it is absorbed between 2-5%. Thus, when the metal is ablated or etched from the dielectric surface, etching continues slightly into the surface. However, the laser is prevented from completely etching through or passing through the dielectric, and inadvertently ablating metal therein or thereunder.
In the preferred embodiment, a 266-380 nm, and preferably a 355 nm, Nd:YAG laser is used to ablate copper or copper/metal shorts on an Advanced Solder Mask (ASM) dielectric surface.
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D.L. Klein, P.A. Leary-Renick & Srinivasan, Ablative Photodecomposition Process for Repair of Line Shorts, Feb. 1984, pp 4669-4671.
Fraley, Esq. Lawrence R.
Heinrich Samuel M.
International Business Machines - Corporation
McGuireWoods LLP
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