System for contactless testing of printed circuit boards

A system for testing for opens in and shorts between conductor traces on two surfaces of a printed circuit board, the conductor traces having a reference potential, said system comprising:

• two electron gun assemblies for generating two electron beams;
• two electron optics assemblies, one of said electron optics assemblies for directing one of said electron beams to the conductor traces on one of the surfaces of the printed circuit board, and the other of said electron optics assemblies for directing the other of said electron beams to the conductor traces on the other of the surfaces of the printed circuit board;
• a raster/vector generator for controlling position and intensity of the electron beams;
• two grids, each of said grids located proximate to and substantially parallel with one of the surfaces of the printed circuit board, said grids capable of collecting secondary electrons emitted from the printed circuit board, at least one of said grids being placed at a first potential before one of the electron beams is directed to a first point on a conductor trace to charge the conductor trace to a first potential, and at least one of said grids being placed at a second potential, the second potential being between the reference potential and the first potential, before one of the electron beams is directed to a second point on a conductor trace to cause emission of secondary electrons;
• and
• signal processing electronics and a CPU for determining whether or not an open or short exists depending on whether secondary electrons are collected by said grids as the electron beams are directed to various locations on the conductor traces on the surfaces of the printed circuit board.

A method for the contactless testing for opens in a conductor trace passing through a printed circuit board from a first surface to a second surface, the conductor trace having a reference potential, said method comprising the steps of placing a first grid located proximate to and substantially parallel with the first surface of the printed circuit board to a first potential;

• directing a first electron beam to a first point on the conductor trace on the first surface to charge the conductor trace to substantially the first potential;
• placing a second grid located proximate to and substantially parallel with the second surface of the printed circuit board to a second potential, the second potential being between the reference potential and the first potential;
• directing a second electron beam to a second point on the conductor trace on the second surface to cause emission of secondary electrons;
• and
• determining whether an open exists depending on whether secondary electrons are collected by the second grid.

A method for the contactless testing for shorts between conductor traces passing through a printed circuit board from a first surface to a second surface, the conductor traces having a reference potential, said method comprising the steps of:

• placing a first grid located proximate to and substantially parallel with the first surface of the printed circuit board to a first potential;
• directing a first electron beam to a point on a first conductor trace on the first surface to charge the first conductor trace to substantially the first potential;
• placing a second grid located proximate to and substantially parallel with the second surface of the printed circuit board to a second potential, the second potential being between the reference potential and the first potential;
• directing a second electron beam to a point on a second conductor trace on the second surface to cause emission of secondary electrons;
• and
• determining whether a short exists depending on whether secondary electrons are collected by the second grid.