Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
2001-04-03
2003-04-29
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S237400, C356S237200
Reexamination Certificate
active
06556293
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to automated optical inspection (AOI) systems and particularly to an AOI system for inspecting and detecting defects in laser-drilled vias in printed electrical circuits.
BACKGROUND OF THE INVENTION
Most multi-layer printed circuit boards (“PCB”s) have vias, i.e., passageways from one layer to another, which generally are plated to provide electrical contact between the layers. Vias may be formed in many ways, for example by means of apertures in the required x-y positions in each layer, such that when the layers are joined together one on top of another and properly registered, the apertures define the via path.
Recently vias have been made by first forming a laminated multi-layer board, and then by drilling the vias by means of photoablating and/or photo-cutting with a laser beam. Two examples of widely used lasers are frequency tripled (or quadrupled) pulsed YAG lasers (wavelength 355 or 266 nm) and CO
2
lasers (wavelength about 10.6 &mgr;m). Both types of lasers can easily cut PCB laminate materials, such as glass-epoxy or polyimide. These two types of lasers have different advantages and disadvantages. For example, the laser beam of a YAG laser cuts through copper. This characteristic offers the advantage of being able to use a YAG laser to prepare vias on substrates that are coated with copper, however one can overdrill the laminate (insulating layer) into the copper of the lower layer if precautions are not taken. Conversely, the laser beam of a CO
2
laser does not cut through the copper, thus there is no danger of overdrilling. However a CO
2
laser can not be used to prepare vias on substrates coated with copper. Consequently, when a CO
2
laser is used, metal must be removed from the top layer where it is desired to drill a via, for example by etching, which does not have to be done for a YAG laser.
FIGS. 1A-1F
illustrate different typical defects associated with laser-drilled vias. The figures show an upper copper layer
2
, a laminate layer
3
and a lower copper pad
4
, wherein it is desired to drill a via
5
from upper layer
2
down to the top of pad
4
.
FIG. 1A
illustrates an underdrilled via, which of course means that although the via is plated with copper, no electrical connection will be made between layer
2
and pad
4
.
FIG. 1B
illustrates an overdrilled via, which means that the via is drilled through pad
4
resulting in too little metal being left on pad
4
for a reliable electrical connection between layer
2
and pad
4
.
FIG. 1C
illustrates “throw out”, i.e., debris
6
from photoablation of the layers being left in via
5
, which can cause plating or electrical connection problems.
FIG. 1D
illustrates the presence of foreign materials
7
in the via, which can cause plating or electrical connection problems.
FIG. 1E
illustrates an underplated via, namely a properly drilled via having plating
8
that is not suitably deposited which can lead to an insufficient electrical connection.
FIG. 1F
illustrates via
5
misregistered with pad
4
, which, although not being a defect in the drilling process per se, nevertheless is a defect which must be detected because it too can lead to an insufficient electrical connection.
Although automated optical inspection (AOI) systems are typically used to inspect PCBs, nevertheless no AOI system is known which can accurately, repeatably and reliably detect various defects in laser-drilled vias, independently of whether or not the PCB under inspection, and the vias thereon, are cleaned prior to inspection. Moreover no laser drill repair station is known to inspect substantially all laser drill vias on a PCB and to automatically repair only those defective laser drill vias having defect types that are repairable.
U.S. Pat. No. 5,216,479 shows and describes an optical inspection system for inspecting a surface of a laminate operative to distinguish between a first material, such as a printed circuit board laminate, and a second material, such as copper formed thereon, and employing a laser illuminator selectively illuminating the surface and signal analyzers operative to sense and analyze fluorescent light and reflected light resulting from illumination by the laser.
SUMMARY OF THE INVENTION
The present invention seeks to provide a novel AOI system which can accurately, repeatably and reliably detect and distinguish various different defects in laser-drilled vias. The present invention can simultaneously inspect through holes and laser-drilled vias. Preferably the detection of the various different defects can be made independently of whether or not the PCB under inspection, and the vias thereon, are cleaned prior to inspection.
Additionally, the present invention seeks to provide a novel AOI system which can accurately, repeatably and reliably detect and distinguish various different defects in laser-drilled vias, and which can automatically repair, for example by redrilling, those defective vias which are repairable. Preferably, if non-repairable defects are detected then no vias, such as vias having repairable defects, are redrilled.
The present invention uses a combination of inputs from at least two optical data channels which sense one or more of three different parameters of a radiation beam impinging on substances found on the PCB: luminescence of the substance (laminate, copper layer, copper pad, debris, etc.) due to the beam impinging thereon, reflection of the beam from the substance, and transmission of the beam at the point of impingement. Each data channel preferably is adjustable independently of the other channels for optimal performance. Although most preferably the beam is a laser beam, nevertheless the invention can be carried out with any coherent or non-coherent monochromatic or polychromatic light beam, or any other radiation, electromagnetic wave or acoustic beam, for example.
It is noted that the term “luminescence” refers to the emission of visible or non-visible electromagnetic radiation as a result of absorption of exciting energy in the form of photons, charged particles, or chemical change. The term luminescence includes both fluorescence and phosphorescence. In “fluorescence”, an atom or molecule emits detectable radiation in passing from a higher to a lower electron excitation state. The term fluorescence relates to phenomena in which the time interval between absorption and emission of energy is extremely short, typically in the range of 0.01-1000 microseconds. The term “phosphorescence” relates to the emission of radiation continuing after excitation has ceased, and may last from a fraction of a second to an hour or more.
The fluorescence of metallic conductors on PCBs, such as copper, under short wavelength visible light (for example 442 nm laser light emitted by a helium cadmium CW laser) is measurably less, generally by an order of magnitude, than the fluorescence of typical PCB laminate materials, such as glass-epoxy or polyamide. In addition, copper reflects light much better than such typical PCB laminate materials. The present invention exploits the large differences in fluorescence and reflectance of laminate compared to conductor in order to distinguish between portions of the via which are laminate and portions which are copper.
The reflectance and luminescence sensors preferably are placed above a horizontal PCB to be inspected, and are angled with respect to the via. The angled orientation of the sensors permits them to view and to provide sensed information along the entire depth of the via. By simultaneously combining and analyzing sensor inputs from at least two channels, the AOI system can detect and distinguish between at some of the various defects shown in
FIGS. 1A-1F
. The combined information can distinguish between defects in the via as opposed to defects on the upper surface of the PCB or at the bottom of the via. Most importantly, the combined information can distinguish between underdrilling or residue debris which can be repaired by further laser drilling, and other defects such
Nguyen Sang H.
Orbotech Ltd.
Stafira Michael P.
LandOfFree
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