Optical comparator method determines the quality and registration of surface features of double-sided printed circuit boards. Color-coded superimposed images of both sides of a printed circuit board are presented to view, clearly showing details and registration of the circuitry

Cridlin, M.

Oconnor, J.

English

NASA Technical Reports Server

January 1968	 Brief 68-10021 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Optical System Facilitates Inspection of Printed Circuit Boards 
Double-Sided Printed 
Circuit Board 
Mirror	 7	 Mirror 
.	 Filters 
TCamera 
\ / Color Film 
Transparent/Reflecting / 
Membrane	 Reticle 
Red & Green Signals 
Superimposed 
The problem: 
To provide a means for accurately determining the 
quality and registration of surface features of double-
sided printed circuit boards. Present methods for 
visual inspection of these attributes on printed cir-
cuit boards, using magnifying devices and a reticle, 
are slow and tedious, particularly for high-density 
circuits. An X-ray superimposition method, which has 
been used, does not register the images from both 
sides of the board with sufficient accuracy for 
reliable inspection.
Ground Glass 
Viewing Screen 
The solution: 
An optical comparator method by which color-
coded superimposed images of both sides of a printed 
circuit board are presented to view, clearly showing 
details and registration of the circuitry. 
How it's done: 
Light sources are arranged to illuminate each side 
of the printed circuit board. Surface features of the 
circuitry are reflected by the two inirrors to primary 
(Continued overleal) 
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	 information contained in this document, or warrants that such use Government nor any person acting on behalf of the United States
	 will be free from privately owned rights.
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(for example, red and green) color filters. The color-
coded images are superimposed by the transparent/ 
reflecting membrane (half-surface mirror) and pass 
into the camera lens. In the camera, the superimposed 
images can be focused onto a ground glass screen and 
viewed actual size or under magnification. A cali-
brated reticle, capable of various magnifications, can 
be used to measure image details. The superimposed 
images can also be photographed on color film (or 
high resolution black and white film) to provide a 
permanent record for viewing directly or as a pro-
jected, enlarged image. Rapid scanning of the super-
imposed images on the ground glass screen will 
immediately bring out deviations from proper regis-
tration and surface details of each side of the printed 
circuit board. The screen displays a dominant color 
mixture for features in registration and a discrete 
primary color (depending on the two filters used) for 
features which are not in precise registration.
Notes: 
1. Various modifications of the basic method are 
possible. These include the use of rotating 
optical devices, stroboscopic lights, prisms in 
place of plane mirrors, and the use of colored 
lights instead of filters. 
2. This method would also be useful for comparing 
other types of surface features on opposite faces 
of a panel. Inquiries concerning technical details 
may be directed to: 
Technology Utilization Officer 
Goddard Space Flight Center 
Greenbelt, Maryland 20771 
Reference: B68-10021 
Patent status: 
No patent action is contemplated by NASA. 
Source: M. Cridlin and J. O'Connor 
(GSC-0797 I) 
Brief 68-10021	 Category 02


Optical system facilitates inspection of printed circuit boards

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