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Probe Accuracy Inspection Tool for In-Circuit Test Fixtures Using PCB Artwork

IP.com Disclosure Number: IPCOM000111412D
Original Publication Date: 1994-Feb-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 4 page(s) / 157K

Publishing Venue

IBM

Related People

Bowen, SO: AUTHOR [+6]

Abstract

This article addresses three In-Circuit Test (ICT, bed-of-nails) mechanical problems: positional accuracy of fixture probes, printed circuit board flex induced by the fixture, and the verification of ICT (bed-of-nails) probe types. The invention uses reverse-image printed-circuit board artwork as an imprint on acrylic (transparent and photo-sensitive) panels, where the imprinted panels are referred to as "Artwork Cards". Different layers, or thin sheets, are epoxied to the artwork card depending on the mechanical problem under investigation.

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Probe Accuracy Inspection Tool for In-Circuit Test Fixtures Using
PCB Artwork

      This article addresses three In-Circuit Test (ICT,
bed-of-nails) mechanical problems: positional accuracy of fixture
probes, printed circuit board flex induced by the fixture, and the
verification of ICT (bed-of-nails) probe types.  The invention uses
reverse-image printed-circuit board artwork as an imprint on acrylic
(transparent and photo-sensitive) panels, where the imprinted panels
are referred to as "Artwork Cards".  Different layers, or thin
sheets, are epoxied to the artwork card depending on the mechanical
problem under investigation.

      Artwork cards consist of photo-sensitive acrylic panels onto
which reverse-image raw card artwork is exposed.  The photo-sensitive
acrylic panels are treated essentially as 35 mm film, and are
machined to the represented printed circuit board size.
Reverse-image artwork of a board's backside, external signal layer is
transferred onto the acrylic, chemically embedding the artwork into
the material, similar to a photograph.  The circular transparent
areas of the artwork represent test points and vias on the board.
Tooling pins orient a printed circuit board relative to the
bed-of-nails (ICT) probes.  The tooling pins serve the same function
on the artwork cards and are drilled according to printed circuit
board specifications.  Fig. 1 presents an artwork card and
illustrates its salient features.

      For probe accuracy inspection, a layer of standard bond paper
(e.g., 20 lb bond) is adhered to an artwork card using an epoxy with
the characteristics (e.g., tackiness) of common rubber cement.  The
adhesive provides the capability of repositioning and removing the
paper layer without affecting (contaminating) the artwork.  To record
probe locations, an additional layer of carbon paper is adhered to
the inspection layer using transparent tape.  Probe actuation of the
bed-of-nails fixture causes distinct dark markings from the
carbon-paper layer onto the paper inspection layer.  Probes in an
accurate fixture will mark the transparent circular areas (test pads)
of the artwork card, while inaccurate probes will leave marks outside
the test pad areas.  When illuminated properly, the markings and
artwork card enable a means of direct visual or optical comparator
verification of probe locations.  Fig. 2 illustrates this inspection
principle.  The paper inspection layer records the mechanical
features and parameters of interest for inspection, while the artwork
card serves primarily as an accurate positioning tool.  Artwork cards
relate the recorded parameters to the test pads of interest for
debugging.  For instance, if a known test pad on the artwork card
does not exhibit a probe mark, the pad location is matched to a
fixture probe for direct resolution of the problem, such as replacing
...