Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-01-05
2002-09-17
Nguyen, Vinh P. (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
Reexamination Certificate
active
06452410
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, to printed circuit board testing, and, more particularly, to the testing of bare printed circuit boards by the use of an electrolyte.
BACKGROUND OF THE INVENTION
Printed circuit boards, or “PCBs,” have been used in the electronics industry for many years as a valuable means of connecting many separate electronic components, such as integrated circuits (“ICs”), resistors, capacitors, and others, to form an electronic circuit necessary to perform a desired function.
Conductive connection points on the boards, called “pads,” are the areas of the board to which the various electronic components are connected. The pads are, in turn, connected to each other with printed wiring to form “nets” so that there are essentially zero ohms of impedance between any two pads on the net. Conversely, two pads on different nets essentially have an infinite amount of impedance between them.
To further define some terminology, a “layer,” as it relates to printed circuit boards, is a collection of pads and associated printed wiring that substantially reside within the same geometric plane. A PCB has one or more such layers, with each layer being isolated from the next by a stratum of nonconductive material. Many PCBs have two or more layers, and some will have several, with only the outermost layer on each side of the board having pads. Some of the layers may even be power or ground distribution “planes,” with those layers consisting mostly of metal, as opposed to a small amount of printed wiring. Portions of a net may reside on several different layers, with “vias,” or plated-through holes, connecting those portions to form the net.
For descriptive purposes,
FIG. 1
presents an edge view of an example of a 2-layer printed circuit board
1
, and
FIG. 2
provides a plan view of the top of board
1
. The conductive portion of board
1
consists primarily of layers
7
and
8
, with the layers being separated by nonconductive material
9
. Printed wires
2
connect solder pads
3
to each other to form nets
4
. Via
5
is utilized on this board to connect two portions of a net
4
that are on opposite sides of board
1
. For purposes of illustration, the height of layers
7
and
8
, as shown in
FIG. 1
, is greatly exaggerated compared to the thickness of nonconductive material
9
.
Prior to attaching electronic components to the PCB, a solder “mask,” or protective layer, is usually applied to the two sides of the board. As shown in
FIG. 3
, solder mask
6
covers printed wiring
2
(FIG.
1
), vias
5
(FIG.
1
), and other areas of the board, essentially leaving only connective pads
3
exposed. Solder mask
6
serves to prevent solder from adhering to unwanted areas of the board, as well as to protect the board from damage due to abrasion or corrosion.
In order to guarantee a certain level of quality in the products that incorporate such boards, the PCBs are usually tested alone prior to being assembled with the rest of the product in which they reside. At least some of this testing is performed after the boards have been populated with electronic components.
Over the years, printed circuit board technology has progressed. More specifically, the number of layers allowed has increased, while the distance between printed wires, and the width of the wires themselves, have decreased. As a result, the number of components that can be attached to a given amount of surface area on a board has increased substantially. At the same time, the electronic components that populate these boards have become more complex, due to advances in IC technology. Therefore, it is now common for the overall cost of a board populated with electronic components to be determined primarily by the cost of the individual components rather than by the cost of the “bare,” or unpopulated, board.
A direct result of these advances is that testing the bare board before attaching the electronic components is essential, since the discovery of a board defect after populating the board may now result in a much greater cost than if the defect were found prior to attaching the components. The two most common defects are “short circuits,” whereby two nets that are intended to be separate are, in fact, connected, and “open circuits,” whereby a portion of a net is inadvertently disconnected from the remainder of the net. In the case that such defects are discovered during testing, the bare board is typically discarded, since repairing the board is generally not economically feasible.
Currently, two basic methods exist for testing bare boards. In the first case, a test fixture is constructed which contains a contact probe for each pad on the board. For boards with pads on each side (called “two-sided boards”), the fixture has two sides and is capable of contacting all of the pads on both sides. Each contact probe is connected in some fashion to a test algorithm controller, which checks for wanted or unwanted electrical continuity between any two probes, thereby checking for any of the short circuit or open circuit board defects mentioned above. Tests can be performed quickly using such a system, but a new test fixture must be manufactured for each board design to be tested. Any changes in the board require commensurate changes in the test fixture. Also, test fixtures for the more complex board designs may require tens of thousands of probes, making such fixtures expensive, fragile, and difficult to modify.
The second method involves a “flying probe” technique, whereby two or more probes-are mechanically maneuvered over, and possibly under, the board under test by a probe movement mechanism. Two or more pads are contacted at a time, and a continuity measurement is made. Due to the need to move the probes horizontally over the board for each measurement, this method will be inherently slower that the first method described. On the other hand, using the flying probe method is generally much less expensive, because little “fixturing,” or modification of the tester specific to the board under test, is required, and design changes to the board under test are easily accommodated by changes to the test algorithm controller software.
One major drawback of both techniques discussed above is that the continuity of each pad is tested with respect to every other pad on the board to guarantee that no unintended short circuits or open circuits exist. Testing of each pad on each net in this manner is typically a time-consuming and relatively expensive process. Therefore, a need currently exists for a method of bare board testing that can measure each net without requiring the probing of each pad on a net, or pads on other, unrelated nets.
SUMMARY OF THE INVENTION
In the embodiments discussed below, the invention provides a way to test a bare printed circuit board without probing every possible pad combination, saving both time and expense during testing. The associated test fixture necessary to implement an embodiment of the invention is simple to manufacture, and easy to modify in response to board design changes.
One embodiment of the bare board tester according to the invention includes a conductive tank, together with a way to support the bare board within the tank. Contained within the tank is a sufficient amount of an electrolyte to immerse the board. One or more probes are then used to contact the pads of the board. The probes are used to measure the electrical resistance between the probe and the conductive tank. In some embodiments, this measurement is performed by applying a voltage across one of the probes and the tank, thereby generating an electrical current through the pad being contacted by the probe, the net to which the probe is attached, and the electrolyte. The amount of current is indicative of the resistance encountered, which, in turn, indicates the total surface area of that net that is exposed to the electrolyte. If the printed circuit board includes a solder mask that leaves only the connection pads exposed, and the pads are all essentially the same size, then the me
Agilent Technologie,s Inc.
Nguyen Vinh P.
Way Kyle J.
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