Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-11-17
2003-01-28
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S761010
Reexamination Certificate
active
06512389
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to devices used for testing electronic components in a mother board/daughter board configuration and the general application of connecting two printed circuit boards (PCBs) in parallel configuration for improving the speed, purity and accuracy of electrical test signals used during component testing.
BACKGROUND OF THE INVENTION
Electronic components such as microprocessor devices, multi-chip packages, memory chip packages, field programmable gate arrays (FPGAs), dynamic random access memory (DRAM) and the like are utilized in packages such as ball grid arrays (BGAs) and chip scale packages (CSPs). These packages are well known in the art and are being used more and more frequently because they offer greater circuit density and higher pin counts than other packages such as quad flat packs (QFPs) and pin grid arrays (PGAs). Packages such as BGAs and CSPs also offer better electrical performance due to the shortened circuit path from the device die to the printed circuit board.
Prior to their being installed onto PCBs, electronic components such as those mentioned above must be thoroughly tested to ensure that the components are electronically correct, and are functioning properly according to specifications. The same is true of other electronic components such as the PCBs that are used to test other components. In an idealized testing regime the goal is to have the component (whether the “component” is a PCB or, for example, a BGA) behave as if it were directly connected to the adjacent PCB and to eliminate any possible fault indications caused by the testing process itself. For instance, in many components certain switches and circuits are designed to react in a specific way to electrical phenomena such as a voltage drop. However, if during the testing process a voltage drop occurs unintentionally, for instance, due to an imperfect signal, a switch may react to the drop in voltage as it normally would. While this may be the proper reaction, in the context of the test it may be interpreted as a fault reaction. In other words, the test regime did not account for a voltage drop due to an imperfect signal. As a result, an error indication is noted where in fact there was no error.
Signal errors and imperfections could be eliminated if the components were directly connected to the PCBs such as by soldering. That, however, is obviously impractical for a test process since the component could not be readily separated from the test board. The test equipment must therefore be designed to minimize signal errors and imperfections while allowing for economical testing processes.
By way of illustration, and with reference to one typical interface that is used to test components, devices called “test sockets” are used as part of the testing procedure and are intended to help ensure that test results are not flawed by errors that occur during the testing process as a result of the test equipment itself. The test sockets define an interface device that holds the component (such as a BGA) and allows the component to be interfaced with a test machine that is programmed to send electric test signals to the component. In this testing procedure the component being tested is referred to as the “device under test” or “DUT.” There are a wide variety of test sockets in use today. Nonetheless, in most instances the test socket comprises a box like compartment that is designed to hold the integrated circuit package such as a BGA in a desired position. For purposes of illustration, the following description refers to a BGA package. It will be understood that the description applies just as well to other packages.
The test socket typically has an internal compartment or window into which the BGA fits. The lower portion of the compartment is called a probe plate. It has a “footprint” of holes drilled through it that matches the array of test points on the BGA. A spring probe is retained in each hole in the probe plate—there is a spring probe associated with each test point on the BGA. Each spring probe extends from the interior portion of the test socket in the compartment where the BGA is held through the bottom of the socket.
The test socket is used in connection with a printed circuit board (PCB) that is specially designed solely for testing the particular BGA. This special PCB has many names, for instance “load board” or “DUT board,” the later meaning the “device under test board.” The later naming convention is used herein. The DUT board has only one purpose, and that is to facilitate testing of a specific component. The DUT board electrically interfaces the component under test with the test machine that sends signals to the component.
As noted, the DUT board is specially designed to test a specific component such as a microprocessor die packaged in a BGA. The board typically has many different circuits and it may have many different electronic components. These vary depending upon the device under test and other factors relating to the testing regime. But the DUT board also always has a footprint of electrical pads on one outer surface of the board that matches the footprint of the spring probes that extend through the bottom of the test socket. In practice, the footprint pattern is drilled through the DUT board according to the array pattern of test points on the device under test. An electrically conductive pad is then deposited on one surface of the board according to well known PCB manufacturing techniques. Each pad is electrically connected through the associated hole and associated traces to other components and ultimately to the test machine interface.
Just as there are many types of test sockets, there are also many types of DUT boards. In some instances multiple DUT boards may be used in combination to test a component. In other instances, multiple DUT boards or other printed circuit boards may be combined in order to provide an interface with a testing machine so that one may vary the combination of boards and/or sockets.
In instances where multiple PCBs are interfaced, or stacked, the multiple boards are oriented adjacent one another. The interface between the two boards may be made with a spring probe holding plate oriented between the boards or through connectors. The spring probes probe respective associated pads on the facing surfaces of the board in order to transfer test signals from the test machine through selected circuits on the boards. But the interfaces between the boards results in an undesirable increased distance between the boards.
For purposes of further illustration, a typical test process involving adjacent PCBs will be described. In the following example the adjacent boards are referred to as the mother board and the daughter board. It will be understood that this example is for illustration only, and that a similar illustration may be made with other board to board interconnections, and with board to socket interconnects. Nonetheless, in this board to board situation mother board is mounted adjacent the daughter board such that the one end of each spring probe held in the probe plate is physically urged against a test pad on the mother board, and the opposite end of each probe is in contact with the associated pad on the daughter board. This compresses the spring probes to insure good electrical contact between the pads and the spring probes. The two adjacent boards are typically bolted to one another with the probe alignment plate sandwiched in between, resulting in physical and electrical contact between the boards through the spring probes. In this way there is an electrical connection established from the test machine through a variety of traces and components in the mother board, through the spring probes, and to the associated test points on the daughter board. This compressive load is referred to as “pre-load” compression.
The paired boards are then connected to a test machine, and the test machine can then begin sending test signals according to a preprogrammed testing routine to the boards to determine whether the
AQL Manufacturing Services, Inc.
Cuneo Kamand
Ipsolon LLP
Tang Minh N.
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