This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to testing the characteristic impedance of integrated circuit package substrates.
As integrated circuits become increasingly faster, the structures used for conducting the signals must be fabricated to increasingly tighter tolerances so that they do not degrade or otherwise introduce unwanted characteristics into the signals which they conduct. Even the electrically nonconductive structures can effect the quality of the high speed signals passing through the conductive elements that are nearby. These tighter tolerances are important not only in the fabrication of the integrated circuits themselves, but also for the structures that are used to feed the signals to and from the integrated circuits, such as package components.
Obviously, if the conductive traces in a package substrate have shorts or opens, then the package substrate will function improperly. However, even seemingly minor imperfections can effect signal integrity at high signal speeds. For example, if such conductive traces are not of a uniform thickness and width along the length of the trace, then the impedance of the non uniform trace changes along its length. Thus, discontinuities in printed traces and connectors can degrade signal integrity. In addition, flaws in the non electrically conductive materials can introduce shorts and capacitances. Once problems like these occur, other problems begin to cascade, such as crosstalk, reflections, logic errors, and clock skew.
Time domain reflectometry is a method that is used to discover such problems in structures like integrated circuit package substrates. Time domain reflectometry uses a high speed digitizing oscilloscope with a built in step generator that launches a fast edge into a device under test, such as an electrical trace in a package substrate. By monitoring the reflected wave from various impedance discontinuities encountered in the substrate, different properties of the substrate, such as those mentioned above, can be sensed and analyzed. Time domain reflectometry uncovers such unwanted signal reflections so that defective substrates can be scrapped or reworked, and design flaws and process flaws can be corrected.
However, because time domain reflectometry is a very sensitive process, it is a relatively difficult and time consuming process. Therefore, only a few traces are typically tested on a substrate, because it is cost prohibitive to test a greater number. Therefore, a certain percentage of problems may remain undetected even after time domain reflectometry, because of the relatively few number of traces that are typically tested. Furthermore, in order to obtain tighter tolerances it is important to consider the variations in the entire package substrate.
What is needed, therefore, is a system by which time domain reflectometry can be performed on a greater number of package substrate traces without an extreme increase in time, so that problems with the traces can be detected prior to attachment and test or use of the integrated circuit.
The above and other needs are met by a probe structure for testing the impedance of a package substrate using time domain reflectometry. A connector electrically connects the probe structure to a time domain reflectometry tester, where the connector has a signal conductor and a ground conductor. A non electrically conductive back side layer is physically connected to the connector. A non electrically conductive probe side layer with electrically conductive contacts is sandwiched with the non electrically conductive back side layer in a layered substrate. The non electrically conductive probe side layer has a centrally disposed signal electrical contact and surrounding ground electrical contacts.
An electrically conductive layer is disposed between the back side layer and the probe side layer. The electrically conductive layer is electrically connected to the ground conductor of the connector. The electrically conductive layer is also electrically connected to the ground electrical contacts of the probe side layer electrical contacts. An electrically conductive signal via extends from the back side layer to the probe side layer. The electrically conductive signal via is electrically connected to the signal conductor of the connector, and is also electrically connected to the centrally disposed signal electrical contact of the probe side layer electrical contacts. The electrically conductive signal via does not make electrical connection with the electrically conductive layer on the back side or on the probe side.
A first of electrically conductive pins is electrically connected to the signal electrical contact, for making an electrical connection with an electrically conductive structure to be tested on the package substrate. Others of the electrically conductive pins are electrically connected to the ground electrical contacts, for making electrical connections with electrically conductive structures on the package substrate that surround the electrically conductive structure to be tested on the package substrate.
In this manner, a single electrical structure on the package substrate, such as an electrical trace, can be probed with the centrally disposed signal pin, while the other ground pins make contact with the surrounding contacts on the package substrate, where the electrical connections to all of the surrounding contacts are tied together. Thus, the package substrate can be quickly and easily probed, and time domain reflectometry measurements can be readily taken. Once this probe setup is attached to an automatic XYZ positioner, time domain reflectometry measurements can be performed automatically. This setup improves the accuracy of the measured wave forms due to the ability of the setup to ground the surrounding conductive structures while probing the structure of interest. Therefore, more accurate measurements can be taken in a given amount of time, providing for a more complete investigation of the package substrate and the package substrate design.
In various preferred embodiments of the invention, the electrically conductive layer is two electrically conductive layers separated by an intervening non electrically conductive layer, where the two electrically conductive layers are electrically connected by electrically conductive vias, with a vertical impedance between the signal via and ground via of about fifty ohms. Preferably, the probe structure is about fifteen millimeters by about fifteen millimeters in size. The probe structure size can be adjusted for other applications. In one embodiment the electrically conductive layer is a metal plane. The connector is preferably a fifty ohm subminiature version A connector. The non electrically conductive back side layer and the non electrically conductive probe side layer are most preferably formed of a solder mask material.
The electrically conductive pins are preferably either cobra pins or pogo pins. Any contact pins with some compliance can be used as well. Most preferably, the electrically conductive contacts on the non electrically conductive probe side layer are disposed at a pitch adapted for testing a ball grid array side of the package substrate, but are alternately disposed at a pitch adapted for testing a solder on pad die side or electrically conductive die side pad of the package substrate. The electrically conductive contacts on the non electrically conductive probe side layer may also be disposed at a pitch of one of about eight tenths of millimeter, about one millimeter, and about one and twenty-seven hundredths of a millimeter.
According to another aspect of the invention there is described a system for testing the impedance of a package substrate using time domain reflectometry. A time domain reflectometry test station is electrically connected to a probe structure via a connector, where the connector has a signal conductor and a ground conductor. A non electrically conductive back side layer is physically connected to the connector. A non electrically conductive probe side layer with electrically conductive contacts is sandwiched with the non electrically conductive back side layer in a layered substrate. The non electrically conductive probe side layer has a centrally disposed signal electrical contact and surrounding ground electrical contacts.
An electrically conductive layer is disposed between the back side layer and the probe side layer. The electrically conductive layer is electrically connected to the ground conductor of the connector. The electrically conductive layer is also electrically connected to the ground electrical contacts of the probe side layer electrical contacts. An electrically conductive via extends from the back side layer to the probe side layer. The electrically conductive via is electrically connected to the signal conductor of the connector, and is also electrically connected to the centrally disposed signal electrical contact of the probe side layer electrical contacts. The electrically conductive via does not make electrical connection with the electrically conductive layer.
A first of electrically conductive pins is electrically connected to the signal electrical contact, for making an electrical connection with an electrically conductive structure to be tested on the package substrate. Others of the electrically conductive pins electrically are connected to the ground electrical contacts, for making electrical connections with electrically conductive structures on the package substrate that surround the electrically conductive structure to be tested on the package substrate.
Preferably, an XYZ stage moves the package substrate under the probe structure, thereby enabling the automated testing of multiple electrically conductive structures to be tested on the package substrate Most preferably, a pattern recognition system aligns the electrically conductive pins to the electrically conductive structures on the package substrate.
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