Testing probe cards that contain complex circuitry
GREG OLMSTEAD and BOB DAVIS, Rudolph Technologies, Inc., Flanders, NJ
State based testing is a simple, highly scalable method to describe activation details for probe card circuitry.
As the complexity and cost of integrated circuits and packaging technologies continue to increase, so also does the need for more sophisticated testing capability. This has resulted in the development of highly advanced probe cards, which provide the mechanical and electrical interface between the tester and the integrated circuit. In addition to tremendous increases in the number of probes and channels, probe density, and probing forces, current generation probe cards also incorporate complex circuitry to support advanced functionality. Both manufacturers and users of probe cards use probe card analyzers (PCA) to monitor and maintain probe cards and ensure the integrity of the testing process. At the most basic level probe card analyzers measure the mechanical position and electrical continuity of the probes. However, on today???s more advanced probe cards, the PCA must also monitor the functionality and performance of on-card circuitry and components.
Evolution of probe card complexity
The first test probes provided no more than a direct conductive mechanical/electrical contact to a test pad or exposed node of the integrated circuit. Multiple probes were soon arranged on a probe card to provide multiple simultaneous contacts. Probe card analysis in these early cards focused simply on measuring the positions (X,Y and Z) and electrical properties (contact resistance, leakage current, capacitance) of the probe tips.
The circuitry on probe cards evolved rapidly, beginning with the addition of passive components such as capacitors and resistors, and quickly expanding to include relays and other active circuitry designed to extend the I/O resources of a given tester platform. The PCA then had to supply power and control to the relays and verify the switched pathways.
Advanced probe cards now often incorporate local power management, digitally controlled analog switching, probe card heating, and non-volatile storage for identification and touchdown history. They may also include devices such as opto-couplers, voltage regulators, PLDs, FPGAs and microcontrollers. Circuits can use latched or sequenced inputs, and serial or other communication protocols to control states and potentially provide feedback on the health or state of the ???on board??? control circuits.
These on board circuits constitute, in effect, an electrical barrier between the tester side of the probe card and the probe tips. They must be manipulated and controlled in order to measure probe characteristics. The circuitry included in many current probe card designs has reached a level of complexity that makes it impractical to control individual components manually. State based testing is a strategy to simplify circuit testing by collecting all of the required activation conditions into a single ???state??? definition. Simply recalling a predefined state allows the PCA to do what is necessary to activate the circuit while the user gets on with testing the probe card. The state definition may be a static definition of an array of I/O conditions or may include sequences of conditions that must be executed in order to bring the circuit to the desired state.
In addition to being able to manipulate the probe card components to enable measurements of probe characteristics, the PCA must be able to test the performance and functionality of the individual components themselves. This requires probe card manufacturers to design-for-test, being sure to provide access to all of the various control elements and signal nodes needed to enable thorough and complete test procedures.
For each probe card circuit, card designers and test engineers must define the required challenge and response, decompose the circuit function, identify a means of circuit activation in the probe card analyzer, and specify control conditions sufficient to achieve activation. With state based testing, the use of predefined activation states greatly simplifies the task of establishing the activation conditions required to make the desired measurements.
Relay switching is a way to increase the number of connections available to a limited number of test channels. Consider a simple multiplexer composed of a pair of double pole, double throw relays configured to switch a single channel among four connections. The connection is determined by the states of the two relays (Fig. 1). Defining a state name for each of the four possible combinations of relay states permits simple measurement of the resistance for each connection.
FIGURE 1. Relay switching can increase the number of connections permitted for a limited number of test channels. Here a pair of relays connects a single channel to any one of four probes (top). State definitions specify activation conditions for two control lines (middle). Results reference the state label for each measurement (bottom).
Simple switching for component validation
In Fig. 2, an FET switch controls a bypass capacitor. There are no probes in this example. It is provided to illustrate the importance of exposing nodes to enable component verification. When CTL14 is at logic 1, FET Q2 is closed. When CTL14 is at logic 0, FET Q2 is open. Capacitor C5 is removed from the circuit by default, but is connected when FET Q2 is on (State = ST_Q2_ON). XCH7, connected between the switch and the capacitor, allows direct and isolated measurement of C5, and direct and isolated measurement of Q2 drain-source resistance in both on and off states.
FIGURE 2. Good design-for-test practice provides connections for component testing as well as probe switching. In this example XCH7 is connected to a node between the capacitor and the FET switch, permitting direct, isolated testing of each component.
Analog IC switching
Analog switching can be much faster, consumes less power and allows greater density than relay based switching. It permits much greater flexibility in configuration but can present challenges to probe card analysis, such as large numbers of connections or the sharing of control returns with other ???grounds???. It has the potential to be configured via on-board logic without much ???driver overhead???, further increasing its flexibility but creating combinations of challenges for probe card analysis.
FIGURE 3. Analog IC switching can be much faster, consumes less power and allows greater density than relay based switching. In this example each switch is controlled independently by the condition of a single CTRL line, providing simple mapping and arbitrary flexibility. State based testing scales readily to the larger number of connections typical of analog IC designs.
In the example shown in Fig. 3, the IC is a quad single pole single throw analog switch connecting XCH1 to any of Probe 61, Probe 62, C17 or C20. CTRL lines 1-4 select the address of these options. Pull up resistors make the hardware default ON. In the State Labels the default condition for all switches is open (contrary to the hardware default). In contrast to the combinatorial control scheme illustrated above for the relay switch, here each switch is controlled independently by the condition of a single CTRL line, providing simple mapping and arbitrary flexibility.
Stateful control satisfies the need for more complex control and monitoring of functions such as switching, power health, identification, verification state and more. It scales easily with probe count, channel count, the amount of switching and the amount of control fanout. It simplifies activation of the desired state through the extension of state based control to include time based control sequences. It can also incorporate feedback from the device under test to the probe card analyzer.
Another important benefit of state based testing is its ability to support thorough and complete testing without exposing details of the circuit design, allowing probe card manufacturers to encapsulate and protect their intellectual property.
FIGURE 4. Current generation probe cards may incorporate very complex circuitry with digitally controlled sequential logic. State based testing scales easily with size and complexity and readily accommodates activation sequences and feedback from the card. This circuit multiplexes 24 I/O lines among 4 separate 24-probe site arrays.
The schematic shown in Fig. 4 illustrates the large number of connections and complexity typical of current probe card designs. As shown in the block diagram, the circuit switches 24 I/O lines among 4 identical 24-probe site arrays. Control and feedback lines operating through a controller IC provide information on the health and status of the circuit. Control lines may be more numerous and may include sequential control schemes.
The complexity and cost of probe cards used to test integrated circuits have increased dramatically, making probe card analysis both more challenging and more valuable. State based testing is a simple, highly scalable method to describe activation details for probe card circuitry to enable traditional probe card testing as well as thorough and complete testing of on board circuitry. Combined with PCA-aware design-for-test, state based testing can maximize PCA verification of probe cards with increasingly complex circuits, while simplifying the process. State based testing also provides important protection for intellectual property by supporting testing without exposing details of the circuit design.
GREG OLMSTEAD is Senior Member of Technical Staff and BOB DAVIS is Staff Software Engineer at Rudolph Technologies, Inc., Flanders, NJ.
Solid State Technology, Volume 55, Issue 7, September 2012