What is the Probe Card?

I. What is a Probe Card?  

### Definition and Function  

A Probe Card is a key interface device used in the wafer test phase of semiconductor manufacturing.  

It is located between the Automatic Test Equipment (ATE, or tester) and the target chips on the wafer surface. Its main function is to complete electrical performance testing before the chips are packaged, screening out good and defective products to prevent waste from entering the packaging process.  

As an electrical test interface, the Probe Card accurately transmits test signals from the tester to the pads or bumps of the chip, and then sends the chip's response signals back to the tester, thereby completing functional testing, parameter measurement, and judgment.  

In a wafer prober, the optical system performs alignment to achieve precise alignment between the probes and the test points on the wafer. The Probe Card is pressed down to make contact with the wafer through mechanical action to complete the test.  

The Probe Card is a core component in controlling chip manufacturing costs and yield. A highly reliable and high-performance probe card can significantly reduce test time, improve test coverage, and maintain good signal quality through accurate contact, thereby increasing production volume and chip quality.  

II. Basic Structural Composition of a Probe Card  

1. Printed Circuit Board (PCB)  

The PCB is the basic structure and carrier of the Probe Card, used for:  

- Physically supporting the probe array and other components;  

- Providing electrical connection paths to transmit test signals and power;  

- Mounting necessary electronic components or spring circuits (ATRE resource sharing).  

PCBs are usually Multi-Layer Organic substrates (MLO), especially in high-end probe cards, which have requirements for precise impedance control and high-frequency characteristics.  

2. Probes (or Needles)  

Probes are the core components of the Probe Card, responsible for direct physical contact with the pads or bumps on the wafer chips to form an electrical test connection.  

- **Materials**: Common materials include tungsten (W), molybdenum-rhenium alloy (ReW), beryllium copper (BeCu), Paliney7, PdCu, etc. The selection is based on performance such as conductivity, elasticity, lifespan, and contact reliability.  

- **Types**: There are Cantilever, Vertical, and MEMS (Micro-Electro-Mechanical Systems) types (micro-spring, micro-bump arrays, etc.).  

  - **Cantilever Probe Card**  

  This type of probe is micromachined to resemble small cantilever arms or blades, arranged along the edge of the chuck. It is suitable for ICs or RF testing with larger pads. Its advantages include a simple structure and good anti-interference ability, but it has lower pin count and density.  

  - **Vertical Probe Card**  

  Probes are inserted vertically into the through-hole structure of the PCB. They typically support high pin counts and high-frequency testing; their structure often includes multi-layer components such as interposers, guide plates, and stiffeners, making them suitable for fine-pitch and high-parallel testing requirements.  

  - **MEMS Probe Card**  

  Micro-probe arrays are precisely fabricated using micro-electro-mechanical processing technology. They are suitable for chips with micro-pitch and micro-bump packaging, featuring extremely high pin count accuracy and repeated positioning performance. This type can currently be used to test entire 12-inch wafers, with full-wafer testing completed in one touchdown.  

3. Functional Components and Structural Elements  

- **Stiffener**: Enhances structural rigidity to reduce PCB bending during testing.  

- **Spatial Transformation Substrates (STF substrates) or interposers**: Bridge PCB signal lines to the probe array, balancing density and wiring complexity.  

- **Guide Plates**: Used for probe positioning and force control, commonly found in vertical structures.  

- **Functional electronic components**: Such as relays, switches, capacitors, and resistors in ATRE, used to realize multi-DUT parallel test resource sharing.  

4. Mechanical Structure Details (Taking Vertical Type as an Example)  

The structure is roughly distributed from top to bottom as follows:  

1. Stiffener (upper rigid support plate): Enhances overall structural strength.  

2. Main PCB: Signal and power transmission layer.  

3. Interposer / Spacer: Intermediate layer for signal spatial transformation.  

4. Upper / lower guide plates: Stabilize the probe array position and control needle stroke.  

5. Probe tips / Cobra or micro-spring: The part of the probe that directly contacts the wafer pads.  

III. Functions and Operation Process of Probe Cards  

1. Electrical Connection and Signal Transmission  

The Probe Card transmits test voltages, currents, and logic signals from the tester to the probe tips through signal paths composed of PCB circuits, capacitors, relays, etc., and then into the corresponding pads of the wafer. The chip's response signals are then returned to the tester through the same path for analysis.  

Tests performed during the process may include:  

- DC testing: Short / open circuit detection, current-voltage measurement;  

- AC testing: Timing, waveform, frequency characteristics;  

- Functional testing: Logic function correctness, parameter boundary judgment, etc.  

2. Multi-DUT Parallel Testing to Improve Efficiency  

To improve test throughput, advanced probe cards often use ATRE (Advanced Tester Resource Enhancement) technology. Through relay switches on the PCB, multiple DUTs can share resources in parallel during a single touchdown, improving test efficiency while saving tester channel resources.  

3. Alignment and Physical Contact Process  

- Loading phase: The Probe Card is installed on the prober, and the wafer is fixed on the chuck.  

- Alignment phase: The visual system identifies the probe tips and reference marks on the wafer, then precisely moves the wafer for alignment.  

- Touch-down: The wafer is slowly moved upward until it touches the probe tips, and then pushed further to break through the oxide layer and ensure all probes make contact to complete the test.  

- Testing phase: Contact is maintained, the tester sends test vectors, and receives the chip's response. In-line cleaning may also be performed to ensure stable probe contact resistance.  

4. Probe Management and Lifespan Maintenance  

As the number of touches increases, probe tips may accumulate dirt or pad material residues, affecting contact resistance and test accuracy. Common practices include:  

- Offline laser cleaning (NWR laser);  

- In-line cleaning structures.  

Probes with different stub types have different lifespans; for example, cantilever probes have a limited lifespan, while MEMS probes are more durable with stable repeatability.  

IV. Type Comparison and Application Scenarios  

Different chip types or test requirements require corresponding probe card types. For example:  

- Cantilever Probe Cards are suitable for ICs or RF testing with larger pads;  

- Vertical or MEMS Probe Cards are used for high-pin-count logic ICs or multi-DUT testing requirements.  

V. How Does a Probe Card Connect to a Wafer Prober and Tester?  

In the entire probing and testing system, the Probe Card is a key intermediate link. The specific connection process is as follows:  

1. The Probe Card is mounted on the test head of the wafer prober;  

2. The prober controls vertical movement to bring the wafer pads into contact with the probe tips;  

3. The PCB part of the Probe Card is connected to the test head, completing the electrical connection with the ATE's signal path;  

4. The tester sends test vectors, which are transmitted to the chip through the probe card;  

5. The chip's response signals are returned to the tester through the same path;  

6. The wafer is sequentially indexed to the next die, and the test process is repeated.  

VI. Further Technical Trends and Challenges  

- **Fine-pitch**: As IC packaging density increases, pad spacing shrinks to tens of microns or smaller, requiring extremely high precision in probe spacing;  

- **High-frequency testing**: RF device frequencies are increasing, raising requirements for impedance control and electrical characteristics of probe cards;  

- **Parallel testing**: Multi-DUT testing to improve throughput requires consideration of force, resource allocation, and test synchronization;  

- **Lifespan management and maintenance**: Probe wear and contamination affect accuracy, requiring regular cleaning and calibration.  

VII. Summary  

A Probe Card is a key interface connecting a wafer tester and wafer dies. Its main role is to accurately send signals from the tester to the chip and receive return signals for functional and parameter testing. Structurally, it includes a PCB, probes, stiffeners, guide structures, and possible electronic functional components.  

Different types of Probe Cards (cantilever, vertical, MEMS) are suitable for different application scenarios (e.g., pad density, test type, and parallel testing capability) and have their own advantages and disadvantages. The performance and quality of a Probe Card directly affect test efficiency, chip yield, and overall manufacturing costs.  

Through reasonable structural design, selection of high-quality materials, and integration of resource sharing technologies and maintenance management solutions, the efficiency and accuracy of the testing process can be greatly ensured. Combined with the alignment of the wafer prober and the testing process of the tester, it forms a complete electrical test loop and is an indispensable core component in semiconductor manufacturing.