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What is a Probe Card, and What Does It Do?

What is a Probe Card, and What Does It Do

The probe card is an integral part of the microchip production process. It acts as a jig, enabling manufacturers to test integrated circuits before slicing wafers to separate the ICs for packaging. This guide intends to introduce you to these PCB types. Learn about them and why they are crucial in the semiconductor industry.

What is a Probe Card?

It’s an advanced circuit board that links an automated machine to a semiconductor wafer for testing. It contains an array of contacts on one side called probes, which explains its name.

The probes make physical contact with the wafer during testing, providing the electrical pathways the tester can use to check and validate individual ICs.

A single assembly can contain a few to several hundred thousand probes. The distance between the pins is usually in the order of microns, small enough to contact the test points of integrated circuits.

Sizes vary, with the typical width ranging from 4.5″ to 8″. It depends on the die under test or DUT, in short. It can also be of different shapes, with circular and rectangular types being the most common.

What Does a Probe Card Do?

It provides the interface the ATE (Automated Test Equipment) needs to test microchip circuits. It facilitates the flow of signals, permitting the checking of larger-scale semiconductor circuits (LSCs) quickly and efficiently.

Because it tests ICs before their packaging, the card helps avoid unnecessary costs that faulty ICs would cause if packaged. It also helps provide critical information about the circuits so the manufacturer can take corrective actions early.

Probe card components
Probe card components
Resource: https://www.researchgate.net

The Anatomy of Probe Cards

Although designs may vary, the main components are generally similar across the different types of this testing tool. They include a printed circuit board, the substrate, needles, or the pins that physically contact the tested device.

PCB

The PCB is the most prominent component, usually made from a particular type of FR4 or ceramic. Apart from providing structural support, it contains electrical pathways called traces.

The board also holds various electronic parts like resistors and capacitors. These control the passing through the board to the wafer and back.

On the edges, a series of contacts connect the PCB to the prober, which is the part that holds the card assembly and connects it to the testing machine.

Substrate

The substrate is a block of material between the pins and the printed circuit board. It holds the pins, supporting them structurally. Their material must be tough, nonconductive, and heat-resistant. A good example is ceramic.

Probes/Needles

These are tiny contacts that touch the wafer during testing. Their function is to connect the test equipment to the wafer electrically. The needles must be rigid to withstand the contact pressures and electrically conductive to pass signals to the circuits.


Different materials are available for use. They include tough ones like tungsten, palladium alloy, beryllium copper, or other conductive materials.

Using a probing card to conduct a wafer test
Using a probing card to conduct a wafer test
Resource: https://www.researchgate.net

How Does a Probe Card Work?

The probe card assembly has needles on one side. The needles match the pattern of the devices on the wafer. It also has terminals on the other side, matching the testing equipment contacts. Here is how it works:

  • During testing, the card is held in a prober, a holder that connects it to the ATE machine
  • The holder uses a high-precision vision technology consisting of cameras to position each device on the wafer under the needles
  • Once the card is in position and has made electrical contact with the wafer circuit, the testing process can begin
  • The tester sends signals to activate the devices on the wafers. It then measures the signals and uses the results to single out defective ICs
  • Faulty ICs are removed and separated from the good ones

Probe Card Cleaning

After repeated use, say 50,000 test cycles, the tips of the pins may be covered by debris or a layer of metal oxide. Tip contamination can negatively impact testing, causing resistance to current and leading to inaccurate results.

Probe tip cleaning typically involves using lasers to burn off any material covering the tips. Offline, abrasive materials help clean the tips and card. These are usually in the form of thin sheets.

The movement of the probes over the wafer surface also helps remove the oxides. For instance, when using a cantilever card, the sharp tips of needles slide slightly, scrubbing the oxides to leave a clean, conductive surface.

In addition to active cleaning, technological advancements have led to the discovery of materials that can help prevent material buildup. When used to make the probes, these materials reduce the cleaning frequency from about 50,000 to 300,000 test cycles or more.

Vertical probe card
Vertical probe card
Resource: https://www.researchgate.net

Probe Card Types

Probe card manufacturers mainly group them by their probe structure and arraignment. Using this criterion, we have the following types today: vertical, cantilever, and MEMS. Each type has application-specific benefits that range from accuracy to cost.

Vertical

It has vertically arranged pins or bumps. The design reduces contact resistance during testing, improving its capabilities and supporting higher volume testing. The vertical contact minimizes damage to the DUT.

The cards are also easy to maintain; individual pins are replaceable when damaged, which means a more durable testing tool. On the downside, they are more expensive to design and fabricate than the cantilever types.

Cantilever

The pins are structured like a cantilever beam and soldered to a printed circuit board in this card type. During testing, they contact the die under test (DUT) horizontally instead of vertically. Cantilever cards are more straightforward to make and, therefore, a cost-effective solution for many applications.

However, they are challenging to align and not best suited for high-density circuits. They also need more care, such as periodic pin height adjustments and other actions. The needles are also more likely to dent the pads of a semiconductor wafer during use.

MEMS

MEMS or Micro-electromechanical System is a card manufactured using very tiny structures (at the microscopic level). It’s the most advanced type and design to cater to the ever-evolving microchip technology, with miniature circuits dominating the electronics industry.

Using MEMS technology, it’s now possible to test a 10-inch semiconductor wafer at a go. The pins are closely ranged, increasing accuracy and supporting high-pin count applications. With MEMS probing cards, manufacturers can meet the testing needs of both 2.5D and 3D packages.

Conclusion

A probe card plays a vital role when manufacturing semiconductor devices. It’s one of the critical components of the testing equipment, enabling testing machines to connect to the IC circuit of a wafer. Different card types are available, each offering a certain level of complexity and application-specific benefits.

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