General Function

1.1 The Integrated Circuit

Semiconductor Integrated Circuits (ICs) are essential in today’s high-tech society. They can be found at the heart of a variety of products, from the simplest calculators to the fastest computers. As a result, the production of ICs has become a billion dollar industry, involving some of the world’s most advanced technology. Probe cards are important in the final phase of this production process, playing a vital role in the testing and measuring of integrated circuits.

Integrated circuits are built from round, thin sheets of semiconducting material. Standard sheets, or wafers, are commonly made of silicon. These wafers can range from 5 cm (~2 in) to 20 cm (~8 in) in diameter and are roughly 0.10 cm (~0.04 inch) thick. On a single wafer, anywhere from 50 to 200 identical integrated circuits, or die, can be made. The process of taking a simple silicon wafer and creating from it circuitry which can use and store electricity is a complex process. In a sense, the circuitry is “embedded” in the silicon, just below its surface. Within this microscopic maze of circuitry, electrical signals flow from one point to the next, much in the same way that water flows in a riverbed. To interact with the world outside of the IC, these signals are passed back and forth through small metal pads attached to the wafer’s surface (see Figure 1-1). The ability to make electrical contact with these metal pads is critical. Without some method of making this contact, the integrated circuit can not be used.

1.2 Testing the IC

In the testing of integrated circuits, probe cards play this vital role of contacting the metal pads on a wafer’s surface. ICs are tested by large machines, called testers, which send a series of electrical signals to each IC. During testing, the probe card and IC are held in place by another machine, called a prober. The prober might be described as the "arm" of a tester, doing the mechanical work of moving and aligning the probe card and IC. The probe card then functions primarily as the "hand" of a tester, allowing it to "touch" the metal pads on a wafer’s surface (see Figure 1-2). This establishes an electrical connection between tester and IC, allowing signals to flow freely between them. An ICs response to these test signals then indicates whether it has been made correctly. Good ICs can then be separated from bad ones. Probe cards are at the center of this testing process.

With the help of the prober, the probe card is lowered onto the IC wafer until the probe tips come into contact with the wafer’s metal pads. Test signals can then be passed between tester and IC.

The movement of the probes as they touch the surface of the metal pads is also important. Generally, the surface of each metal pad is covered by a very thin layer of glass-like material, called an oxide. In order to make contact with the metal underneath, the probe tips must break through this thin oxide.
Because the probes are very thin, roughly 0.250 mm (~0.010 in) in diameter, they flex when touching the wafer. As they flex, the probe tips slide, or scrub, across the surface of the pads (See Figure 1-4). This scrubbing action causes the probe tips to break through the surface oxide, helping make good electrical contact with the metal underneath.

Although its basic function is relatively simple, building a probe card requires precise and careful work. Probe cards are an assembly of a variety of parts, some fragile and some sturdy. In the following chapters, you will be introduced to the primary components which make up a standard probe card. Because no two probe cards are exactly the same, each component is described in its most common form.

Probe

2.1 Function

Earlier in section 1.0, a probe card was described as the “hand” of the tester. If we think of a probe card in this way, the probes then function as its "fingers." As was shown in Figure 1-4, the probe is the part of a probe card that actually makes contact with the integrated circuit.

2.2 Straight Probes

Generally, if probes are purchased from a vendor, they arrive just as shown above in Figure 2-1. They are straight, thin, needle-like pieces of metal, with one end that tapers down into a sharp point. Probes are most commonly made of Tungsten (W) and Rhenium Tungsten (ReW), although materials such as Beryllium Copper (BeCu) and Palladium (Pd) are also used.  A straight probe is defined by four basic parameters: material, probe length (L), probe diameter (D), and taper length (T). Common material and dimensions are listed below in Table 2-1.

Table 2-1: Straight Probe parameters

Figure 2-2: Bent Probe

Before a probe card is assembled, the tapered end, or tip, of these straight probes must be bent. Again, depending on a customer’s specifications, these tips can be bent at a variety of angles, although 103 degrees is standard. Throughout the assembly process, the shape of a probe remains just as it has been bent, with a sharp, pointed tip. After a probe card has been fully assembled, however, these probe tips are sanded down to create a desired tip diameter (d) and tip length (l). Including the parameters from Table 3-1, a bent probe is defined by seven parameters, which are listed below in Table 2-2.

Table 2-2: Bent Probe parameters

Ring

3.1 Function

Figure 3-1: Assembled Ring

3.2 Material

Although a ring’s basic function is purely mechanical, the type of material it is made from is very important. A ring must be

• sturdy, to give the probes a firm foundation,
• electrically non-conductive, to protect the test signals being passed through each probe,
• able to withstand high temperatures, sometimes as high as 200 deg C (~392 deg F).

To meet all of these requirements, ceramic is often used.

3.3 Shape

The size and shape of a ring depends on the IC device which will eventually be tested. It is most common for the pads to be arranged along the outer edge of an IC device. When probing a single die, this results in a ring that is most often square or rectangular, as shown in Figure 3-2.

Figure 3-2: Standard Rectangular Ring

Rings, too, come in all shapes and sizes. A few examples are shown below in Figure 3-3.

Figure 3-3: Ring Examples

3.4 Epoxy

In order to attach the probes to the ring, a special glue, or epoxy, is used (see Figure 3-1). Although the ring provides support for the probes, it is the epoxy which actually holds the probes in place. After curing, an epoxy generally has properties similar to those of the ring. It must be very sturdy, electrically non-conductive, and be able to withstand high temperatures. In addition, an epoxy must adhere itself well to both the ring and the probes, in order to hold the probes in position.
A variety of epoxies are used in the probe card industry. Some are designed for high temperature applications, while others are designed for low leakage applications. The epoxy which is used for any individual probe card depends primarily on the customer’s requirements.

Printed Circuit Board

4.1 Function

The largest of the components which make up a probe card is the board. It functions both as a support for the assembled ring, as well as the point where test signals are passed from the probes to the prober. Because the test signals pass through the board, it is also a convenient place to mount components, such as capacitors, resistors, and relays. These components can then help in manipulating and controlling the test signals as they are passed back and forth between tester and IC.

4.2 Material and Shape

A few different materials are used in making a board, the choice of which depends on a customer’s testing temperature and leakage requirements. As was true of both the ring and epoxy, the main board material must also be electrically non-conductive. A type of fiberglass, FR-4, is commonly used, as well as polyimide. To carry the test signals, thin strips of metal run along the top and bottom of the board.
Unlike the ring and probes, which are designed entirely to match the IC, the board is designed to match the prober. For this reason, boards come in many sizes. Although there are many sizes, they come in two basic shapes: round and rectangular. Round boards commonly range in diameter from 5.7 cm (~2.25 in) to 34 cm (~13.5 in), while rectangular boards are roughly 11.5 cm (~4.5 in) X 21.5 cm (~8.5 in).

Figure 4-1: Common Boards (bottom side view)

Figure 4-2: Round Board, Trace and T-Pin Location

4.3 Traces

The thin strips of metal located on the bottom-side (or ring-side) of a board are called traces (see Figure 4-1). It is here that the probes are soldered to the board (see Figure 4-2). These traces are very much the same on most boards, regardless of size or shape.

4.4 T-pin, Pogo Pad, and Edge Connector

As was mentioned in section 4-1, the board must also connect to the prober. While the probes are generally soldered to the traces near a board’s center, the prober connection is usually made near a board’s outer edge. The way this connection is made depends on the type of prober, but there are generally three methods which are used. They are as follows:

Table 4-1: The Board/Prober Connection

Key Parameters

Here is a general diagram of a completed probe card, labeled with a few of the more important parameters. See the following table for additional explanation.