Blade probe cards can offer a cost-effective solution for less complex wafer test applications. Thanks to their inherent design, blade probe cards offer good visual and physical access to the device under test (DUT). This makes them ideal for tests which require the use of optics to view the device during probing or where a diagnostic probe is needed in addition to the probe card.
Many options are available to tailor the blade to meet the requirements of the test area. Most blade shapes are available in several sizes, and all can be combined with a selection of needle probes to provide custom needle holder assemblies (NHAs) to suit your application.
PROBE CHARACTERISTICS
When choosing needles for probe cards, several conditions may be considered:
- Material
- Gram force
- Tip diameter
- Tip length
- Tip shape
- Probe extension
The following are the probe specifications used in most probing applications:
These specifications have been set up for standard probing situations. However, there are many available options which may affect probe yields. In an attempt to offer some advice, this section reviews some of these options.
PROBE MATERIAL
Probe needles are available in tungsten (WNP), tungsten rhenium (WRNP), beryllium copper (BC), and palladium alloy (Pd), also known as Paliney®.
Tungsten is the most common material used for integrated circuits with aluminum pads. This material provides good wear characteristics in small geometries. Contact resistance for a typical clean tungsten probe averages 250 milliohms, which is suitable for most applications.
It should be noted that buildup of aluminum oxides on the probe tip can become a factor in increasing contact resistance between the probe and test pad.
Tungsten rhenium is often used where high contact resistance becomes a problem. The addition of rhenium to the tungsten probe material helps to prevent oxide buildup and to stabilize the contact resistance levels.
Beryllium copper is not normally used for probing aluminum pads. Beryllium copper probes are much less durable, and a probe card with BC probes with small tip diameters will have a much shorter life. In addition, aluminum oxide causes excess wear. However, if there is a problem of arcing due to high current flows, it is sometimes worthwhile to substitute this material for the sensitive probes. Power probes or ground probes are likely candidates for this material, as the probe tip diameter can often be increased which helps to increase the life of the probe.
The material of choice for probing gold pads or large geometry hybrid circuits is beryllium copper. There can, in some cases, be a transmigration of material from a tungsten probe to a gold pad. Because gold is a much less abrasive material than aluminum oxide, the excess wear problem associated with the softer beryllium copper is not as great a factor for these applications.
Palladium alloy is sometimes used when high contact resistance becomes a problem, but has not found high acceptance in the market due to its high cost, quick wear and difficulty in controlling tip shape and diameter.
The probe material should be chosen after consideration of the above factors.
GRAM FORCE
The industry standard of 1.5 or 2 grams per mil pressure is recommended for most applications. This will apply enough contact pressure to provide good electrical contact on the device pads. A lighter gram force may be selected to minimize pad damage for sensitive circuits. A higher gram force may be selected to minimize contact resistance, help break through oxides or to carry higher electrical currents.
Beryllium copper or palladium alloy probes generally use 1 gram pressure for IC (integrated circuits) probing. Because of the lower contact resistance of these materials, this force is usually sufficient to provide good electrical contact, and it helps to alleviate the wear factor for these materials when using small diameter probe tips.
Gram force is determined by the material, the shaft diameter and the taper length of the probe selected, in combination with the length of the needle extension from the blade. Each of the blade styles selected has a standard probe extension. * The shaft diameter and taper of the probe needle will be selected by Wentworth after reviewing the probe material and the standard extension of the probe as determined from the blade style selected.
* It is possible to custom tailor the probe extension if the user wishes. Contact us if special probe extensions are required.
TIP DIAMETER
As a general rule, the probe tip for IC devices should be 50% of the size of the pad to be probed. For example, any 0.004″ device pad would use a probe tip diameter of 0.002″. Normal tolerance ranges for a probe tip are ± 0.0002″.
For probing “bumped” IC wafers, the tip diameter should be increased to 0.004”. Smaller tip diameters may actually imbed into the bump and lift it off of the wafer. The 0.004″ probe will sit on top of the bump and minimize damage. For high current IC pads, a larger tip diameter is recommended. In this case, the tip diameter might be 1 mil smaller than the pad. The tolerance recommended would be -0.0003, +0.
For probing hybrid circuit devices, a tip diameter of 0.006″ is usually sufficient, even if the pad geometry is larger.
TIP LENGTH
IC probing
Standard tip length in IC probing is 0.007″ ± 0.002″. Any increase in bend length creates an increase in the “scrub”, or forward motion of the probe on the device pad. This can be an advantage in breaking through oxides. Tip lengths of 12 to 14 mils may be used to increase this oxide penetration. However, this increase in tip length will increase the scrub length and may increase pad damage. If the tip length is increased substantially, it is even possible for the probe to scrub completely off the device pad. The 0.007″ length provides sufficient scrub to break through most oxides.
Hybrid circuit probing
Standard tip length is 20 mils. Any increase in bend length creates an increase in the “scrub”, or forward motion of the probe on the device pad. If your pad size is smaller than 10 mils, then the tip length can be decreased to minimize scrub. If the probe must fit into a packaged device, the tip length should be increased so that it clears the top of the package by at least 30 mils.
TIP SHAPE
Standard blade cards are largely made with radius probe tips. Customer preference has generally been the deciding factor in making the decision for flat or radius tip shape. This decision is normally made by the process test engineering department based on their experience with the effect on wafer yield. As a general guideline, radius tips do less pad damage and pick up less oxides. Flat tips will break through oxides more easily, but will pick up oxides quicker than radius probes and will require more frequent cleaning.
PLATING
It is possible to purchase probes with gold or rhodium plating, although generally, test probes are rarely plated. Where the pads to be probed are gold plated, there is a small advantage in plating the probe. However, in most cases, the plating on the probe tip is quickly worn away by the harder aluminum oxides. Gold plated probes are typically used to minimize contact resistance between edge sensor probes.
EDGE SENSOR PROBES
Edge sensor (ES) probes were originally designed to determine the edge of the wafer in IC probing. With the advent of programmable probe stations, the usage of ES probes has changed. They are more often used as an open/close switch to send a message to the probe station that the probes have made contact to the device. They can be used to help control the amount of overdrive placed on the probes. The edge sensor switches are opened during the overdrive position of the wafer, and closed when the probes are not contacting the wafer. They are manually adjusted during the planarization process to open and close in tandem with the probe contact. They most often are set to contact the device 0.0005″ before the first probe, and open 0.0005″ after the last probe. The probe station is programmed to recognize the open switch, and to count the overdrive from that point.
Most edge sensors are gold plated to provide good electrical contact of the switch. Edge sense probes come in three types: standard, 2-wire isolated, and 3-wire isolated.
Isolated edge sensors are used when a non conductive section of the wafer is not available for probe placement. The 3-wire isolated edge sensor is more durable than the 2-wire. The epoxy/glass bead on the 2-wire edge sensor is subject to breakage, and generally exceeds 10 mil diameter, with the potential for damaging the circuit. It is used only when spacing constraints prohibit the use of the 3-wire edge sensor.
Standard edge sensors
Standard edge sensors consist of a contact probe, and a switch probe. Current is applied to the contact probe, and flows through to the switch probe. The contact probe is generally positioned on a non conductive section of the wafer. The switch probe is placed under the contact probe, just making contact.
When the device is overdriven, the contact probe raises to separate from the switch probe and opens the circuit.
Isolated edge sensors, 2-wire
These consist of a contact probe, a switch probe, and an isolation bead. Current is applied to the contact probe, and flows through to the switch probe. The contact probe is generally positioned on a conductive section of the wafer. The switch probe is placed under the contact probe, just making contact. When the device is overdriven, the contact probe raises to separate from the switch probe and open the circuit.
These edge sensor probes are gold plated. They contact the device .0005″ before the first probe and break .0005″ after the last probe.
Isolated edge sensors, 3-wire
The isolated 3-wire sensors consist of a contact probe with an isolation bead, a switch probe and a straight probe. Current is applied to the straight probe, and flows through to the switch probe. The contact probe is isolated from the switch and straight probe. It is generally positioned on a conductive section of the wafer. The switch probe is placed over the isolation bead on the contact probe, just making contact. The straight probe is placed under the switch probe, just making contact. When the device is overdriven, the contact probe raises the switch probe to separate the switch probe from the straight probe and open the circuit.
The straight probe and the switch probe of 3-wire edge sensors are gold plated. The contact probe is not gold plated.
