Moore’s Law continues to prove itself, as leading semiconductor manufacturers develop smaller devices and even smaller pads. Probing pads with dimensions of
100 μm x 100 μm or 80 μm x 80 μm is no real challenge with most conventional probe technologies. However, trying to probe pad sizes of 50 μm x 50 μm or less becomes a challenge and a source of frustration when reliable and repeatable contact is not made on all the pads. There are many factors for poor measurement results when probing small pads that can lead to probes not making good contact.
Here are seven solvable small pad probing challenges:
1. Large probe tip contact area. Depending upon the type of contact such as DC or RF, the probe tip of a mixed-signal probe may be relatively large compared to the area it needs to contact. It is common for a Tungsten needle probe to have a tip diameter of ~20 μm and for a RF contact to have a diameter of 40 μm. In order to probe small pads the tip contact area needs to be significantly smaller than the pad.
2. Poor tip planarity. When a multi-contact probe has poor tip planarity the amount of overtravel needed to make contact with all needles, including the last one to touchdown, results in excessive skate across the pads of the needles that contact before it. Poor planarity can result in the first contacting probe to skate off the pad before the last one has touched down. The better the planarity, the less unnecessary skate is encountered.
3. Overtravel-to-skate ratio. The aforementioned problem is amplified when the overtravel-to-skate ratio is too small. If a probe has a 1:1 skate ratio then each additional micron of overtravel needed to bring all tips into contact will result in an extra micron of skate across the pad. However, if a probe has a 4:1 skate ratio then that same micron of overtravel only results in 0.25 μm of skate.
4. Side-skating probes. When a conventional multi-contact probe has many contacts, the needles need to be routed in the shape of a fan in order to bring all the tips into the tight pitch arrangement of the device under test. This configuration means that the needles mounted in the center will skate forward in the direction of the probe, and the probes on the outer ends enter from an angle and will skate to the side as well as forward when brought into contact (see photo). This is known as ‘side skate.’ Side skate limits pad size and pitch.
5. Relative tip accuracy. Conventional probes will have mechanical errors in positioning of each tip relative to each other. These errors can be in the X, Y and Z axis (Z-axis error is essentially tip planarity error) and will lead to missing or skating off a pad, if too extreme. The better the accuracy of the tip position, the greater the chance of hitting small pads.
6. Yaw-axis error. Considering the multi-contact probe may have up to 25 contacts and pitches as wide as
250 μm, the overall length of the probe contact area can be as much as 6 mm. Unless the probe is aligned perfectly in the yaw-axis (probe rotation), then the probe tips at the outer ends of the probe will not be in-line with each other. This needs to be mechanically corrected by loosening the screws and manually aligning the yaw-axis by sight, before tightening the probe mount screws. This method is problematic and time-consuming, and if not done correctly the probes will not align with small pads.
7. High contact resistance on aluminum pads. Aluminum pads grow a layer of oxide when exposed to the oxygen in the air. This layer of Aluminum Oxide needs to be penetrated by the probe in order to make a low contact resistance measurement. In order to break through this oxide layer, probes require additional skate to scrub down to the Aluminum metal below the oxide. Again, the more scrub the greater the chance of skating off the pad and losing contact altogether.
We’ve recently launched our new InfinityQuad™ multi-contact probe that successfully addresses these small pad probing challenges. Pads as small as 30 μm x 50 μm become a reality in automated over-temperature probing applications. This allows the user to reduce pad sizes, saves device real estate space and lowers pad parasitics – both saving money and improving measurement accuracy.
Have you faced these same small pad probing challenges? We’d love to hear about your experience.