As more functionality is included in integrated circuits, there is an increasing need to make extremely accurate voltage measurements or force extremely accurate voltages to perform tests such as RDSON and VDO. The only way that these tests can be performed accurately and repeatably in a high-volume test environment, over hundreds of thousands of insertions, is to contact the device through a Kelvin connection.
The challenge of wafer-scale test is performing final test, with its high currents, accurate measurement requirements and full-speed functional, at the wafer level. This requires a high-performance contact method that fits into the wafer-probe environment. If Kelvin contact is required, the challenge is dramatically increased.
After outlining the issues related to Kelvin contacting for wafer-scale test, this paper presents a solution to these obstacles.
The best high-volume Kelvin contacting solutions use offset probes. The offset probes provide the capability of landing on small targets, including solder balls, while having board-side spacing that allows board layout to coarser-pitch pads. This simplifies the performance board layout to keep its cost down.
Actual examples of Kelvin contactors for wafer-scale test will be shown. These contactors use a Kelvin (offset) probe that has been used in hundreds of designs to test millions of devices worldwide.
Historically, the Multitest contactors that include these probes were pitch-limited and could only make contact to full arrays of solder balls at 0.65 mm pitch, and partial arrays at 0.5 and 0.4 mm pitch. Because most wafer-scale devices are 0.4 mm, and moving to even finer pitch, this solution will not work for all.
A smaller version of the offset (Kelvin) probe is successfully employed in beta site evaluations. This probe is small enough that two probes can make contact to a full array of solder balls at 0.4 mm pitch. These probes are also capable of making Kelvin contact to a subset of solder balls at finer pitches. And they are usable with a floating alignment plate so the same probe can be used for singulated devices. The presentation will include data from the beta sites.
To summarize, the paper shows that Kelvin solutions are not limited to coarse-pitch, singulated, or in-line devices, such as QFPs, QFNs, and SOs. Probes and contactors that are proven production-worthy can be used for Kelvin contact to BGA devices, both singulated and at wafer-scale test. And the latest design extends the capability to finer pitches.