Xcerra is continuously improving the spring probes it manufactures for Wafer-Level test. This presentation describes an improvement that extends the life of a WLCSP probe that is used in a harsh environment.
In 2015 Xcerra introduced spring probe technology capable of contacting WLCSP devices down to 0.3 mm pitch. Probeheads employing this technology shipped to multiple sites. The technology performs well, providing excellent yields and long life.
At one user site, the probe had intermittent mechanical issues. At random times (typically after 300 k – 500 k touchdowns), some probes would be damaged – bent or broken. Worse yet, bent probes damaged the probe housing on subsequent touchdowns. Investigation revealed that the application is not a true wafer-level test, but rather a film-frame application. Wafer-level test and film-frame test are very similar, with one important difference: the positional accuracy of the devices on film is not as good as those on a wafer. The damage was being caused by the probes landing on the edges of solder balls causing the probes to bend and/or break.
There is now way to improve the positional accuracy of the devices on film. The only reliable solution would be to use a single-site probehead and align to each device individually – very impractical for high volume test. A sturdier probe would help, but the probe manufacturing technique does not accommodate this – things cannot simply be made thicker or larger with this technology.
Xcerra's research and development team was working on an inexpensive method of manufacturing a 3D probe out of flat components to resolve a different issue (that was also related to making the probe more robust). It was quickly determined that this solution could be applied to the 0.3 mm pitch probe, and samples were fabricated, tested, and sent to the field.
The technical benefits of these developments are improved electrical performance at these finer pitches. Because spring probes provide a shorter path length through the probehead than vertical probes, they typically have a higher bandwidth, lower inductance, and higher current carrying capacity. This new probe provides these performance benefits with a more-mechanically-robust probe. The presentation will describe the improvements, and the test results, in detail.