Pin Reduction DUT boards
In part 1 of this post series, we learned that high pin count devices can be tested in sections but that only unpowered curve tracing is practical. In this second part, we will see how modifications to the DUT board design can cope with that limit and permit powered curve trace. The trade-off with this solution is that the design will be dedicated. If a universal solution is possible, it will be limited to similar packages in the same product family that have similar power pin locations.
Please note that these solutions and methods in this series may not always be a viable option for your particular situation and test requirements. Upgrading your curve tracing system to accommodate higher pin counts is still the best option to address high pin count devices. Contact us to discuss your curve tracing needs.
Bussing power pins to power planes and dropping NC pins
One way to fit a big device into a smaller tester is to find ways to reduce the number of pins that need to be contacted. Eliminating no connect or NC pins is a no-brainer but not many devices waste pins on NC anymore, so that opportunity will be very limited.
Fortunately, it is very common for high-pin count devices to have many redundant power pins that are shorted internally. Any pin with the exact same name as another is likely to have an internal path inside the device and are one in the same node electrically speaking. Such devices may have 50% or more pins dedicated to the power supply. This is done to improve power distribution across the silicon when supply currents are high. This leads to a strategy where either a limited number of select representative pins can be contacted or all power pins can be connected but shorted together on the DUT board to a power plane. A bus refers to a common electrical connection often a trace on the DUT board connecting multiple pins together in this context. The common connection can be in turn connected to the curve tracer switch matrix or external banana jacks using a manual test method.
Extended octopogo board with large BGA socket for MegaTrace
Consider an example of a 1,444 pin 38×38 ball device and the user owns a 1,080 pin MegaTrace with an Octopogo fixture. Perhaps it has 375 VSS pins, 150 VDD core pins, and a variety of other power pins in smaller groups, and the rest are IO pins. By placing just the VSS on a power plane, the pin count is now 1,069 plus at least 1 pin for that ground group. Now it fits on the MegaTrace and all other pins are testable – meaning that powered, and most unpowered test conditions are possible. The only blind spot is the potential for some VSS pins to be open since they are grouped. But as long as some are working, it will appear that all are connected.