High Temperature Flash Memories

A challenge in high temperature electronics is the availability of flash memories. To address this, Tekmos is offering up-screened flash memories that are rated at 175C and 200C. More specifically, the first part is the TK29H400AT-70, which is a 4MB, 3 volt, top-boot sector, 70 ns NOR flash memory, This article will discuss the issues in up-screening flash memories to work at elevated temperatures.

To up-screen a part, it is necessary to test each parameter to guarantee compliance with the specification. And in the case of high temperature operation, it is also necessary to address reliability issues and technical issues.

Functionality

The first question to be addressed is if the part is functional at high temperature. Flash memories have a fair amount of analog circuitry which is used in the sense amplifiers, the programming charge pump, voltage references, and the brownout detect. This circuitry was typically designed for functionality over a -55C to 125C range. But will it still work at 175C or 200C? The only way to find out is to run a part at those temperatures.

We took some of the 4M flash die we use in our microprocessor parts, and assembled them in ceramic packages. We then wrote a test program for the flash, and verified correct operation at room temperature. Next, we used our temperature forcing probe, and characterized the part at increasingly higher temperatures going up to 200C.

To the left is a picture of the test hardware we used to characterize the flash memories. The area was constrained by the physical size of the temperature forcing probe, which is shown by the circle. And we opted to solder the parts to the board, which temporarily bypasses the problem of high temperature sockets.

Reliability

The main reliability concern in a flash memory is data retention. How long will a flash hold its memory? As a very rough rule of thumb, the data retention time halves for every 10C rise in temperature. The 10 year guarantee at 85C starts to look more like 1 week at 175C, and a few days at 200C. Fortunately, this can be addressed by refreshing the flash. There are strong limits on the number of times a flash can be erased, but the same data can be re-written many times without degrading the parts performance.

JEDEC specification JESD22-A117E covers the testing of flash endurance and data retention. We are running these tests now to verify the upscreened part reliability.

Testing

When up-screening, each part has to be tested at temperature. Unfortunately, there are no commercially available test sockets that will work in the 200C range. As a result, the test socket has to be custom made out of high temperature materials. The socket difficulties were one factor in our choosing a 56 pin package for the high temperature flash parts. That way, we can reuse the same hardware for a number of different sized flash memories.

Packaging

The plastic used in packages decomposes at elevated temperatures, so the only practical solution is to use ceramic packaging. There are also reliability issues with gold bonding on aluminum pads, so the bonding has to be done either with aluminum wires, or the pads have to be replaced with a nickel-palladium-gold coating. We are choosing to go with aluminum bonding.

Future Work

While we are currently working with 4MB flash memories, we plan to extend this work up into 64 MB flash after we finish with the 4MB parts.

Availability

The TK29H400A is priced at $225 in 100 piece volumes, and is available now in limited quantities.