Damage to Wire Bonds
To date (Dec. 20, 2001), we have exposed four PreFPIX2Tb chips to the proton beam at IUCF. One of these boards has been irradiated twice, for a total dose of 43 MRad. Three others have been irradiated once, to ~26 MRad. These three boards were fabricated after the original printed circuit board, and the chips were mounted & components soldered at a different time from the original board. The first board is still in good condition. All three of the other boards have developed some kind of growth on the wire bonds, which has either eaten through many of the wire bonds, or lifted them off of the pcb. Click Here to see results which make me believe that the wires were eaten through, probably by chlorine from imperfectly removed solder flux.
Here are pictures of one of the three damaged boards:
The junk, whatever it is, can be seen on the wire bonds near the bottom of this picture. On this board, the junk looks white. On one of the other two boards, it is mostly clear.
This picture shows a number of wire bonds which are no longer touching the printed circuit board.
This close-up shows that most of the growth on this board is in the form of white filaments, reminiscent of coral.
Here is another close-up, showing clear blobs as well as "white coral."
This picture shows two of the bond wires which are no longer bonded to the pcb.
This is a close-up of the printed circuit board itself, near the wire bonds. Smooth, pearl-like blobs can be seen, as well as hairy structures. Some of the blobs are greenish. It looks like the green color probably comes from the nearby green solder mask.
Last updated on 12/21/01
by David Christian