Hi, We found that those particular nylon washers by themselves did not hold up well with (overly?) enthusiastic tightening of the screws.
We mount the iBob J20 and J21 Infiniband 4x connectors to the PCB using 1) a metal #0 washer, McMaster 92141A002, $1.70 for 100. This metal washer goes under the screw head and above the nylon washer. This is to spread out the load so the nylon washer doesn't get too deformed as the screw is tightened. This particular washer is selected so that it just fits the screw but has an OD well matched to the nylon washer. And isn't so big as to touch the nearby resistors. 2) a plastic M2 washer, McMaster 95610A110 $2.36 for 100. This plastic washer goes under the metal washer and above the PCB. This provides the electrical insulation for the etch. 3) M2-12mm screw, McMaster 92000A019 $4.14 for 100. We have selected these long screws so that we can readily see the screw from the top of the PCB. Matt Dexter On Thu, 15 May 2008, Andrew Siemion wrote: > Hello Casperites, > > It has come to the attention of the CASPER group via diligent investigation > by Jason Ray and Randy McCullough with NRAO that, under certain conditions, > the fastening screws securing the CX4 connecters to the IBOB printed circuit > board (PCB) can impinge on a bottom layer power trace causing a short to the > grounded CX4 connector housing on the affected traces. This problem will > manifest itself as data integrity and reliability issues in systems with > gateware designs utilizing the CX4 connectors for any purpose, including but > not limited to IBOB-IBOB, IBOBBEE2 and IBOB-Workstation communication using > either 10 Gigabit Ethernet or XAUI interface blocks. > > For further information on the problem and fix, please visit > http://casper.berkeley.edu/papers/Science_Safety_001.pdf > > > - Andrew > > >
J20_J21_install.pdf
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