[SI-LIST] : Backside metal

Tom Zimmerman (zimmerman@fnal.gov)
Fri, 17 Sep 1999 17:01:28 -0500

I just wanted to make a couple of comments about a thread from about a week
ago, namely backside metal on a die.

In my experience, without backside metal, a good low resistance connection
is not made to the backside with the conductive adhesive. Just glueing the
die down as is results in several Kohms or more between die pad and chip
substrate. On one large die that I worked on, we actually used the
substrate (very low resistivity substrate in a radiation hard process) to
conduct the (substantial) analog ground current. These chips were
background and plated, and glued down with silver epoxy. I was amazed at
the very low resistance connection we obtained (a few milliohms between die
pad and back side). By using this technique we obtained lower resistance
and inductance to system ground than by using bondwires.

But I think an important question is whether the backside really needs to
be grounded. If topside substrate connections are made properly, there may
well be no need to ground the backside. In our case, we required a really
good backside connection, and to achieve that backside grinding and metal
application was necessary.

Tom Zimmerman
Fermilab

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