Corrosion of the aluminum conductors within the device which can lead to
dendrite formation as opposed to purple plague can be driven by moisture
that is captured within the device and phosphorous that is used in the
glass passivation over the IC to make it spread more easily.  Together,
they form phosphoric acid which corrodes aluminum quite well.  Other
contaminants need not be present, but will accelerate the corrosion
process if present.  This was a big issue with "canned" devices because
all moisture had to be excluded from the can prior to hermetically
sealing it.  The problem was reduced with the plastic encapsulation
because the plastic is molded around the IC and bondwires at
temperatures higher than the boiling point of water.  So most vapor is
excluded during the molding process.

Historically, the amount of phosphorous in the glass has not been well
controlled and an excess amount in the glass will damage the component
within.  Early ICs carried their own destruction mechanism!  Perhaps
control has gotten better in recent years.  I have been out of touch for
a while with this aspect of failures.
mark

Mark Woolley
PTRL Laboratory
Avaya,  1300 West 120th Ave,   Area 33,  Westminster, CO, 80234

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Syed Ahmad
Sent: Thursday, November 06, 2008 2:41 PM
To: [log in to unmask]
Subject: Re: [TN] FW: [TN] AL CU DENDRTIES INSIDE PLASTIC IC

Purple plague at the Au/Al interface would be aided by moisture and
contaminants like halides, Sb, etc., sometimes part of the molding
compound
as flame retardants.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey
(EHCOE)
Sent: Thursday, November 06, 2008 7:44 AM
To: [log in to unmask]
Subject: [TN] FW: [TN] AL CU DENDRTIES INSIDE PLASTIC IC

 

 


Subject: Re: [TN] AL CU DENDRTIES INSIDE PLASTIC IC

 

Purple plague is an intermetallic alloy that is formed at the junction

of the Aluminum pad where the Gold bondwire is bonded.  But this is not

a dendrite, it is a "Barny"cle.

Dewey

 

 

  It is a multi-crystalline growth that appears as a dark or

purple ring around the ball of the gold bondwire.  Typically dendrites,

which will grow within a contaminated plastic IC, do not contain gold.

All it takes is the aluminum, moisture, and an electric field.

 

In the past purple plague has been generally caused by high temperature

whether or not the IC is powered.  It usually causes an open circuit

(eventually) due to Kirkendahl voiding at the interface.  With today's

very closely pitched bonding pads the purple plague growth may be able

to bridge between two adjacent bonding pads. 

 

But again, this is not a dendrite.  Dendrites need an electric field to

induce growth, purple plague occurs in the absence of an electric field.

It occurs at some rate continuously, but at normal operating

temperatures grows so slowly that it is not considered a primary factor

in calculating the lifetime of an IC. 

 

mark

 

Mark Woolley

PTRL Laboratory

Avaya,  1300 West 120th Ave,   Area 33,  Westminster, CO, 8023


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