MadSci Network: Computer Science

Re: how pure is the gold used for the small wires inside microchips.

Date: Fri Jul 7 09:17:46 2000
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Computer Science
ID: 961022742.Cs

Hello John,

An interesting question, with a convoluted answer!

As I'm sure you know, gold is extracted from ore using the 'amalgamation 
process' or 'cyanide method'; both quite dangerous. Pure gold is an 
excellent conductor and is not subject to corrosion (Aqua Regia being the 
exception), but has some downsides to it - the main ones being its 
extreme softness and relatively high melting point. For many years, 
aluminum was used in the production of microchips; now it is (slowly) 
being replaced by copper because of its better electrical conductivity; 
necessary in today's higher speed processors. Gold would be a better choice 
(better conductor), but its higher cost and lack of physical strength 
make it less desirable than copper. The processor (CPU) goes thru 
some extreme temperature changes at the metal/silicon junctions, and the 
thermal expansions/contractions would lead to early failure. Besides, being 
a sealed unit, the corrosion resistance of gold is not required. Gold is, 
however, used heavily in the production of LED's. In a LED, the anode wire 
junction with the active region of the diode must be very small and gold, 
being extremely ductile, fills the bill very nicely. And, because a LED is 
encapsulated in epoxy resin and generates little heat, thermal expansion 
is not a major concern.  It is also used extensively on circuit board 
contacts, where good electrical contact is a necessity. Even so, the gold 
is usually alloyed with another metal such as copper or platinum, which 
gives it greater resistance to the abrasion that comes from repeated 
removal and insertions.  

So, to answer your question, while very pure gold is necessary to ensure 
high electrical conductivity, it is almost always alloyed with other 
metals with desireable physical properties; namely strength and abrasion 

At some time in the not too distant future, I'm sure gold, in its pure 
form (24K) will be needed to handle the ever increasing speed of 
processors. Even now, manufacturers of CPUs are switching to '.15 micron' 
technology, where the connection path between the internal transistors is 
reduced in order for them to operate at 1GHz and above speeds, since 
propagation delay through connections/('wires') can severely limit the 
speed at which a CPU can operate. I put wires in quotes because they are 
not actually wires, but heat/vacuum depositions of the conductive material 
on the substrate.

I hope this answers your question.

Your (not-so) Mad Scientist,

Karl Kolbus

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