To all that have inquired about 30 m gold thickness:

This question would be better answered with a required application.  
Let me try to cover most of this subject and apply various 
applications.

Many years ago (too many to mention), I performed an experiment to 
find out how many insertions I could get from a gold plated board 
into a connector.  Carefully selecting the same thickness boards (.
062") I found the approximate following results, always with 100 M 
base nickel.  100m gold = 500 insertions;  50 m gold = 300 insertions;
 30 m gold 50 to 75 insertions;  20 m gold 10 insertions.  As you can 
see, the insertion count reduces rapidly as the gold thickness 
reduces.  

Now let's look at cost!  Though gold is expensive, the labor cost to 
prepare the board for plating is much greater.  For instance, let's 
look at the actual gold cost for these above thicknesses.  Remember, 
the cost of labor to prepare the board for plating is the same for 
all thicknesses.  Obviously, you don't need 100m of gold.  Therefore, 
we will start with 50m.  To reduce a typical set of 6" long fingers 
from 50m to 30 m will save you about 30 cents at today's gold prices. 
 To reduce it from 30m to 20m will only save about 10 cents.  On an 
expensive multilayer board, is this small savings worth potential 
quality problems.  By the way this gold always should be a hard gold 
(99.96%-99.97% pure) with a knoop hardness range of 130 to 200 
(optimum 170).

When using gold for a surface finish that will be soldered you would 
like to use a soft gold (99.99% pure) and a knoop hardness of 70 to 
130 and an optimum of 90.  The thickness here should should range 
between 5 and 15.  Closer to 5 than 15.  This process makes 
electroless (or immersion) gold more desirable than electrolytic.  
The nickel thick should be maintained to a minimum of 50m.

Wire bonding:  Always a minimum of 50m nickel.  For gold wire bonding 
the gold thickness wants to be a minimum of 50m and a purity of 99.
99% (no thalium).  Purity is very important for any type of wire 
bonding process.  For aluminum wedge wire bonding the same type of 
gold will be used.  However, for aluminum wedge wiring bonding the 
nickel becomes critical.  No contaminates and it wants to be plated a 
soft as possible.  Requires good control of Ph and plating chemicals 
in the plating bath.

When specifying gold thickness, you must ask how much you will save 
with a lessor thickness and who will benefit from the savings, you or 
your vendor.  After nowing the cost for preparing the board for gold, 
it is a very simple task to calculate gold thickness variations.  
Contact a gold supplier for the amount of gold, and cost, per square 
inch per micro inch.

Norm Einarson
Printed Circuit Technology 

 

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