Colin,
Once again, to coin the popular phrase,'it depends'.
First, what size wire are you speaking of? If you are speaking of wire
>75µm you likely will not have to worry about anything. Heavy Al wire to Ni
has been done for many years and is considered to be quite reliable. One
'trick' that has been used, is to be driving the ultrasonics at the time of
touch-down to help 'break up the oxides' on the Ni. The larger the wire,
the less of an impact this plays in the overall bond reliability - IMHO
Unfortunately, that is likely not your situation.
Now then, if you are speaking of plain ol' 32 µm Al wire, my answer gets
different. Having the bonding tool ultrasonically 'hot' at the time of
touchdown can definitely help [check with your equipment manufacturer].
I get a bit uneasy when the Ni gets down below the 3.5-4 µm range [but that
is my personal preference]. Would rather have it in the 4-5 µm range
myself. Not being sure which electroless Ni process you are dealing with
[Ni Phos, Ni Sulfamate, or Ni Boride], I definitely prefer the low phos over
a high phos, for bondability, but you may experience some soldering issues.
The happiest tradeoff there is a low/med phos bath, still in the 5-8% phos
range...
Whatever the Ni thickness, just drive into your plater that it MUST be
consistent, and as tightly controlled in thickness and contaminant
concentration as possible. Not knowing your PWB layout or anything, make
sure they definitely check thicknesses in your wire bond area! Check
elsewhere too, for comparison, but being a bonder, I know which areas I am
MOST interested in :-) Possibly be willing to give up thickness control in
non-wirebondable areas so that you can get the thickness control you want -
specifically, where you want it!
There are a multitude of games that you can play with the wire specs, etc.
Bottom line - nail down as many variables as you can - plating being one of
them.
Don't sweat the gold thickness too much. Keep it as thin as you can without
losing solderability, due to oxidized Ni. If you have solderability
problems, you will definitely [at least] have variability in bonding
characteristics. You are actually bonding THRU the Au to get to the Ni. The
gold is only there to passivate/protect the nickel from oxidation. Too much
gold there and you will get weak bonds over time.
Now then, Al wire on non-oxidized Cu [in a non-hermetic environment] is
pretty good as well! Really good for heavy Al.
In general, the finer the wire, the more control one requires in the bonding
variables. You can readily dial your process into a set of conditions -
just don't let those conditions be wandering all over the countryside!
My Saturday comments...
Steve Creswick - [vastly under-employed wire for hire]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of colin mcvean
Sent: Thursday, March 11, 2010 6:51 PM
To: [log in to unmask]
Subject: [TN] Aluminium wire bonding
We have an application which is asking for a ENIG finish with Ni spec of 2-4
um and Au of 0.05-0.1um.
What is the industry expectation of NI specification for Aluminium
wirebonding? I think 2-4 is a little on the low side, and am worried about
possible Nickel corrosion and poor bonding adhesion, especially if the
Nickel is towards the lower side of the 2-4um.
Colin McVean
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