Brain,
I agree with you. You have defined a good plan for Steve to define the cause.
I have found many times that the nickel will be driven to the surface as a result of prior thermal processing of the PWB, and subsequent oxidation of the nickel. As the total time interval between high temperature exposures (cummulative effects) and wire bonding increases, the difficulty in bonding increases. Basically, we verified this once by Auger analysis. Pores in the gold plating exist and nickel will come to the surface.
Resolutions were to increase the plated density of the gold (it was quite porous). They did this by changing the rate at which the (electroplating) was performed - maybe the plating gurus can chime in here - and reducing the Work In Process time, storing in Nitrogen at all possible opportunities, and more agressive plasma cleaning prior to wirebonding.
In the hybrid field, we routinely plasma clean with Argon, or Argon/Nitrogen, or Argon Hydrogen, or oxygen (if we suspect organics).
The argon/hydrogen would be settled on for this type of incoming (to wirebonding) problem. The heavy Argon atom mechanically smacks the surface and dislodges whatever is there (atomic layer/s only - this is not a very agressive plasma cleaning). In theory, the debris is sucked out the vacuum port. Also in theory, the hydrogen, reduces [converts the metal oxide back to the base metal] the nickel oxides to nickel - with the argon freshening that up a bit more. While bonding to a gold/nickel surface is not as good as bonding to pure gold, it certainly is far better than bonding to a hard nickel oxide/gold surface.
Lastly, if one were not concerned about gold embrittlement of solder joints, or cost, one could simply plate more gold on the surface. This results in a delaying factor. Usually, not a viable approach if there were SMT components involved.
My concluding comments are to minimize the thermal excursions and get the parts into wire bonding as soon as possible.
Steve Creswick - Gentex Corporation, Zeeland - MI
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Friday, December 12, 2003 5:26 AM
To: [log in to unmask]
Subject: Re: [TN] Cleaning of Wire Bondable Gold
The first thing to try is to bond to an unassembled, as received, board.
a) If that works, then you know that the problem lies with the
subsequent process. b) If not, the boards are unsuitable for bonding or
have been inappropriately stored.
If a) then one or both of the following may help:
a1) 35 - 40 µ" Au is next to bugger-all and is certainly porous. The Ni
underneath is probably oxidising and, as the oxide occupies more volume
than the metal, it is exuding to the surface. This can be proven by
trying to immerse a piece in 10% hydrochloric acid solution for 10
minutes at room temperature. Then rinse copiously in very pure DI water
(>5 Mohm-cm) and dry with a cold nitrogen blast. If that sample bonds
OK, you have your answer. This treatment may discolour the solder
joints, but will cause no harm to them (for your first trial, you can
try it on an unmounted board.
a2) your subsequent processes (pressing, soldering etc.) have
contaminated the surface, which is quite possible. The most likely
candidates are sublimated flux, contact materials during pressing or
residues from the cleaning (if any) process after soldering. It is not
possible to categorically recommend any cleaning process without knowing
a lot more about all your processes. However, I suggest you narrow it
down by trying to bond after each step of each process, in order, until
the bonding fails. The previous process then becomes the responsible one
and the cleaning process should become evident.
If b) then one or both of the following may be considered:
b1) if the boards are fresh, then take it up with your supplier to
correct the problem
b2) store the boards in nitrogen under slight pressure to prevent
oxidation (do not vacuum-seal, as this can draw contaminants out which
can settle on the bonding pads).
Hope this helps
Brian
Steve Kelly wrote:
> Fellow Technetters,
>
>
>
> I need some recommendations on how to clean wire bondable gold pads before
> bonding. We are electroplating 35-40 microinches of soft gold over 130
> microinches of nickel. After gold plating we have to go through 2 press
> cycles at 350 F for approx. 1.5 hours each to attach stiffeners onto the
> flex. We then surface mount components and reflow. After all these heat
> cycles we see some oxidation on the gold pads. What is a recommended
> cleaning process to have a "fresh" surface to wire bond to. Thanks for the
> help.
>
>
>
> Regards Steve Kelly
>
>
>
> PFC Flexible Circuits Limited
>
> Ph: (416) 750-8433
>
> Fax: (416) 750-0016
>
> Cell: (416) 577-8433
>
> E-Mail: [log in to unmask]
>
>
>
>
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