The more the flux is 'cooked', the harder it is to remove. As you 
apparently use a rosin-based flux, it may become very difficult. It is 
not helpful to say you use an ultrasonic cleaner without telling us your 
soldering time/temp profile, cleaning products and procedure, ultrasound 
data etc.

I suggest you consult your flux manufacturer for advice on how HIS flux 
residues may be best removed. For cleaning to wire-bonding standards, 
you will probably have to go to more than a solvent, such as Vigon or 
Kyzen products, if the flux maker cannot help.

There is no 'best' product.

Brian

jonathan noquil wrote:
> Hi All
> What is the best flux cleaning agent that can effectively remove flux on
> ENiAu bond pads?
> We solder a copper strap on the silicon with PbSn5 at 350 degc temperature,
> the flux contaminates the bonding pads.
> Paste is from Heraues. We used an ultrasonic cleaning equipment.
> During Au wirebonding, we have lots of bond off occurences? We tried to do
> FTIR on the surface and found some traces of that resin contamination.
> 
> Is the flux difficult to remove with flux cleaning agent when it was
> reflowed at high temperature?
> 
> 
> Thanks
> 
> 
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