Thank you Brian for you insight.
No elemental Chlorine—so when I read from the NEC Electronics website and the Recommended Soldering Conditions that “Maximum chlorine content of rosin flux (percentage mass) 0.2% or less” they are meaning a chlorine compound or a chloride.  Which makes some sense when I read from a Agilent Versatile Link fiber optic Tech Data sheet which recommends “nonhalogenated .. fluxes (0% chloride)”. 
The reason for my initial question was component failures. The company I work for has experienced failures of an opto coupler. The opto coupler component manufacturer found in their analysis chlorine residue in the package . I questioned and suggested the source might be from the encapsulation process or other assembly process which they claimed was not possible because they realize the detrimental effects of chlorine to the wirebonds and leadframe. 
The company who solders the printed circuit assemblies uses a flux that contains chlorine (1.5% by wt). Plastic encapsulated IC packages are not considered hermetic, hence the conclusion of chloride in the flux migrating into the IC was a possible cause of the component failures.
Other thoughts are appreciated.
 
Mike B
-------------- Original message from Brian Ellis <[log in to unmask]>: -------------- 


> I'll take the example of an SOIC, but you can extrapolate the points to 
> other package types. 
> 
> Firstly, no flux contains elemental chlorine. Some fluxes contain 
> organic chlorides or bromides which, under the influence of heat, 
> decompose into hydrohalide gases. 
> 
> Plastics may also contain chlorides or bromides, some added 
> deliberately, some there as a result of a prepolymerisation reaction. 
> 
> Then there is the leadframe: is it clean and burr-free (if stamped) and 
> exempt from halides (if plated or chemically etched). 
> 
> The integrity of a plastic casing around an IC chip is often doubtful. 
> It depends on the good flow of the material round the leadframe and the 
> adhesion to the leadframe. If either of these is doubtful, then you have 
> means of ingress of hydrohalide gas molecules, especially at soldering 
> temperature, remembering that there is a stark TC difference between the 
> frame and the polymer. Then how porous is the polymer structure itself? 
> The hydrogen chloride molecule itself is quite small and can penetrate a 
> polymer to a few molecules depth at temperatures above the glass 
> transition temperature of the plastic (i.e., above soldering 
> temperature), the same as for board substrates. 
> 
> Various stock phrases of these netlists come to mind: "It depends!", 
> "How long is a piece of string?" etc. IOW, there is no hard and fast 
> rule because it is up to the IC manufacturer to provide the quality you 
> require to minimise the problem or, if it happens, to ensure that it has 
> no effect on the IC function over its lifetime. Most of them do a 
> reasonable job, but there are variations within a single manufacturer 
> from batch to batch, otherwise called working tolerances. The point I 
> wish to make is that, even if you use a halide-free flux, you cannot 
> guarantee that the manufacturer used the same if the leads are 
> hot-tinned. In fact, he probably used a highly aggressive 
> chloride-containing water-soluble flux, especially if the leadframe is a 
> nickel alloy. 
> 
> Hope this helps (but I'm afraid it won't!) 
> 
> Brian 
> 
> Mike B wrote: 
> > Do anyone have information regarding solder fluxes that contain chlorine 
> > migrating into plastic IC packages? 
> > 
> > 
> > 
> > 
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