Brian,
It might not "help", but it is an excellent answer.
Bev
RIM
----- Original Message -----
From: Leadfree <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Sent: Thu Nov 13 03:22:18 2008
Subject: Re: [LF] solder fluxes that contain chlorine
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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