There are two ways you can go at this:
1) mask the areas
2) Accept that even if you mask them you gotta clean them so just drop the masking and
clean.

1) for masking you could try out the various temporary stops off that are available, I
suggest the most likely to work are the UV curable materials that go on as a gel or paste
and after a UV zap can be peeled off.
Alternatively I came across a sticky tape once which was used in semiconductor wafer
processing. This is sticky to hold wafers for dicing, after dicing you zap with uv and the
stickiness goes and all the die fall off. As it's for wafer fab should be OK for you.
Thats the maybe good news, the bad news is it will take me a while to plough through my
"Must file this useful stuff when I've got time"  heap to find it. Let me know.

2) Alternatively and this is proven process. You can simply select a solder paste cleaner
combination which will clean off well enough for wire bonding. Your test would be wire
pull strength. You are looking for min deviation max to min with highest mean. There is
some but not total correlation with these numbers and ionic cleanliness. You need to speak
to a supplier who'se local tech rep knows more than just conventional SMT processing, as
otherwise likely they won't know how to identify the suitability of their own products.

Mike Fenner


----- Original Message -----
From: Ingemar Hernefjord (EMW) <[log in to unmask]>
To: <[log in to unmask]>
Sent: 06 October 1999 14:02
Subject: Re: [TN] Wire(?) Bonding, was: Re: [TN] Temporary Solder Mask


> Hi Matthias,
> don't use anything "wet" or "chemical" or "plastic" on the bond pads! There will be a
powerful reaction from jk or Mike F or worse..from Aussilek! We have exactly your
wonderings in a line just now, to protect bond pads from flux splatting around during oven
process. We simply let the robot drop a plastic cover down, because all bondings are
concentrated to one area. After oven soldering, the cover is lifted away.  If your bond
pads are spread out, of course this method doesn't work. May I ask what kind of wire
bonding you aim at? If it's heavy wire bonding it should not be so sensitive to minor
residues, thin gold wire more apt to disasters then. In principle, you should make your
process such, that flux is stopped within such ranges that your bond pads are not
involved. Well, to say more is not meaningful, I don't see your problem with a video
camera, hope that comes soon in TechNet.
>
> Cu
>
> Ingemar Hernefjord
> Ericsson Microwave Systems
>
> I'm looking for a reliable process to protect wire bonding areas
> (mostly immeesion NiAu for Al wire bonding) with something like a
> temporary soldermask against any dirt from SMD process, instead of
> expensive cleaning processes like plasma...
>
> Thanks, regards
> Matthias Mansfeld

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