Hello UGH!

In response to your first message, water based flux, let me run this by you...

One of the most important reasons for fluxing is to remove surface oxides and 
render the surface suitable for a good alloy between the copper and the solder 
media.

All fluxes leave residues! 

If you are using a no-clean process, then the residues are to be left on the 
board surface and they should be a benign influence on circuit reliability - 
but how do you know?

If you have no-clean flux/paste, then the level of activity cannot be 
particularly good and that is why you and others have to pre-tin component 
leads. The high volume guys just have to come on strong with their component 
and board suppliers to keep things under some degree of control.

If you were to use an aggressive water soluble flux/paste, you could, very 
probably, solder black leaded components or at least, heavily oxidised 
surfaces. This would mean that your process "window" suddenly stretches to the 
size of a barn door compared to the "porthole" of no-clean.

HOWEVER, it is essential that you clean off the residues and if you do not 
have the right cleaning machine, you are going to suffer. Look at modern 
aqueous machines in the USA, and you will find minimum 15HP pump technology.

So no-clean residues wouldn't be as harmful? NOT SO! Most of these modern 
process chemistries have non-ionic additives that are not detectable using 
ROSE testing (ionic extract) but that are harmful contaminants. 

For example, how do you get a low or zero solids product to stick? Answer, by 
adding "wetting agents". During soldering everything expands with the heat 
(boy it must be hot in here!) and these additional liquids are more readily 
absorbed into the board surface through the solder resist. Of course when they 
cool down, the surface closes up, and the residues are trapped.

You need to trial different types of flux and paste to find the ones that are 
the most sympathetic to your process.

As you may be aware, new test methods are being applied to more accurately 
determine circuit reliability rather than circuit cleanliness which is always 
somewhat impirical. Refer to the IPC/ANSI-J-Std-001, IPC-TM-650 and IPC 9201, 
and you will get some better information.

You go on to ask about N2 bags.

First, maybe just maybe, using water soluble fluxes will remove the need for 
this?

Whatever, I recommend that you contact Dr Chris Hunt of the National Physical 
Laboratory in the UK. ([log in to unmask]). They are into year 2 or 3 of a major 
long term research project into this, and he may well be able to give you 
detailed information.

Hope this helps, Graham Naisbitt
[log in to unmask]

Tel: +44 (0)1276 691100
Fax: +44 (0)1276 691227

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