Flux specs such as IPC J004 are qualitative rather than quantitative. In
other words what the residue does/doesn't do is measured, not how much of it
is there. I offer the following for information.

Some paste suppliers give percent residue remaining numbers for their
products but this is by no means consistent. Not all do and those that do
don't do it on every product! As a guide a resin based no clean will leave
about half its starting flux content as residue, usually slightly more. The
number will go up and down slightly according to product. Small changes can
also be made with profile. 
Low residue types are, but require inert atmosphere to make up the loss of
protection from the resin. They are not much used in SMT.
When evaluating residues by TDS number you need to make sure that your
"units" are the same. Some suppliers reference a percent of the flux and
others of the paste.
The 50% number I quoted is of the flux. Remember that in round numbers the
flux is only 10% of the paste so as a proportion of the paste that would be
5%, which sounds pretty good. It will look a lot more though.
This is by weight; by volume the flux/solder ratio is closer to 50:50.
So if the starting flux is half the printed volume and half of that remains
after reflow that's 25% of the print volume remaining. The residues can also
be in slightly different places. They can spread out, sit on the joint and
so on. Sod's Law (Murphy's in US) predicts that if its important residues
will be where you don't want them. In your case under devices :) 
Whether this has any significance in manufacturing or service life you will
have to determine.


Regards 
 
Mike Fenner 
Bonding Services & Products
M: +44 [0] 7810 526 317 
T: +44 [0] 1865 522 663
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne - IS
Sent: Monday, November 05, 2012 11:50 PM
To: [log in to unmask]
Subject: [TN] Limits on flux residue

Is there a spec somewhere which limits the amount of residue allowed for no
clean processing?  I just saw a board where a BGA was about 70%
"underfilled" by flux residue.  No, this was not a flux/underfill product!
A quick look at JSTD-001 Section 8 didn't seem to have specific guidance on
this condition.

Wayne Thayer

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