Following are portions of several notes regarding test probing of solder 
paste (start at the bottom and work up). Can anyone provide support for 
Bill. Maybe a process change that will reduce the flux collecting dimple in 
the vias??
 --------------------------------------------
(amplifying info)
 --------------------------------------------
According to our test fixture people, we have tried "all that is available" 
in "Flux Buster" type probes.  Perhaps I should explain the problem a little 
further:

In many cases where dual reflow assembly is used, the design is so dense 
nodal access may only be achieved by probing vias.  The preference here is 
for vacuum fixtures which require the vias to be filled with solder paste. 
 The result of vias plugged with solder from paste is a concave fillet where 
the flux residue more or less "pools" and is several times thicker than on a 
surface mount pad.  In these cases, we have been unable to find a probe 
which can break through and release with out accumulating flux build up.

I assume this is not an uncommon problem as paste manufacturers are now 
starting to market products which aleviate this problem.
 --------------------------------------------
(my initial response)
 ---------------------------------------------
. . . the booth next to me was a test fixture company, and I remember 
talking about some of their "sharp" probes with them. (We were discussing 
the aspects of testing the very thin PCMCIA boards and associated mechanical 
stress problems, didn't really talk about probing through flux residue.) 
Have you asked this question of any test fixture companies?
 ---------------------------------------------
(original query)
 ---------------------------------------------
I was wondering if EMPF has performed or is aware of any testing on how easy 
it is to probe solder paste flux residue.  We have seen that even though we 
have a qualified no-clean process, there are case where we must still clean 
in order to get repeatible test probe contact.  I have been in contact with 
most paste supplier and have gotten the impression that there testing of 
probeability has been very limited.  Any input put would be greatly 
appreciated.

Bill Barthel         [log in to unmask]
Process Engineering Manager
Electronic Assembly Corporation
(414) 751-3651