Ha!
The glib answer is till the results are not good enough, but this is really
the ultimate it depends....
Tack time is a reasonable guide. If nothing else the number gives you an
indication of expectation by setting a dimension in minutes, hours or days.
All properties of paste such as tack time, cold slump, print release,
fluxing efficiency and so on are related to each other and will degrade
roughly together with time. The paste will also be affected by external
factors such as deposit size - the smaller the deposit the greater the area
to volume ratio so the more susceptible it is to degradation. Also not
specific to paste, but to you/your working environment: Higher humidity,
higher temp and even air movements across the paste will also impact.
Just keep in mind that manufacturers data on tack life, cold slump etc are
determined under standardized conditions so may not be an absolute
representation of what will actually happen in your shop. [They do though
give a relative indication to other pastes tested the same way.] Some
manufacturers attempt to give in addition more practical results on
"distressed" paste. In real life paste is churned around on the printer and
then left and then printed etc, so they do tests on paste which has had some
simulation of use in an attempt to be more representative of reality.
In practice in a high volume line, printed paste life is not too relevant,
as it doesn't hang around, except possibly after line downs. On small volume
lines it is usually taken care of by SOP on loading paste and starting
afresh with each work period. So it may not be important unless you need to
determine by empirical trial how much 'engineering margin' you have between
SOP and time to fail.
Hope this helps.



Regards

Mike Fenner 
Bonding Services & Products
T: +44 [0] 1865 522 663 E: [log in to unmask]

 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Wednesday, February 22, 2012 3:11 PM
To: [log in to unmask]
Subject: [TN] Measure of "How long after printing can parts be placed."

Hi,

I was wondering "How long after printing a boards can components still be
placed with no decrease in joint consistency."

I was looking at some paste specs looking for an indication of how long a
printed image will remain "Wet" and support the placement of additional
components after a period of time. 

I was looking at IPC TM-650 2.4.44 and it describes the force required to
remove a test probe from a paste deposit over time. Not the same thing.

 Can anyone point me in a direction to resolve the issue.

Bob K.


______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________



______________________________________________________________________
This email has been scanned by the Symantec Email Security.cloud service.
For more information please contact helpdesk at x2960 or [log in to unmask] 
______________________________________________________________________