(1) viscosity one point does not tell you if the organic part of the paste
is degraded or not: shear rate should be the one.  if you do not have
shear rate of the paste in qualification stage as reference - tough luck
(2) slump may not be good - you need to shorten the process window if you
know what you are deal with (see item 1)
(3) solder ball might occur - depend upon your design margin - clearance
to be specific.  due to the potential lower activity of the flux - if you
have residue on your pwb surface as solder fine - if you do not have good
vision for inspection - you are running risk.  since you are running class
III product, assume you have qualified vision inspector look at the extra
high mag for visual.
(4) ionic extraction might need on the sampling basis - just in case the
thermal degradation resulted some un-known species as small molecules that
oxidized and left on the PWA that was not the chemist design intended.  -
always a risk when you deal with unknowns.  if nothing pop out, all the
better.  Class III should not take any risk work.
(5) with all the work required - as mentioned above, you might as well
order a new batch provided you are not impact your delivery date.  (for
class III, I would delay the delivery even ... depend upon what is the
product).  the vendor - or the shipper, should split the cost if the
contractual agreement clearly stated the responsibility of the shipment -
if it is you, too bad.. (next contract should get that clause in - talk to
your legal department and might add 5% cost as part of it... you will
sleep well)
(6) always carry minimum 2 date code - so you will never run out (3 month
apart for shelf life), specifically designed to address some issue like
you just mentioned.  - payment should not be issued until receive the
parts in good condition (at least 50% of the payment) - logistic issues
depend upon your company policy.  Good luck.  (money well spend)
my 1.6 cents.
              jk
> Hello Leland,
>
> There are a couple of potential issues when using solder paste that has
> been thermally stressed.   Reactions between the flux and solder powder
> are accelerated by heat.  Some solder pastes are not stable at room
> temperature, and higher temperatures can cause dramatic degradation in
> performance.
>
> If the viscosity is within specification, then the solder paste will
> likely print fine.  The stencil life may be shortened, so the print
> characteristics of the paste may worsen more quickly than normal.   The
> solder paste may also have issues in reflow.  Wetting may not be optimal,
> and you might see solder balling and graping issues.
>
> I agree with a previous response that it is best to check with your
> supplier before using the solder paste.
>
>
> Best regards,
>
> Tony Lentz
> FCT Assembly
> Field Application
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Leland Woodall
> Sent: Friday, June 5, 2015 11:06 AM
> To: [log in to unmask]
> Subject: [TN] Solder Paste Question
>
> Everyone,
>
> We just received 25 tubes of solder paste that got misplaced by the
> carrier and was delivered in 6 days instead of the usual two.  When it
> arrived it was warm to the touch.
>
> We performed a viscosity test after allowing it to stabilize to ambient,
> and the readings returned were within spec and very close to our typical
> results.
>
> Is there any danger in using this shipment?
>
> Thanks for your help!
>
> Leland
>
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