We use a lot of Through-hole  X7R, and Z5U 20% tol.  in the range of 1nF to
330nF.  After Wave the capacitance may increase to 40%  over nominal value.
 This increase lasts several weeks, and all manufacturers I talk to confirm
this is normal.  This has no real affect on our applications, as they are
used for  power supply bypass.  Kemet has a published application note:
 "AGING - Capacitance Change Due to Time or Exposure to High Temperature".
 This was written by Dick Thompson, Apr, 1995.
     Pete Hanamann
     EAC/Plexus
 ----------
 ----------
From: TechNet-request
To: TechNet
Subject: RE: ASSY: Problems with 50V ceramic capacitor at Wave Soldering
Date: Tuesday, February 04, 1997 8:45AM

Alain,

All capacitors will experience a capacitance shift when ran through any
heating cycle.  Your problem could be at what time  you actually measure
the capacitance.  My first question is if you leave the board sitting
for 24 hours are the capacitors in tolerance?  If  that is the case,
then you will know that your problem is a temporary heating situation.

I do not know what type of ceramic capacitor you use.  There are some
real gotcha's when measuring hot capacitors though.  For example, if you
use a NPO type capacitor, the capacitance change is usually in the 30
PPM region and is linear over temperature.  However, X7R capacitors can
vary +\- 15% over temperature and are highly non linear in the
temperature region.  Then you get into Y5U +20% to -55%, Y5V +22% to
 -82%, and Z5U which is +22% to  -56% over operating temperature.  Since
a solder pot is way hotter than any operating specification on a
capacitor, I am not surprised that you would find a lot of capacitors
25% or more out of tolerance.  We have found in the past when using Z5U
material that we had to let the board cool for almost 24 hours before
the capacitors would come back into tolerance.

I hope this helps your problem.

Max Bernhardt
Woodward Governor Co.
970-962-7376
 ----------

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